Environmental Correlates of Aquatic Faunal Distribution in the Namib Desert

J. A. Day Freshwater Research Unit, Zoology Department University of Cape Town, Rondebosch, 7700 South Africa

The very limited literature on the limnology of the Namib Desert is reviewed. The major habitat types are described: permanent systems include two relatively large lakelets, a number of freshwater springs that form shallow trickles or pools, and a series of brackish to hypersaline mineral springs on gypsous crusts; ephemeral systems include rainpools that form in endorheic basins, pools left in river-beds after they have stopped running, and the rivers themselves, which contain water for very short periods.

A checklist of the aquatic fauna is provided. Analyses of the entries in the list show that ephemeral waters are dominated by crustaceans and permanent waters by insects, especially immature forms. The number of taxa is greatest for permanent lakelets and streams and least for endorheic rain pools and gypsous springs. The number of taxa does not correlate with salinity for any of the habitat types nor with depth of water in any except the gypsous springs. The major determinant of faunal richness is isolation, the number of taxa showing strongly significant negative correlations with distance from a given pond to another of the same kind, or another of any kind, for all types except the gypsous springs.

INTRODUCTION limnology, documents what is known of the distribution and the phYSical and biotic features of the major types of surface .

) The characteristics of athalassic (non-marine) surface waters waterbody in the Namib Desert and examines some of the 9

0 in any region depend primarily on the geology of their catch­ determinants of species richness in the aquatic fauna. The 0

2 ments and on the climate. In arid areas, by far the more area under discussion is limited to that from the Munutum River

d 0 0

e significant of these two is the climate, to the extent that one in the north (Fig. 1) to LOderitz (27 N, 15 E) in the south and t a might expect deserts to have no surface waters at all except extends inland to about the 100 mm isohyet because, although d (

for ephemeral pools that form, or rivers that run, for a brief the Namib Desert extends well into Angola, no limnological r e period after rain. This is far from the truth, although the most information is available on that region. Further, with the excep­ h s

i obvious waterbodies in deserts are certainly temporary. tion of ephemeral pools forming after rain at Sossusvlei and in l b The Namib Desert is no exception. Climatic conditions are a few small depressions near LOderitz, there are virtually no u

P severe, to the extent that evaporation rates may reach athalassic surface waters (Le., those not influenced by the sea)

e 4000 mm y-1 (Lancaster, Lancaster and Seely, 1984, give a in the dune field south of the Kuiseb River. h t mean pan evaporation rate of 3168 mm y-1 for the central Largely for logistic reasons, including the unpredictability of y b

Namib). Rainfall is spatially erratic and highly unpredictable rainfall, the Namib Desert is remarkably poorly known lim no­ d

e from year to year, while the mean annual rainfall over most of logically so that the literature on the topic is very limited. A few t

n the Namib is considerably less than 100 mm y-1 (Lancaster et papers provide some limnologically pertinent information on a r al., 1984). Winds are high. Thus it is not surprising that, as in climate (Lancaster et al., 1984), historical climatology (Seely g

e deserts everywhere, although surface waters do exist in the and Sandelowsky, 1974; Sandelowsky, 1983) and geology c

n Namib, they are small, scattered, and often ephemeral. and soils (Martin, 1965; Scholz, 1972) or a combination of e c

i Despite their small sizes and scattered distribution, the these topics (Logan, 1960; Goudie, 1972). Some aspects of l

r inland waters of the Namib are chemically very diverse, result­ the distribution of surface and subsurface waters are men­ e

d ing in a variety of permanent or semi-permanent pools and tioned in, for example, Hellwig (1988). n springs with a more diverse aquatic fauna than may otherwise Somewhat more published information is available on the u

y be expected in the region. Because the rate of evaporation chemistry of surface waters on the gravel plains of the central a

w exceeds rainfall, permanent waterbodies cannot form unless Namib (Kokand Grobbelaar, 1985), on gypsous crusts (Martin, e t they are fed from underground sources. Thus all the surface 1963; Watson: 1979), on hot springs (Gevers, Hart and Martin, a

G waters in the Namib can be divided into ephemeral ones that 1963), on waterholes in the Kuiseb River canyon (Kok and t

e form as a result of rainfall, and semi-permanent or permanent Grobbelaar, 1980), and on Kuiseb River water (Grobbelaar n i ones fed by groundwater. and Seely, 1980). Some aspects of the limnology of b a This paper briefly reviews the literature pertinent to Namib Sossusvlei, a large ephemeral interdune lake, are described S y b

d Day, J. A., 1990, Environmental correlates of aquatic faunal distribution in the Namib Desert. In: Seely, M. K., e c ed" Namib ecology: 25 years of Namib research, pp. 99-107, Transvaal Museum Monograph No, 7, Transvaal u

d Museum. Pretoria, o r p e R 100 AQUATIC FAUNA IN THE NAMIB DESERT

19~

eOUTJO eKHORIXAS ~ N ~ ~ . ) 9

0 ~ 0 2 d

e Ct, t USAKOS a rain pools in river beds d o ~ (

r • endorheic rainpools ~ e

h permanent lakelets s * i l freshwater springs

b a u • gypsous springs P

e 2305 .. permanent running streams h t y b d e t n a r

g 100 km e c n e c i l

r Fig. 1 e d

n Map of the northern and (inset) central Namib Desert showing the positions of the waterbodies referred u to in the text. Each symbol may refer to more than one site. y a w e t a by Grobbelaar (1976) and of Hosabes, a hypersaline spring, nothing has been published on the biology of the Namib G

t by Day and Seely (1988). A number of features of the Kuiseb aquatic biota, although a number of systematic papers (see e

n River are described in the report edited by Huntley (1985) and references in Table 1) deal either exclusively with, or include i b by Stengel (1964), who also examines the Swakop River. information on, Namib forms. The paper on the flamingoes of a S

With the exception of several articles on frogs, including Etosha Pan (Berry, 1971) and that on the diatoms of thermal y

b those by Channing (1976) and Jurgens (1985), it appears that springs near Windhoek (Schoeman and Archibald 1988), for d e c u d o r p e R AQUATIC FAUNA IN THE NAMIB DESERT 101

example, contain some information on taxa that probably also depressions after rain. occur in the Namib itself. The borders of that part of the Namib under discussion are more or less coincident with the Orange River in the south and MAJOR TYPES OF AQUATIC ECOSYSTEM the Cunene River in the north, the only permanent rivers in the region. Nothing has been published on the Cunene, while In most climatic zones, surface waters are conveniently di­ Cambray, Davies and Ashton (1986), Agnew (1986) and Skel­ vided into standing (lentic) and running (lotic) systems. In ton (1986) address various aspects of the limnology of the deserts, though, a more appropriate and fundamental division Orange River. is between permanent and semi-permanent, and temporary One of the characteristic features of the Namib north of about (ephemeral) waters, a division that is also reflected in the major 22° S (roughly Walvis Bay) is a series of dry river-beds running elements of the biota. Below is a classification and brief perpendicular to the coast. They act more as canals than as description of the major types of aquatic habitats in the Namib. rivers in the usual lim no logical sense, in that they carry water The localities of those known to the author are indicated in infrequently and do not develop a lotic biota. Their waters are Fig. 1 and Table 1. allogenic (derived from elsewhere, being precipitated in the more mesic uplands and channeled towards the sea via the Permanent and semi-permanent waterbodies rivers) and in many cases little or no water actually reaches Such systems are fed by groundwater springs. Their surface the sea, except during periods of unusually heavy rain. The areas and fluctuations in depth are thus determined by the more northerly rivers run to the sea almost every year, while relationship between the rate of flow of spring water (they are those in the south are less predictable. Of all of the rivers, the often recharged by subsurface flow from rain that has fallen Kuiseb, which divides the dune fields of the southern Namib many kilometres inland), the rate of evaporation, and the from the gravel plains of the north, is the best known. The occasional influx of rainwater. upstream damming of both the Kuiseb and the Swakop Rivers, Although no systems in the Namib fit the definition of a true and extraction of water from below the bed of the Kuiseb lake (a waterbody too deep for rooted plants to grow on the (Huntley 1985), has resulted in these rivers flowing for shorter bottom), at least two lakelets (Ausis and 'The Oasis', both in distances and for shorter periods, and consequently in less the region of Mowe Bay) cover several hundreds of square frequent and less intense flooding in recent years. Nonethe­ metres, reach a depth of a metre or more, and appear to be less, when flood-waters recede, pools of very fresh water are permanent. They can be classified as permanent endorheic left in the river-beds (RR in Table 1). In deep canyons, these (closed-basin) lakelets (PL in Table 1). may persist for months.

. On some geological formations, the water in smaller perma­ Although surface flow is seasonal and ephemeral, the river­ )

9 nent springs remains fresh, or nearly so. On others, usually on beds act as permanent courses for subsurface water. Where 0

0 gypsous crusts, evaporation from the surface, together with this water is close beneath the surface of the river-bed, the 2

d precipitation of CaC03, results in brackish to highly saline rivers form longitudinal oases supporting a variety of plants e t surface waters. (often including large trees) and their associated fauna. while a d Freshwater springs (FS in Table 1) are found in dry river­ larger mammals are able to excavate waterholes in the river­ (

r beds or on the open plains and vary from small trickles to beds. e h standing pools up to a metre or so deep. Most are permanent. After rain, water will also collect in rainpoo/s in endorheic s i l In a few places (near the mouth of the Uniab River and at basins (RE in Table 1). which may be depressions in rocky b u Wolfwasser, for instance) enough groundwater escapes to outcrops or on any other sealed surfaces such as clay or salt P

e produce very fresh, fast-running, permanent streams (St in pans. Depending on the intensity and duration of rainfall, the h t Table 1). pools may persist for as little as a day or two, or for as long as

y The small, highly mineral springs on gypsous crusts (GS in several weeks or, exceptionally, months. Although the water b

d Table 1) are common in several deserts, where they seep from in most pools remains fresh to brackish. in some it may e t faults, extend for a few tens or hundreds of metres, and then become very saline. n a disappear. A fault-line running along much of the coast of the r g

Namib provides a series of these springs, one of which has THE AQUATIC FAUNA OF THE NAMIB e c been described in detail by Day and Seely (1988). It appears n e that the groundwater supplying the springs is fresh, or nearly Table 1 is a preliminary list of the taxa known so far from c i l

so, but owing to the intense solar radiation and the hygroscopic athalassic waters of the Namib. It has been compiled from the r e gypsous soils, the water evaporates until enough CaC03 available literature on the taxonomy and distribution of the d n precipitates for the water to become a slightly calcium­ aquatic fauna of the Namib Desert, together with records from u enriched NaCI brine. One of these systems in the central my personal collections and hatching experiments on dried y a Namib is unique in possessing the only known athalassic mud, and from the State Museum, Windhoek. As is frequently w e species of the protozoan group Foraminifera (Brain and the case in Africa, the taxonomy of most groups is poorly t a Adams, 1984). known, so that many of the taxa are incompletely or tentatively G

t identified. Forthis reason, the term 'species diversity' has been e n Ephemeral waters avoided in the following discussion, where 'richness' refers i b Ephemeral waters include rivers that run for short periods merely to the number of taxa. Further, the apparent distribution a S

after rain has fallen high in their catchments, the pools thatthey of the various taxa must reflect collecting bias, in that most y b leave behind as they cease to flow, and pools that form in samples were obtained from the central Namib and the d e c u d o r p e R 102 AQUATIC FAUNA IN THE NAMIB DESERT

Table 1 A preliminary checklist of the aquatic metazoan animals known from athalassic surface waters in the Namib. RR =rainpools in river-beds; RE =endorheic rain pools; PL =permanent lakelets; FS =permanent freshwater springs; St = permanent streams; GS =springs on gypsous beds. 'Sp. indet.' refers to species considered to be new by the authorities who have examined them.

RR RE PL FS St GS RR RE PL FS SI GS

COELENTERATA 13 Polamocypris mastigopllora + Hydrasp. + Sarscypridopsis ?glabrata ? S. ocIlracea + + PLATYHELMINTHES 13 S. cl. pygmaea + Turbellaria + + Sclerocypris coomansi + S. dayae + + NEMATODA '3 + S. dedeckkeri + Strandesia cf. vinciguerrae + ROTIFERA Brach/onus calyciUorus dorcas 12 + INSECTA Others 13 + + EPHEMEROPTERA 13 Baetidae ANNEUDA Cloeonsp. + + OLiGOCHAETA 13 + Other + + + HIRUDINEA ODONATA Umnatis sp, indet.'2 + Libellulidae ?Crocothemissp. (nymph) 13 + ARACHNIDA ?Nannotllemissp, (nym~hl'3 + + HYDRACARINA 13 ?Paltoillemissp, (nymg ) 3 + + Sp.A + + Panlala Ilavescens (a Ult) 5 ? Sp.B + + Tri/Ilemis kirby; arclens (adutt) 5 Sp.C + + + Sp,D + Gomphidae Sp. E + Paragomphus genei'2 Sp, F + Paragomphus sp, (nymph) 13 + + + ARANEIDA 12 Coenagrionidae Pardosasp, + Enallagma sp, (nymph) 13 + + Theridion sp, + Enallagma glaucum (aduH) 4,5 ? Ischnura senegalensis Ifdult) 5 ? CRUSTACEA Ischnumsp, (nymphs) ? NOTOSTRACA 1,2,13 Triopsgmnarius + HEMIPTERA

. NotoneClidae 6,13 ) CONCHOSTRACA 13 Anisops sardea + + +

9 +

0 Caenestheriella cf. australis +

0 Eocyzicussp. + + Gerridae 13 2

Eulimnadiacf. a/ricans + Limnogonussp. + d Leptastheriellacf. inermis + + e Hebridae 13 t Leptastheriaci, rubidgei + a L cf, slTiatoconcha + Sp,A + d ( ANOSTRACA 12 Naucoridae 13 r e Bmnchipodopsiscf. kaokoensis + ?Pelocorissp. + + + h B,lridens + + s i Branchipodopsissp, indeJ. + Saldidae 13 l Straptocepllalus caler Sp, A b + +

u S, ovamboensis +

P Slreptoceplla/ussp, indet. + + Corixidae al3 e Sigaracf, contortuplicata + + + + + h CLADOCERA Sigamsp,13 + + + + t Alona karua '2 + y Alona sp, (raclangu/a group) 12 + + COLEOPTERA b Ctenodallhnia comula rigaudl'2 + d C, dubia l2 + Gyrinidae e 9 t Ctenodaphniasp, 7 + Dineu/us sUbfinosus + n Daphniasp7 + Dineutus sp, I + + a Macrotllrixcf, gouldi'2 r + + g M. Iriserialis 12 Hydrophilidae

+ e Moina belli 12 + + Berosus sp, A 13 + c M. dubia7 t Berosussp, B 13 + n e M. cf. llartwigi'2 + Berosus sp, C 13 + + + c M. cf. micrura 12 Cee/os/oma rufitalSe 12 i + + + + + + l

M, cf, reticu/ata 12 + He/ochares (He/ocllares) sp, 12 + r Oxyurella singa/ensis 12 He/oCllaras (Hydrobaticus) sp, 12 e + + d Tropistemussp,13 + t n COPEPODA u Eucyc/ops (Afrocyclrp) gibsoni '2 + + Dytiscidae y Mesocyclops major + Cybis/er llipunctatus afncanus 12 + + a M. (Tllermocyciops) oblongatus 12 + + Ereles stictus 13 + 7 + + + w Metadiaptomus meridianus + Graphoderus Sp.'2 + e t Microcyc/ops inopinatus 12 + Herophydrus Sp,12 + + + a Hydroglyphus ,nlirmus 12 +

G OSTRACODA 3,12 H, linea/a/us 12 t

+ + t Ampllibolocyprissp,lndel + H, zanzibarensis 12 + e Apate/ecypris schultze! + + + H droglyphussp, A,12 n + + i Eucypriscf, t~ona + tus 12 + b He/erocypris ,congenera + + L pficis/riatus 10 + a H. cf. giesbracllti S + + lsocypris perangusta + y

b Plesiocypridopsis /naequiva/va + Hydraenidae 13 Plesiocypridopsissp,lndel. Ochthe/ius sp, A t d + + + + e c u d o r p e R AQUATIC FAUNA IN THE NAMIB DESERT 103

Table 1 (continued) particular habitat type to the number of sites in that category allows the different types to be ranked according to richness RR RE PL FS St GS of the fauna (i.e., number of taxa per site). Although the data on which the numbers are based are far too incomplete to use Ochthebius sp. B + for anything other than such ranking, these numbers do indi-

Dryopidae 12 cate that permanent lakelets and streams are richest in taxa, Heterocerus sp. + + followed by freshwater springs and rainpools in river-beds, with Elmidae 13 gypsous springs and endorheic rainpools supporting relatively Sp.A + few taxa. Ptilidae 13 Endorheic rainpools are dominated by crustaceans, espe- Sp.A + cially euphyllopods and ostracods, which are able to withstand Staphylinidae 13 long periods of desiccation within 'egg' shells (which are Sp.A + + actually cysts containing young larvae). Typically, they are DIPTERA 13 uncommon in, or absent from, permanent waters and prelimi- Chironomidae + + + + + + nary personal observations suggest that a dry resting period Culicidae is obligatory before the eggs can hatch. Although this appears Anopheles listeri 12 + + A. fonffnalis 11 + not to be true for the copepods or the cladocerans, these, too, Anopheles spp. 13 + + + + Culex (Culex) theileri 12 + generally occur in ephemeral waters and must also have a C. (C.) cf. theileri 12 + + desiccation-resistant phase in their life-cycles. Sixteen of the Culexsp. indet. 12 + Culiseta (Allotheobaldia) longiareolata 12 + 48 species of Crustacea are ostracods, reflecting both their Culiseta sp. indet.12 + species richness in the ephemeral waters of the Namib and Psychodidae 13 + + + + the extensive recent systematic work on the group by Martens 13 Empididae + (1984, 1986, 1988). Further information on the crustaceans of Tipulidae 13 + + + Tabanidae 13 + + desert waters is available in the reviews by Hartland-Rowe Stratiomyidae 13 + (1972) and 8elk and Cole (1975). Ceratopogonidae 12 The fauna of rainpools in river-beds comprises an element Culicoides schultzei + c. herero + typical of ephemeral pools and, if the river has a distant origin, Culicoides sp. + may also include lotic elements brought downstream from Dasyhelea sp. + + + + Leptoconops dixi + further inland. Thus some such pools may have a rather L. interruptus +

. diverse fauna for this, and other, reasons. First, rainfall is more )

9 Syrphidae 13 + predictable inland, so that organisms able to withstand only 0

0 Ephydridae seasonal desiccation can survive in river-beds because they 2 Ephydra stuckenbergi (adult) 12 + +

d are usually flooded annually. Second, at least some pools

e Sp. A llarva) 13 + + t sp. B larva) 13 + persist for long enough to be colonized by flying insects and a d propagules imported on the fur and feathers of vertebrates ( VERTEBRATA

r AMPHIBIAB visiting the pools. Third, a number of the rivers have been e Bufo vertebralis + h dammed in their upper reaches; the resulting permanent lakes s Phrynomerus annectans + i l Tomoptema delalandii + provide habitats for a variety of aquatic organisms that may be b u washed downstream in floods to join the temporary pond P Number of systems sampled 16 35 2 17 3 16 fauna. Even the obligatorily aquatic frog Xenopus laevis has e Number of taxa 48 51 10 43 21 31 h t Number of taxa separable to species 41 45 9 42 13 25 been recorded from pools in the Kuiseb River (Channing,

y Ratio of taxa: systems 3,0 1,5 5,0 2,5 7,0 1,9 1976), where fish have also been found in pools. b

d The insects, especially immature forms, are poorly known e 1 Longhurst (1955) t systematically so that the number of species must exceed the 2 Barnard (1929) n

a 3 Martens (1986 or 1988) number of taxa listed. Some seem to have wide distributions, r 4 Ris(1921) g 5 Longfield (1936) occurring in several types of waterbody. The beetles, which e c 6 Hutchinson (1929l can generally be separated into different species or placed n 7 Grobbelaar (1976 e BChanning (1976) fairly reliably into genera even by non-coleopterists, are wide- c i 9 Brink (1955) l

spread in all types except endorheic rainpools. The only spe- 10 Omer-Cooper (1965) r e 11 Gillies and De Meillon (1969) cies SO far found in these pools is the cosmopolitan Eretes d 12 Identifications of taxa belonging 10 the State Museum, Windhoek, or to the author and n stictus satisfactorily identified by systematists. (Dytiscidae), a strong flier occasionally found even in u 13 Identifications of taxa belonging to the State Museum, Windhoek, or to the author and very isolated and hypersaline gypsous springs, the fauna of y tentatively identified by her, often from unsatisfactory non-African sources. a which is otherwise restricted to surface-dwelling spiders and a w e few aquatic beetles and dipterans. t a Interestingly, only one of the 31 taxa that occur in the G

t Skeleton Coast. Most of the ephemeral waters have been gypsous springs is crustacean. A third of the taxa (mostly e

n sampled only once, so that temporal variations are entirely insects) appear to be restricted to this type of habitat, while i b unknown. Despite these limitations, some points of interest somewhat less than a third co-occur in gypsous and freshwa- a S

arise. ter springs. Five species of beetle occur both in gypsous and y

b Calculation of the ratio of the number of taxa recorded for a ephemeral waters, reflecting their ability to withstand either d e c u d o r p e R 104 AQUATIC FAUNA IN THE NAMIB DESERT

desiccation or extremes of salinity (or both), and their strong Table 2 flying abilities. Comparison of Sorensen's community coefficient (expressed as per­ centages) for the six habitat types. RR rainpools in river-beds; RE The faunas of the two permanent lakes are, characteristi­ = = endomeic rainpools; PL = permanent lakelets; FS = permanent cally, dominated by planktonic crustaceans with the odd spe­ freshwater springs; St = permanent streams; GS = springs on gypsous cies of beetle, hemipteran and chironomid. beds. Taxa higher than species have been omitted from this analysis. The permanent freshwater springs are dominated by in­ sects, particularly beetles, while crustaceans are virtually ab­ sent. The relatively low ranking of taxa per pool may be an RR RE PL FS St GS artefact caused by the number of insect taxa identified only to RR 36,8 20,0 41,0 40,1 21,2 family. The faunal richness in freshwater springs seems to be RE 7,2 11,4 6,8 2,8 positively correlated with their degree of permanence, al­ PL 11,8 18,2 0 FS 32,7 38,8 though insufficient information is available to confirm this sus­ St 15,8 picion. Certainly the number of taxa is greatest in the true GS streams, which are also unusual for the Namib in that their fauna is typically lotic rather than lentic. The relative similarities of the faunal communities of the six habitat types can be simply analysed (Table 2) using Sorensen's Community Coefficient (Southwood, 1966), which LaVigne, 1984); and that the number of taxa will be inversely expresses the number of shared species in any two habitats proportional to environmental 'harshness', usually some mea­ relative to the total combined number of species: sure of a variable such as salinity (e.g., Ranta, 1982) or oxygen content of the water (e.g., Morton and Bayly, 1977). cc= It has generally been found that the number of taxa is most a+b closely related to pool size, however (e.g., Weir, 1966; Dehoney and LaVigne, 1984). This is easily explained for the where CC is Sorensen's Community Coefficient, j is the num­ inhabitants of temporary pools, where size, especially depth, ber of shared species, and a and b are the number of species is proportional to longevity and therefore to the likelihood of an in each habitat type. animal being able to complete its life-cycle. It is less easily There is an interesting contrast between the two ephemeral explained for permanent systems, though, since increased habitats: while those in river-beds share a good proportion of size per se does not necessarily result in increased habitat

. their species with all other habitat types (for reasons men­ diversity, the commonest explanation in island biogeographic )

9 tioned above), endorheic rainpools, which are generally iso­ theory for increased species diversity in 'islands' of increasing 0

0 lated habitats, show a close similarity only with rainpools in size (but see, for instance, Fryer, 1985). 2

d river-beds. The similarity between these two types is deter­ Even with the rather limited data presently available to me, e t mined largely by those crustaceans that are confined to it is possible to make a preliminary assessment of the relation­ a d ephemeral habitats. Of the permanent running waters, the ship between faunal richness (Le., number oftaxa) and salinity, (

r freshwater springs share 30 % of their species with both the size of system, and distance from other systems. The analyses e h streams and the gypsous springs. described below and/or shown in Fig. 2 are based on data from s i l all the sites indicated in Fig. 1 for which measurements of the b u FACTORS DETERMINING FAUNAL RICHNESS OF THE other variables are available. Those from the Skeleton Coast P HABITAT TYPES were all collected in January 1983, while data from the central e h

t Namib Desert (inset in Fig. 1) have been collected by me from

y Because of their small sizes and isolated positions, and the time to time over several years. Thus a major source of error b

d generally perceived harshness of their physical and chemical in these analyses, especially for the ephemeral pools, lies in e t environments, desert waters, especially temporary ones, have the fact that samples were collected after different intensities n a often been considered to be useful models for an improved of, and at different times after, rainfall. r g

understanding of distribution patterns, dispersal, isolating Linear regressions CZar, 1984) of the number of taxa upon e c mechanisms, island biogeography and competition theory. salinity (measured in the field with an American Optics refrac­ n e The resulting hypotheses often propose that the number of tometer) are not statistically significant for the data-set as a c i l

taxa in desert and/or ephemeral pools will be determined by whole or for any of the habitat types individually (P> 0,05). r e the size of the waterbody (see, for example, Weir, 1966; Indeed, although a plot of the number of taxa versus salinity d n Pajunen and Jansson, 1969; Dehoney and LaVigne, 1984; shows considerable scatter, the trend reflects, if anything, an u Ebert and Balko, 1984), its isolation (e.g., Pajunen, 1986) and increase in the number of taxa at least at intermediate y a its chemical composition (e.g., Weir, 1966; Ranta, 1982). salinities. w e

t These hypotheses presuppose that: either dispersal from Surface area of each system was calculated from measured a pond to pond is limited (e.g., McLachlan, 1983a, b) or that or estimated lengths and widths, and depth was measured in G

t dispersal is common (e.g., Dimentman and Margalit, 1981) situ, both within an accuracy of about 10 %. Distances shorter e n and 'easy' (on the feet of birds, for instance: Maguire, than about 1 km were estimated in the field and those exceed­

i 1963); b that species richness will be greater in larger pools because, ing 1 km were measured from appropriate maps. The results a S

in the case of ephemeral waters at least, larger pools are likely can be considered only to be preliminary because of the y b problem of incomplete identifications of the animals, resulting to persist for longer (Ebert and Balko, 1984; Dehoney and d e c u d o r p e R AQUATIC FAUNA IN THE NAMIB DESERT 105

ralnpools in freshwater 0---0 river beds 0---"000 springs permanent v------.., streams ... - -.endorheic .... _._.. gypsous rainpools springs o 15 o o 0 DV D o 10 •

• • D D. ~._._~--.-.-.-~-.-.-.-.':...-.-.:-.-.-.: o _._ .•. ..-.tl 0 0 5 • _.. _._.~.-.- • 0 _._~:.~.-.-.-.-.-;-.:.-~.:.-.-.-~:-.-. D·· OL-~L-~~LUU-___~~~~UL __-L~~_T~IUIIW,IL-~ ___~I~I~IUlitt~1 __-i~-ilil~I~II~d 0,1 10 100 1000 10 000 Surface area m 2

15 '. "'" .... -.. ..•..... _...... - CIII )( CIII ••...... •. _-_ .. -10 -- - ~ '...... 0 , . -- ~ ...... -...... • 0 0 0 II (j- .. - ..a 5 .~~. 0 0·-·· ...... _ =------E .... :- .. t_..... · .. _.. - .. _-- .. _.. _.. _ .. _ :s z •• • .... ' ..... __ c. "-""'f) .... -.------,-----... :1------+------.1 0 0 20 40 Distance km

15 '--

. " ) ------'-';;------. ---. 9 - 0 ...... _----- 0 10 - ...... 2 - - ...... --...... "' ...... -- .. - ... ~~-P d e t a d 5 ( ..,...... _.. .. -- -- r "-., • e _ •• _ •• _ ...... _ .. __ .• _ .• _ .. _ ...g •• - •• _ •• _ •• - h s i l 0 b 0 100 150 u P

Distance km e h t Fig. 2 y b The relationships between number of taxa recorded from each site and (a) surface area, (b) distance to d e any other surface water, and (c) distance to any other surface water of the same type. t n a r g e c in the use of various taxonomic levels in the analyses. crease with increasing distance apart of individual systems. n e Regression of the number of taxa per site upon depth was Further. the regression lines for different habitat types should c i l not significant (P > 0,05) for the data-set as a whole and for reflect their permanence: the faunas of permanent waters r

e each habitat type individually. Regression upon surface area should be less influenced by distance from other systems than d

n (Fig. 2a) was significant only for the gypsous springs (n = 13, should the faunas of ephemeral pools. Finally. where faunal u P < 0,01, y = 3,03 + 0,67 In x). Thus these data suggest that, richness is greatly influenced by isolation, one might expect y a for most habitat types in the Namib Desert at least, size (as the relationship to be exponential. w

e reflected by depth and surface area) is not a primary determi­ Figure 2b illustrates the regression of the number of taxa t a nant of faunal richness. upon the shortest distance from a given system to any other G

t If, on the other hand, the geographical isolation of individual and Fig. 2c illustrates the regression of the number of taxa e

n pools or springs has an effect on faunal richness, and if the upon the shortest distance from a given system to another of i b individual elements of the fauna are at all able to select, or the same type. Although no significant relationships exist in a S

differentially to survive in. different types of habitat. then one either case for the fauna of the gypsous springs, all the other y

b can predict that the number of taxa in any pool should de- habitat types show strongly significant (P < 0,01) negative d e c u d o r p e R 106 AQUATIC FAUNA IN THE NAMIB DESERT

correlations. The regression equations are shown in Table 3. Table 3 These results are fascinating, not least because they differ Regression equations for the relationships shown in Figs 2b & 2c. quite markedly from those of several of the authors referred to above (e.g., Dehoney and LaVigne, 1984; Ebert and Balko, 1984). n Number of taxa Number of taxa vs vs distance to distance to nearest It is not clear why the richness of the fauna of the gypsous any waterbody waterbody of the springs should be more strongly related to surface area (and same type therefore size) than to degree of isolation, or why these sys­ tems should be different from the others in this respect. It is possible that the habitat diversity of the larger gypsous springs Permanent streams 3 Y =18,58 - 0,29x Y= 17,26e-Q,004X is greater than that of the smaller ones; perhaps, on the other Freshwater springs 13 Y =26,02 - 6,54ln x Y= 1Q,16e-Q,015x hand, the relative permanence of these systems may allow Y= 9, -O,108X Y = 3,90e-Q,Q30X even isolated ones to accumulate species over long periods Rainpools (plains) 17 73e of time. Rainpools (river-beds) 7 Y= 14,8ge-O,034x Y= 15,81e-Q,042x Certainly a comparison of the results for the other habitat types suggests that the richness of the fauna increases with increasing permanence and predictability, in that the curves for both types of ephemeral pool lie closer to the axes in Fig. 2c (i.e., the exponent is larger) than does the curve for the pool. permanent springs, with that for the permanent streams lying In conclusion, the data strongly suggest that, with the excep­ farthest away. Finally, a comparison of the curves for the tion of the gypsous springs, the various types of aquatic habitat permanent springs and the rainpools in river-beds in Figs 2b in the Namib Desert are strongly dependent on neighbouring and 2c suggests that the fauna of the springs is more depen­ systems as sources of 'new' species. This in turn suggests that dent on the closeness of other springs of the same kind, while extinction in individual pools plays a significant role in the that of the rainpools responds to the closeness of any type of determination of community structure.

ACKNOWLEDGEMENTS I should like to thank the following systematists for identifying organizing or aSSisting in various collecting trips: Dr Mary various components of the fauna: Dr Denton Belk (Anostraca); Seely and her staff at the Desert Ecological Research Unit of .

) Prof. Jim Green (Cladocera) and Mrs Nancy Rayner Namibia, Gobabeb; Barbara Curtis, Charmaine Meyer and 9

0 (Copepoda and Cladocera); Dr Koen Martens (Ostracoda); Dr Shirley Bethune at the State Museum, Windhoek; Mike Griffin 0 2

Bruce Mcintosh (Culicidae); Dr S. EndrOdy-Younga (some and Rudi Loutit of the Directorate of Nature Conservation and d

e Coleoptera); Dr A. Prins (adult Ephydridae), and Belinda Day Recreation Resorts, Namibia. I also thank Barbara Curtis for t a for drawing the figures. I am also most grateful to the following providing some of the information included in Table 1. d (

people for their kindness, helpfulness and hospitality in r e h s i l b REFERENCES u P e h t AGNEW, J. D., 1986. Invertebrates of the Orange-Vaal system, with DAY, J. A. and SEELY, M. K., 1988. Physical and chemical conditions

y emphasis on the Ephemeroptera. In: DAVIES, B. R. and WALKER, in an hypersaline spring in the Namib Desert. Hydrobiologia 160: b K. F., eds, The ecology of river systems, pp. 123-134. Junk, 141-153. d

e Dordrecht. DIMENTMAN, Ch. and MARGALlT, J., 1981. Rainpools as breeding t

n BARNARD, K. H., 1929. Contributions to the Crustacean fauna of and dispersal sites of mosquitoes and other aquatic insects in the a South Africa. No. 10. A revision of the the South African Central Negev Desert. Journal of Arid Environments 4: 123-129. r g Branchiopoda (Phyllopoda). Annals of the South African Museum DEHONEY, B. and LAVIGNE, D. M., 1984. Macroinvertebrate distri­

e 29: 181-272. bution among some southern California vernal pools. In: JAIN, S. c n BELK, D. and COLE, G. A., 1975. Adaptational biology of desert and MOYLE, P., eds, Vernal pools and intermittent streams, pp. e c temporary pond inhabitants. In: HADLEY, N. F., ed., Environmental 154-160. Institute of Ecology, University of California, Davis, Pub­ i l

physiology of desert organisms, pp. 207-266. Dowden, Hutchinson lication No. 28. r e and Ross, Inc., Stroudsburg, Pa. EBERT, T. A. and BALKO, M. L., 1984. Vernal pools as islands in d BERRY, H. H., 1971. Flamingo breeding on the Etosha Pan. Madoqua space and time. In: JAIN, S. and MOYLE, P., eds, Vernal pools and n

u 1: 5-32. intermittent streams, pp. 90-101. Institute of Ecology, University of

y BRAIN, C. K. and ADAMS, C. G., 1984. Observations on Foraminifera California, Davis, Publication No. 28. a living in a Namib spring. South African Journal of Science 80: 194. FRYER, G., 1985. Crustacean diversity in relation to the size of water w e (Abstract). bodies: some facts and problems. FreshwaterBiology15: 347-361. t

a BRINK, P., 1955. Gyrinidae. In: HANSTROM, B., BRINK, P. and GEVERS, T. W., HART, O. and MARTIN, H., 1963. Thermal waters

G RUDEBECK, G., eds, South African Animal Life 1: 329-518. along the Swakop River, South West Africa. Transactions of the t

e CAMBRAY, J. A., DAVIES, B. R. and ASHTON, P. J., 1986. The Geological Society of South Africa 66: 157-189. n Orange-Vaal River System. In: DAVIES, B. R. and WALKER, K. F., GILLIES, M. T. and DE MEILLON, B., 1969. Anophelinae of Africa i b eds, The ecology of river systems, pp. 89-122. Junk, Dordrecht. south of the Sahara (Ethiopian zoogeographical region), 2nd edn. a

S CHANNING, A., 1976. Life histories of frogs in the Namib Desert. South African Institute for Medical Research Publication No. 54.

y Zoologica africana 11: 299-312. GOUDIE, A., 1972. Climate, weathering, crust formation, dunes and b d e c u d o r p e R AQUATIC FAUNA IN THE NAMIB DESERT 107

fluvial features of the Central Namib Desert, near Gobabeb, South MARTIN, H., 1963. Suggested theory for the origin and a brief West Africa. Madoqua (2)1: 15-31. description of some gypsum deposits of South West Africa. GROBBELAAR, J. U., 1976. Some limnological properties of an Transactions of the Geological Society of South Africa 66: 345-350. ephemeral waterbody at Sossus Vlei, Namib Desert, South West MARTIN, H., 1965. The precambrian geology of South-West Africa Africa. Journal of the Limnological Society of Southern Africa 2: and Namaqualand. Precambrian Research Unit, University of Cape 51-54. Town. GROBBELAAR, J. U. and SEELY, M. K., 1980. The composition of McLACHLAN, A., 1983a. Habitat distribution and body size in rain-pool water collected from the Kuiseb River, Namib Desert, at Gobabeb. dwellers. Zoological Journal of the Linnean Society 79: 399-407. Journal of the Limnological Society of Southern Africa 6: 46-48. McLACHLAN, A.. 1983b. Life-history tactics of rain-pool dwellers. HARTLAND-ROWE, R., 1972. The limnology of temporary waters and Journal of Animal Ecology 52: 545-561. the ecology of Euphyllopoda. In: CLARK, R. B. and WOOnON, R. MORTON, D. W. and BAYLY, I. A. E., 1977. Studies on the ecology J., eds, Essays in hydrobiology presented to Lesley Harvey, pp. of some freshwater pools in Victoria with special reference to the 15-31. University of Exeter Press, Exeter. microcrustacea. Australian Joumal of Marine and Freshwater Re­ HELLWIG, D. H. R., 1988. Challenging the Namib Dune Sea. Journal search 28: 439-454. of the Limnological Society of Southern Africa 14: 35-40. OMER-COOPER, J., 1965. A review of the Dytiscidae of southern HUNTLEY, B. J. (ed.), 1985. The Kuiseb environment: the develop­ Africa, being the results of the Lund University Expedition 1950- ment of a monitoring baseline. South African National Programmes 1951, with which are incorporated all other records known to the ReportNo. 106. C.S.I.R., Pretoria. author. In: HANSTROM, B., BRINK, P. and RUDEBECK, G., eds, HUTCHINSON, G. E., 1929. A revision of the Notonectidae and South African Animal Life 2: 59--214. Corixidae of South Africa. Annals of the South African Museum 25: PAJUNEN, V. I., 1986. Distributional dynamics of Daphnia species in 359-474. a rock-pool environment. Annales zoologici fennici 23: 131-140. JURGENS, J., 1985. The frogs of South West Africa. SWA 1985: PAJUNEN, V. I. and JANSSON, A., 1969. Dispersal of the rock pool Annual Yearbook. SWA Publications (Pty) Ltd, Windhoek. corixids Arctocorisa carinata (Sahib.) and Callocorixa producta KOK, O. B. and GROBBELAAR, J. U., 1980. Chemical properties of (Reut.) (Heteroptera, Corixidae). Annales zoologici fennici 6: 391- water-holes in the Kuiseb River Canyon, Namib Desert. Journal of 427. the Limnological SOCiety of Southern Africa 6: 82-84. RANTA, E., 1982. Animal communities in rock pools. Annales KOK, O. B. and GROBBELAAR, J. U., 1985. Notes on the availability zoologicifennici 19: 337-347. and chemical composition of water from the gravel plains of the RIS, F., 1921. The Odonata or dragonflies of South Africa. Annals of Namib-Nauklufl Park. Journal of the Limnological Society of South­ the South African Museum 18: 245-452. ern Africa 11: 66--70. SANDELOWSKY, B. H., 1983. Archaeology in Namibia. American LANCASTER, J., LANCASTER, N. and SEELY. M. K., 1984. Climate Scientist 71: 606-615. of the central Namib Desert. Madoqua 14: 5-61. SCHOEMAN, F. R. and ARCHIBALD, R. E. M., 1988. Taxonomic LOGAN, R. F., 1960. The Central Namib Desert. South-West Africa. notes on the diatoms (Bacillarophyceae) of the Gross Barmen National Research Council Publication No. 758, National Academy thermal springs in South West Africa/Namibia. South African Jour­ of Sciences, Washington DC, U.S.A. nal of Botany 54: 221-256. LONGFIELD, C., 1936. Studies on African Odonata with synonymy SCHOLZ, H., 1972. The soils of the central Namib Desert with special and descriptions of new species and subspecies. Transactions of consideration of the soils in the vicinity of Gobabeb. Madoqua (2)1: .

) the Entomological Society of London 85: 467-498. 33-51. 9

0 LONGHURST, A. R., 1955. A review of the Notostraca. Bulletin of the SEELY, M. K. and SANDELOWSKY, B. H., 1974. Dating the regres­

0 British Museum (Natural History) (Zoology) 3: 3-57 sion of a river's end point. South African Archaeological Society, 2 MAGUI RE, B., 1963. The passive dispersal of small aquatic organisms Goodwin Series 2: 61-64. d e and their colonization of isolated bodies of water. Ecological Mono­ SKELTON, P. H., 1986. Fish olthe Orange River System. In: DAVIES, t a graphs 33: 161-185. B. R. and WALKER, K. F., eds, The ecology of river systems, pp. d

( MARTENS, K., 1984. Annotated checklist of non-marine ostracods 143-162. Junk, Dordrecht.

r Crustacea Ostracoda from African inland waters. Koninklijk Mu­ SOUTHWOOD, T. R. E., 1966. Ecological methods with particular e

h seum voor Midden-Afrika, Zoologische Dokumentatie N20 Ter­ reference to the study of insect populations. Methuen and Co., s i vuren, Belgium. London. l b MARTENS, K., 1986. Taxonomic revision of the subfamily STENGEL, H. W., 1964. The rivers of the Namib and their discharge u Megalocypridinae Rome, 1965 (Crustacea, Ostracoda). Ver­ into the Atlantic. 1. The Kuiseb and Swakop. Scientific Papers of P handelingen van de Koninklijke Academie voor Wetenschappen, the Namib Desert Research Station e 22: 1-49. h Letteren en Schone Kunsten van Belgie. Klasse der Weten­ WATSON, A., 1979. Gypsum crusts in deserts. Journal of Arid Envi­ t schappen, Jaargang 48, No. 174. ronments 2: 3-20. y b

MARTENS. K .. 1988. Seven new species and two new subspecies of WEIR, J. S., 1966. Ecology and zoogeography of aquatic Hemiptera d SclerocyprisSars, 1924 from Africa, with new records of some other from temporary pools in Central Africa. Hydrobiologia 28: 123-141. e t megalocyprinids (Crustacea, Ostracoda). Hydrobiologia 162: 243- ZAR, J. H., 1984. Biostatistical analysis, 2nd edn. Prentice-Hall, n Englewood Cliffs, N.J.

a 273. r g e c n e c i l r e d n u y a w e t a G t e n i b a S y b d e c u d o r p e R The Biology and Ecology of Namib Desert Ants

Alan C. Marsh Department of Zoology, University of Namibia, Private Bag 13301, Windhoek, 9000 Namibia

Current knowledge of Namib Desert ant biology and ecology is reviewed. The review covers the composition of the ant fauna of the central Namib in qualitative and quantitative terms at the level of subfamily, genus and species. The dietary pattems of the ants are described and adaptations that facilitate species persistence are discussed. These adaptations include granivory and behavioural plasticity. Ecologically dominant ant species from the dune sea and gravel plains are compared in an attempt to identify those characteristics that have enabled these species to become particularly successful desert inhabitants. The reasons that the thermophilic genus Ocymyrmex forages during the heat of the day are considered. The reproductive biology and social structure of dominant ants such as Messor denticornis and Camponotus detritus are compared with that of relatively rare ants that have small colony sizes such as Ocymyrmexspecies and members of the Monomorium Salomon is-group. Marked differences occur between these two groups of ants and the adaptive value of these differences is discussed in relation to survival and reproduction in a desert environment.

INTRODUCTION currently recognized as occurring naturally in the central Namib. The central Namib has, however, not been compre­ Numerically and in terms of biomass. ants represent one of the hensively sampled and the species list will inevitably grow with dominant animal taxa in hot deserts throughout the world time. Of the three major habitats in this region, the gravel plains (Crawford. 1981; Pisarski, 1978). Seed predation is probably have been the most comprehensively sampled, followed by the most important ecological effect that ants have on desert the dune sea. The ant fauna of the Kuiseb River bed has been ecosystems (Stradling. 1978; Buckley, 1982) but they are also sampled sporadically and remains largely unknown. Ant col­ .

) important in the conservation, localization and turnover of lecting in other regions of the Namib has barely commenced. 9

0 nutrients (Crawford. 1981) by virtue of their central place Many of the ant species of the central Namib are new to 0 2

foraging and subterranean nesting habits. science. Nine of the 13 Monomorium species were only re­ d

e Despite their ecological importance and biological interest, cently described (Bolton, 1987). Furthermore, the ant fauna t a Namib ants remained unstudied until the present decade. elsewhere in the Namib Desert is likely to be quite different. d (

Barbara Curtis commenced a study of the ecology of the large, For instance, based on brief and unsystematic collecting, four r e conspicuous dune ant Camponotus detritus in 1980, and in species of Ocymyrmex new to science have been recorded in h s

i 1981 I commenced a general study of the ecology of Namib regions outside the central Namib (0. tachys and O.afradu in l b Desert ants in which attention was focused largely on the ants the Skeleton Coast Park, O. engytachus from the top of the u

P of the gravel plains to the north of the dune sea. As the work Gamsberg Pass, and O. okys from the Naukluft Mountains). e on C. detritus is reviewed elsewhere in this volume (Curtis, Clearly a comprehensive survey of ants in the Namib Desert h t 1990), the present paper focuses on my own work and that of will reveal a wealth of new material and our current knowledge y b

a few other researchers who have subsequently commenced of the most basic issue, namely the species composition of the d

e studies on the Namib ant fauna. In this review I examine overall desert, is far from complete. t n ecological patterns and trends that characterize Namib ants Nevertheless, sufficient is known about the composition of a r and emphasize particularly interesting aspects of the biology the ant fauna of the plains and sand sea in the central Namib g

e of some of these ants. Desert to make four general statements that will probably c

n remain true even when more detailed information is available: e c

i THE ANT FAUNA OF THE NAMIB l

r 1) More species occur on the gravel plains (33 species) than e d To date 38 species of ants have been discovered in the central in the dune sea (13 species) (Marsh, 1986a) despite the n

u Namib Desert. Thirty six species are listed in Marsh (1986a) existence within the dune sea of extensive interdune

y and additional taxonomic notes are given in Appendix I. The valleys that resemble the gravel plains. A possible expla­ a

w remaining two species are Monomorium rufulum, which occurs nation involves the existence of large inselbergs on the e t on the eastern edge of the dune sea, and Iridomyrmex humilis, gravel plains. The runoff from these inselbergs is concen­ a

G the Argentine ant, which was discovered subsequently at trated into water courses that support perennial vegeta­ t e Mirabib. As I. humilis is a tramp species, distributed by man's tion, and hence several species of ants that are dependent n i activities, and as it has been located once only it has been on this vegetation. In contrast, the interdune valleys lack b a disregarded in this review. Thus, 37 species of ants are these more productive habitats. The dunes themselves S y b

d Marsh. A. C., 1990. The biology and ecology of Namib Desert ants. In: Seely, M. K., ed., Namib ecology: 25 e c years of Namib research, pp. 109-114. Transvaal Museum Monograph No.7. Transvaal Museum. Pretoria. u d o r p e R 110 BIOLOGY & ECOLOGY OF NAMIB DESERT ANTS

support perennial plants but the unstable nature of the cool, coastal fog zone and the other two species are substrate probably prevents most ant species from being apparently rare, having been collected at one or two sites able to colonize the dunes. Only specially adapted ants only. such as C. detritus, which make use of the roots of these plants to provide structure to their sandy nests (Curtis, Endemism 1985a), are able to occupy the dunes. Greater species It is not possible to make a reliable statement as regards the richness of ants in gravel plain habitats as opposed to degree of endemism of the Namib ant fauna. Although many sand sea habitats has also been reported for the Sahara previously unknown species have been discovered in the Desert (Bernard, 1964; Delye, 1968). Namib, the ant fauna of the rest of Namibia and surrounding countries, including South Africa, is largely unstudied. It is 2) Four of the eleven subfamilies of Formicidae occur in the conceivable that many species occurring in the Namib also Namib Desert. Twenty-eight species belong to the occur, and may be more abundant, in the semiarid habitats to Myrmicinae, seven species belong to the Formicinae, one the north, south and east of the desert. Monomorium species, Tetraponera ambigua, belongs to the Pseudo­ mantazenum appears to be confined to the fog belt on the myrmecinae (Marsh, 1986a) and an unidentified Tech­ western edge of the Namib and it is therefore probably a Namib nomyrmex sp. belonging to the Dolichoderinae has been endemic. Similarly, C. detritus is confined to the sand sea and collected in the Kuiseb River bed (Robertson, personal is undoubtedly a Namib endemic. communication). The arboreal T. ambigua was found once only on an Acacia sp. tree on the eastern border of Dietary patterns of Namib ants the desert (Le., the most productive edge of the desert). Since most ants are opportunistic foragers it is unrealistic to Subsequent searches of other Acacia trees were un­ assign any species to a particular trophic group without some successful and it appears that this species is a marginal clarification. Here I have assigned a species to a trophic group occupant of the Namib. Thus, at the subfamily level, the if most of its diet consists of a specific food type. The diet of Namib is almost identical to the Sahara, but unlike deserts 27 Namib ants, from a wide variety of habitats, is sufficiently in the United States of America and Australia which con­ known to make some generalizations. Ten of the species are tain an additional two subfamilies, the Dorylinae and the granivorous (harvester ants), ten honeydew/nectar feeders, Ponerinae (Briese and Macauley, 1981; Chew, 1977; three omnivores and three insectivores (MarSh, 1986a). One Greenslade and Halliday, 1983; Whitford, 1978). Marsh species, Pheidole tenuinodis, can be classified as either a (1986a, b) suggests that the most likely reason for the granivore or a honeydew/nectar feeder depending on where it

. exclusion of these subfamilies in the Namib and Sahara occurs, and illustrates the problems encountered when as­ )

9 is the greater aridity of these deserts. This results in lower signing a species to a trophic group. 0

0 productivity and greater unpredictability in production, Restricting the analysis to 12 sympatric species from the 2 which effectively excludes trophic specialists and barren gravel plains on the eastern edge of the desert, is more d e t conversely only permits opportunistic, desert-adapted illuminating. Seven of the species are granivorous (i.e., 58 % a d species to occupy these regions. of the species), two are honeydew/nectar feeders, two omniv­ (

r orous and one insectivorous (Marsh, 1985a). Most individuals e h 3) The genus Monomorium, comprising small to medium­ (94,7 %) are granivorous making up 96,7 % of the total forager s i l sized monomorphic ants, is the dominant ant genus in the biomass. Harvester ants are therefore overwhelmingly the b u Namib. Of the 37 naturally occurring species in the Namib, predominant trophic group in this ant assemblage. Their pre­ P 13 are Monomoriumspecies, I.e., 35 % of all ant speCies dominance in deserts is attributed to the fact that they are e h

t belong to this genus. Furthermore, the majority (10 spe­ primary consumers reliant upon a relatively dependable, nutri­

y cies) belong to the arid-adapted Salomonis-group (Bolton, tious food resource which can be stored for extensive periods b

d 1987). This is the dominant group of Monomoriumspecies (Carroll and Janzen, 1973). Within this species assemblage, e t in Africa in habitats ranging from savannah through to the relative abundance of species ranges from rare to abun­ n a desert. Bolton (1986) has postulated that the dominance dant, with two harvester species, Messor denticomis and r g

of this group in deserts is partly related to the occurrence Tetramorium rufescens, representing 90 % of the forager bio­ e c of apterous and ergatoid females in it. The importance of mass (Marsh 1985a). n e this aspect of their reproductive biology is elaborated upon Numerically T. rufescens is the dominant species making up c i l

elsewhere in this paper. 42 % of the assemblage, whereas, in terms of biomass, M. r e denticomis is dominant, comprising 58 % of the assemblage. d n 4) On the gravel plains, ant species richness is strongly The above assemblage occurred on a barren gravel plain u correlated with mean annual rainfall, an indirect indicator devoid of perennially green vegetation and characterized by y a of productivity (Marsh 1986b). Thus, species richness episodic pulses of superabundant seed. In other habitats, w e increases progressively from the coast inland. A particular however, other trophic groups may be dominant. For example, t a site on the gravel plains supports anything from six sym­ in the dune sea it is apparent, even without quantitative data, G

t pat ric species (14 mm rain p.a.) to 23 species (87 mm rain that the dominant ant in terms of biomass is C. detritus, a e n p.a.). With three exceptions, all ant species occur at the honeydew feeder. This habitat suppo rts perennial grasses that i b eastem, more productive edge of the desert but penetrate support large numbers of honeydew-secreting homopterans a S

the desert towards the west to different extents. One (Curtis, 1985b). Although Tetramoriumjordaniis a numerically y b species, Monomorium mantazenum, is restricted to the common granivore where grass occurs in the dunes, in terms d e c u d o r p e R BIOLOGY & ECOLOGy OF NAMIB DESERT ANTS 111

of biomass, granivores are probably not very important in the thereby persist through time is by definition an evolutionary dunes. success; however, some species are much more abundant than others and could be construed as being more successful Adaptations facilitating species persistence ecologically than rarer species. Investigations of these domi­ An important characteristic of many Namib ants that facili­ nant species may reveal adaptations that are particularly im­ tates their persistence is their use of storable seeds. After portant in a given environment. episodic rainfall events, seeds are superabundant, enabling On the eastern edge of the gravel plains there is evidence harvester ants to collect in excess of their immediate require­ that the dominant ant species is M. denticornis (Marsh, 1985a), ments, to store the surplus and later to use these seeds during whereas in the dune sea the dominant species is C. detritus periods of drought. Another important characteristic of Namib (Curtis, 1983). There are many characteristics that these two ants is their degree of opportunism and behavioural flexibility. species share and that are generally not shared with other These characteristics are particularly evident in terms of activ­ sympatric species. Both species are the largest ants in their ity and diet (Marsh, 1985a, c, 1986c, 1988). Desert ants have respective habitats (M. denticornis total length 5,5-11,0 mm, several attributes that make behavioural plasticity advanta­ C. detritus total length 7,0-16,0 mm); they exhibit continuous geous if not essential. Colonies are relatively long-lived, per­ size variation; their colonies are polydomous, comprising up manently 'rooted' in the soil and individuals are small. to four widely scattered nests; colony size is large comprising, These attributes restrict exploitation of the surrounding hab­ several thousand individuals; both species derive most of their itat by a colony to a small area. From this habitat patch, a nourishment from temporally and spatially predictable food colony has to obtain all the resources necessary for its survival, sources (Curtis, 1985b, c; personal observations). There are growth and reproduction. also some obvious differences between the two species. In an environment where food is spatially and temporally Camponotus detritus is a diurnal, highly aggressive honey­ patchy, the ability to use diverse food types is clearly advan­ dew-feeding ant whereas M. denticornis is a nocturnal, tageous. Thus, although most species are classified as har­ unaggressive seed-harvesting species. vester ants, in reality they are highly opportunistic foragers The ecological dominance of these species is indicative of taking a wide range of food types depending on availability and their success as foragers. Large body size and hence long legs hunger. For example, when grass seeds are abundant, M. enables the ants to cover long distances efficiently and their denticornis and T. rufescens exploit seeds exclusively but foraging range is therefore large. In a desert environment switch readily to non-seed food, especially insects, when seed where food sources are typically patchy, increasing the forag­ availability is low (Marsh, 1986c). Shifts in diet are often ing range will increase the number of food patches encom­

. accompanied by changes in foraging behaviour. For example, passed by the colony. Polydomy further increases the foraging )

9 when seeds are abundant, M. denticornis uses extensive trunk range substantially. An additional advantage that polydomy 0

0 trails that lead to specific patches up to 80 m away, but when confers on these ants is that of spreading the risk of extinction 2

d seeds are rare, it switches to more localized diffuse foraging (sensu Den Boer, 1968) and thereby minimizing the effects of e t to facilitate the discovery of less spatially predictable food catastrophic events. For example, a M. denticornis nest that a d items such as mobile termites (Marsh, 1986c). was partially excavated by an aardvark resulted in mass (

r Epigaeic desert ants also forage in a region where thermal emigration and resettlement of the nest occupants at a sister e h conditions vary considerably on a diel and seasonal basis. nest (personal observations). The presence of sister nests that s i l Flexibility in the periods used for surface activity (mainly for­ remain untouched by predators means that even in the event b u aging) is therefore adaptive in that it enables ants to select of total destruction of one nest and its occupants, the colony P appropriate microclimatic conditions. For example, in summer remains alive. Furthermore, survivors from the destroyed nest e h

t the majority of foraging excursions within a particular ant rapidly find a place of refuge. Similarly, C. detritus nests

y assemblage occur at night, whereas in winter, diurnalism become unsuitable for occupation from time to time when wind b

d predominates (Marsh, 1985a). Most strictly diurnal species either deposits excessive sand on the nest or alternatively e t have a unimodal pattern of activity (centred on mid-day) in erodes too much sand from the nest and mass migration of n a winter and a bimodal pattern in summer (Marsh, 1988). like­ adults and brood ensues (Curtis, 1983). r g

wise, species that are nocturnal in summer become crepuscu­ Large colony size is advantageous for much the same e c lar in winter and thereby avoid thermally stressful conditions reasons advanced above with reference to catastrophe and n e (Marsh,1988). spreading the risk. Furthermore, in a successful polydomous c i l

In view of the foregoing discussion on opportunism and species each sister nest must be sufficiently large to facilitate r e behavioural plasticity, it is conceivable that, at different locali­ ergonomic efficiency; a polydomous colony is therefore inevi­ d

n ties, ants from one species could be consuming different food tably relatively large. u types and doing so at different times within the diel cycle. This The advantages of continuous size variation are more elu­ y a is particularly likely in the central Namib where rainfall is sive. It may ensure a more efficient division of labour in the w e extremely patchy (Seely, 1978), creating a mosaic of produc­ colony (Oster and Wilson, 1978), but this aspect of social t a tive/unproductive habitats, and where there is a steep climatic structure has not been rigorously quantified in either species. G

t gradient. This dynamic aspect of ant ecology is worth investi­ For example, it is possible that size matching between forager e n gating. and food item occurs in M. denticornis. However, the impor­ i b tance of size matching remains questionable in an environ­ a S

Ecological dominance ment where most seeds retrieved are of one size only (Marsh, y b Any species that can survive, grow and reproduce and 1986c), but it could be important during prolonged droughts d e c u d o r p e R 112 BIOLOGY & ECOLOGY OF NAMIB DESERT ANTS

when seeds are scarce in the environment. Size matching individuals that have not previously been outside of the nest, between forager and food in the predominantly honeydew­ can be recruited to exploit the resource (Marsh, 1985c, 1986d). feeding C. detritus is only evident on those rare occasions Heat is probably the dominant component of environment in when large solid items are retrieved (Curtis, 1983). Therefore, the ecology of the Namib Ocymyrmex species, and definitely it seems more likely that continuous size variation has evolved is for the O. robustior population living in the dry Kuiseb River in C. detritus in response to the need to defend food-bearing bed. Heat generates their food, regulates the speed of loco­ host plants against con specifics. motion and hence foraging range, regulates forager activity at In M. denticornis continuous size variation may be an adap­ the colony level, regulates the behaviour of individual foragers, tation for trail construction. By clearing small pebbles off the regulates the intensity of predation from ant-lion larvae and trail, workers construct permanent trunk trails, leading from the acts as a stochastic malentity, killing foragers that are too slow nest to food patches (personal observations). These trails to locate thermal refuges (Marsh, 1985b, c, 1987, 1988). appear to be constructed in part as an incidental consequence offoraging activity (unsuccessful workers retrieve pebbles and Reproductive biology and social structure thousands of foraging trips along the pathway inevitably dis­ The ultimate measure of success for individuals of any place loose pebbles to the side) and in part as a deliberate species is whether they are able to reproduce themselves and effort whereby workers pick up pebbles in their mandibles, thereby perpetuate the species. Desert environments pose deposit them off the trail and return to the trail to repeat the particular reproductive problems to ants. The important com­ operation (personal observations). Large individuals may be ponent of environment for most desert organisms is rainfall. best suited to carry these relatively heavy pebbles, whereas The rarity and unpredictability of rainfall ensures that mating smaller workers would be adapted to carry seed. Work cur­ and colony founding conditions are similarly rare and unpre­ rently in progress indicates that trail-using foragers travel dictable. The pattern of rainfall also ensures that food re­ substantially more rapidly than non-trail users (Seely, personal sources are sparse. Although detailed investigations of repro­ communication), suggesting that the construction and use of duction in Namib ants has just commenced, certain patterns trails enhances foraging efficiency and will be adaptive pro­ are evident from the little information that is currently known. vided the costs of building the trail are more than offset by Dominant ant species, such as M. denticornis, T. rufescens increases in food retrieval. and C. detritus, reproduce in the standard forrnicid manner. Colonies produce large numbers of fertile, alate males and Thermophilic ants females which typically have nuptial swarms after rainfall Perhaps the most fascinating ants in the Namib are mem­ events (Curtis, 1985c, personal observations). Colonies are

. bers of the strictly diurnal, insectivorous genus Ocymyrmex presumably founded by inseminated females immediately )

9 which have adopted a unique temporal niche, specializing in after the swarming event when the earth is still moist and 0

0 foraging during the heat of the day. Ocymyrmex robustior, for capable of being excavated with relative ease. The majority of 2 example, commences foraging activity when surface temper­ swarming reproductives are usually consumed by predators d e t ature approaches 30°C, has maximal activity at 52°C and (Bolton, 1986; personal observations) and it is also possible a

d only ceases activity as the temperature approaches 70 °C that suitable conditions for swarming and colony foundation (

r (Marsh, 1985b, c). The ants possess a suite of morphological, may not occur during the lifespan of certain reproductives. e

h physiological and behavioural features that enable them to Such a pattern of reproduction is expensive and can represent s i l forage during thermally stressful conditions. They have long a huge drain on colony resources. To reproduce successfully b

u legs, facilitating rapid locomotion and the exploitation of the in this manner requires a large colony that has access to a P steep temperature lapse profile above the desert floor, and low dependable food supply. e h thermal inertia enabling them to rapidly offload excess body The reproductive pattern that has evolved in the genus t

y heat in cool thermal refuges (Marsh, 1985b). They can repeat­ Ocymyrmex and in certain species belonging to the Monomor­ b edly tolerate high heat loads, in excess of 50°C, for short ium Salomon is-group stands in marked contrast to the stan­ d e t periods of time (Marsh, 1985b). They very effectively exploit dard formicid mode of reproduction. In these species the n

a the thermal mosaic, which even a barren desert floor presents reproductive females are apterous and ergatoid, and au­ r g to a small organism, employing thermal respite behaviour toparasitism followed by colony fission occurs (Bolton, 1986; e c whenever heat stressed (Marsh, 1985b). Marsh, 1986d). That is, worker-like reproductive females re­ n e Ocymyrmexspecies are thermophilic because they usually main in the mother colony for variable lengths of time after c i l scavenge heat- and desiccation-stressed prey, and by forag­ insemination and then start a colony by recruiting nest mates r e ing during the heat of the day, obtain exclusive access to their from the mother colony. Bolton (1986) suggests that auto­ d

n prey (Marsh, 1985c). Foragers voluntarily tolerate potentially parasitism and subsequent colony fission is adaptive in envi­ u lethal heat loads on particularly hot and dry days when carrion ronments where food is sparse as it facilitates the production y a is more abundant than usual (Marsh, 1985b, c). of a few females with high survival potential, thereby w e In addition to their thermophilic behaviour, Ocymyrmexspe­ minimizing wastage of scarce resources. t a cies exhibit highly plastic behaviour that facilitates their sur­ Ocymyrmex colonies are monogynous but produce large G

t vival in desert environments. The ants are normally diffuse numbers of ergatoids; on average O. robustior colonies com­ e

n foragers, reflecting their spatially unpredictable food resource prise 13 % virgin ergatoids (maximum 40 %) and O. tumeri i b (Marsh, 1985c), but when a particularly rewarding food patch colonies have a mean of 14 % virgin ergatoids. Furthermore, a S

is located (e.g., a patch of active termites or a large carcass) ergatoids and alate males are produced throughout the year. y

b a large proportion of the normal forager force as well as Such a system would be very wasteful of resources were it not d e c u d o r p e R BIOLOGY & ECOLOGY OF NAMIB DESERT ANTS 113

for the fact that uninseminated ergatoids adopt roles that are serve special attention are the dune sea and the Kuiseb River. indistinguishable from those of sterile workers (Forder and Beyond this relatively well-studied area of the Namib, surveys Marsh, 1988). The efficiency of this role flexibility is enhanced are urgently needed in the northern and southern Namib as by the fact that reproductive females are indistinguishable from well as in the regions immediately to the east of the Namib. sterile workers in size and gross morphology. Accordingly, it Surveys are also required in all semiarid and arid areas of appears that Ocymyrmexspecies have evolved a reproductive southern Africa. Only once these have been carried out will we system that is highly adaptive in an energy-poor, unpredictable be able to see the Namib fauna in context. From such surveys environment. It is hypothesized that virgin ergatoids and males it will become possible to answer fundamental questions such are produced on a regular basis to take opportunity of any as: Why are the Dorylinae and Ponerinae excluded from the suitable mating or colony founding conditions that may occur Namib and how close to the Namib do they occur? Why is the (MarSh, 1986d). If such conditions do not occur by an as yet genus Monomorium so prevalent in the Namib? How many unknown age, the ergatoid adopts worker roles and no longer Namib endemics are there? represents a drain on colony resources but contributes posi­ More detailed ecological studies at specific sites would be tively to their acquisition. This hypothesis will undoubtedly appropriate to answer questions such as: What is the relative require modification as new information is obtained. For exam­ importance of different trophic groups in the dune sea and how ple, it remains uncertain that excavation of new nests by a does this compare to the situation pertaining on the gravel relatively large worker force can only take place after rainfall plains? If granivorous species are not dominant in the dune events. In fact, the evidence collected to date indicates other­ sea, why? How does diet and activity of widespread species wise. Information on reproductive phenology is required to (e.g., Monomorium viator, Anoplolepis steingroeveri and Oc­ facilitate a deeper understanding of the evolution of the unique ymyrmexturnerivary in space and time? How important is heat social structure found in Ocymyrmexspecies. Similar studies and desiccation stress in generating arthropod carrion for of the reproductive biology of species of the Monomorium Ocymyrmex species? Salomonis-group in the Namib, could be very rewarding. Possibly the most exciting field of study at present concerns the adaptive value of the reproductive biology and social structure of the genus Ocymyrmex and the Monomorium CONCLUSION Salomonis-group. The obvious questions to address are: How does male and ergatoid female production vary through time Studies of ant ecology and biology in the Namib Desert are in and are there any environmental correlates with this variation? their infancy. Inevitably, the pioneering research has tended How are colonies founded in these species? What are the

. to be descriptive in nature. While there remains a need for similarities and differences in reproductive biology and social )

9 further basic descriptive work, it would be opportune to start structure in these two groups of ants? What is the energetic 0

0 building upon the initial foundation using a more rigorous cost of the social system in the above ants and how does this 2

d approach that attempts to answer certain specific questions. compare to that found in ants that breed in the conventional e t Many questions could be addressed from further systematic formicid manner such as Messor denticornis and Camponotus a d collecting of ants. In the central Namib, the regions that de- detritus? ( r e h s i ACKNOWLEDGEMENTS l b I am grateful to B. A. Curtis, H. Robertson, B. A. Marsh, M. the field work and the synthesis was made possible through u P

K. Seely and an anonymous reviewer for their comments on financial support from the Bureau for Research of the Acad­ e

h this manuscript. Most of the field work, upon which this syn­ emy, Windhoek. The Directorate of Nature Conservation and t

y thesis is based, was carried out with the financial support of Recreation Resorts gave permission to live and work in the b the Foundation for Research Development of the C.S.I.R., Namib Naukluft Park. d e

t South Africa, and the Transvaal Museum. The remainder of n a r g REFERENCES e c n e c i BERNARD, F., 1964. Ecologiques sur les Fourmis des sables CARROLL. C. R. and JANZEN. D. H .• 1973. Ecology of foraging by l

r sahariens. Revue d'Ecologie etde Biologie du So11: 615-638. ants. Annual Review of Ecology and Systematics 4: 231-257. e BOLTON, B., 1986. Apterous females and shift of dispersal strategy CHEW, R. M .• 1977. Some ecological characteristics of the ants of a d

n in the Monomorium salomonis-group (Hymenoptera: Formicidae). desert shrub community in southeastern Arizona. American Mid­ u

Journal of Natural History 20: 267-272. land Naturalist 98: 33-49. y BOLTON, B., 1987. A review of the Solenopsis genus-group and CRAWFORD, C. S .• 1981. Biology of desert invertebrates. Springer­ a

w revision of Afrotopical Monomorium Mayr (Hymenoptera: Verlag, New York. e t Formicidae). Bulletin of the British Museum (Natural HistorY! Ento­ CURTIS, B. A.. 1983. Ecology of the Namib Desert dune ant, a mology Series 54: 263-452. Camponotus detritus. M.Sc. thesis, University of Cape Town, Cape G BRIESE, D. T. and MACAULEY, B. J .• 1981. Food collection within an Town. t e ant community in semi-arid Australia, with special reference to seed CURTIS. B. A.. 1985a. Nests of the Namib desert dune ant, n i harvesters. Australian Journal of Ecology 6: 1-19. Camponotus detritus. Insectes Sociaux 32: 313-320. b

a BUCKLEY, R. C., 1982. Ant-plant interactions: a world review. In: CURTIS. B. A.. 1985b. The dietary spectrum of the Namib Desert ant. S

BUCKLEY, R. C., ed .• Ant-plant interactions in Australia. pp. 111- Camponotus detritus. Insectes Sociaux 32: 78-85. y 141. Dr Junk Publishers, The Hague. CURTIS. B. A .• 1985c. Observations on the natural history and b d e c u d o r p e R 114 BIOLOGY & ECOLOGY OF NAMIB DESERT ANTS

behaviour of the dune ant, Camponotus detritus, in the central MARSH, A. C., 1986a. Checklist, biological notes and distribution of Namib Desert. Madoqua 14: 279-289. ants in the central Namib Desert. Madoqua 14: 333-344. CURTIS, B. A., 1989. Behaviour and ecophysiology of the Namib dune MARSH, A. C., 1986b. Ant species richness along a climatic gradient ant, Camponotus detritus (Hymenoptera: Formicidae). In: SEELY, in the Namib Desert. Journal of Arid Environments 11: 235-241. M. K., ed., Namib ecology: 25 years of Namib research, pp. 129- MARSH, A. C., 1986c. The foraging ecology of two Namib Desert 133. Transvaal Museum Monograph No.7, Transvaal Museum, harvester ant species. South African Journal of Zoology 22: 130- Pretoria. 136. DELYE, G., 1968. Recherches sur physiologie et I'ethologie des MARSH, A. C., 1986d. Adaptations of the myrmicine ant genus Fourmis du Sahara. Ph.D. thesis, University of Marseille. Marseille. Ocymyrmexfor exploiting a hot arid environment. In: EDER, J. and DEN BOER, P. J., 1968. Spreading of risk and stabilization of animal REMBOLD, H., eds, Chemistry and biology ofsocial insects, p. 542. numbers. Acta Biotheoretica 18: 165-194. Verlag J. Peperny, Munich. FORDER, J. C. and MARSH, A. C., 1988. Social organization and MARSH, A. C., 1987. Thermal responses and temperature tolerance reproduction in Ocymyrmex foreli (Formicidae: Myrmicinae). In­ of a desert ant-lion larva. Joumal of Thermal Biology 12: 293-300. sectes Sociaux 35: 400-410. MARSH, A. C., 1988. Activity patterns of some Namib Desert ants. GREENSLADE, P. J. M. and HALLIDAY, R. B., 1983. Colony disper­ Journal of Arid Environments 14: 61-73. sion and relationships of meat ants Iridomyrmex purpureus and OSTER, G. G. and WILSON, E. 0., 1978. Caste and ecology in the allies in an arid locality in South Australia. Insectes Sociaux 30: social insects. Princeton University Press, Princeton. 87-99. PISARSKI, B., 1978. Comparison of various biomes. In: BRIAN, M. MARSH, A. C., 1985a. Forager abundance and dietary relationships V., ed., Production ecology of ants and termites, pp. 326-331. in a Namib Desert ant community. South African Journal ofZoology Cambridge University Press, Cambridge. 20: 197-203. SEELY, M. K., 1978. Standing crop as an index of precipitation on the MARSH, A. C., 1985b. Thermal responses and temperature tolerance central Namib grassland. Madoqua 11 : 61-68. in a diurnal desert ant, Ocymyrmex barbiger. Physiological Zoology STRADLING, D. J., 1978. Food and feeding habits of ants. In: BRIAN, 58: 629-636. M. V., ed., Production ecology of ants and termites, pp. 81-106. MARSH, A. C., 1985c. Microclimatic factors influencing foraging pat­ Cambridge University Press, Cambridge. terns and success of the thermophilic ant, Ocymyrmex barbiger. WHITFORD, W. G., 1978. Structure and seasonal activity of Chihua­ Insectes Sociaux 32: 286-296. hua Desert ant communities. Insectes Sociaux25: 79-88.

Appendix I . ) 9 0

0 In several of the early papers by Marsh, many of the ant species were either not 2 described or their taxonomy was in the process of revision at the time of going d e

t to press. Consequently, code letters were used for these species. The correct a

d species names for Monomorium species are: ( r e

h B = vatranum s i l C = alamarum b

u D = mictilis P

E= drapenum e h F = mantazenum t

y G = katir b H= esha"e d e t 1= marshi n

a J = nirvanum r g K = kitectum e c Ocymyrmex barbiger= O. robustior. n e c i l r e d n u y a w e t a G t e n i b a S y b d e c u d o r p e R The Biology of Leucorchestris arenico/a (Araneae: Heteropodidae), a Burrowing Spider of the Namib Dunes

Johannes R. Henschel Desert Ecological Research Unit, P. 0. Box 1592, Swakopmund, 9000 Namibia

The ecology, distribution, behaviour and life history of Leucorchestris arenicola Lawrence, 1962 (Araneae: Heteropodidae), a 2-5 g endemic spider of the central Namib Desert, were studied over a 20-month period. These spiders live in 33 ± 13 cm long silk-lined burrows, which are constructed in firm dune sand on dune bases and lower dune plinths. Foraging activity is inversely related to light intensity, wind speed and fog condensation. They capture prey on the surface about once a month and usually consume it in the burrow. Coleoptera form the bulk of the diet, followed by Lepidoptera and conspecific spiders. Small vertebrates are rarely eaten. Foraging activity, reproduction and development rate are seasonal, with a 4 to 5-month period of quiescence in winter. Females lay an average of 76 eggs per clutch and tend offspring for about 75 days. On average, nymphs moult every 85 days in summer and every 156 days in winter. Adulthood is reached in the 10th instar at an age of 2 years. Adult males are short-lived (1-2 months) and travel long distances (20-450 m) in pursuit of mating opportunities, while adult females are philopatric, long-lived (6-15 months) and produce up to three egg clutches in a breeding season. Territoriality and cannibalism of juveniles favour site fidelity and avoidance of neighbours at distances of about 4 m. Predation by gerbils and con specific spiders could limit populations in a density-de­ pendent manner. This spider appears to have a K-selected life history pattern, in which relatively constant annual food availability, variable risk of predation and climatic seasonality favour slow development, high longevity, small brood size, iteroparity and extended brood care.

INTRODUCTION and, ultimately, their long-term life history strategies. . ) In the present paper, I present details on the biology of L 9

0 Desert sand dunes, such as are found in the Namib, are arenicola, including morphology, distribution, habitat use, bur­ 0 2

characterized by sparse vegetation, loose substrate, strong row structure, foraging behaviour and predator-prey interac­ d e winds and a dry and thermally fluctuating surface climate tions, reproduction, development, population ecology, intra­ t a (Robinson and Seely, 1980; Louw and Seely, 1982). Such specific and intraguild relationships, and mortality. d (

desert climates do not favour aerial web-building spiders so Characteristics and environmental determinants of these as­ r e that nocturnal wandering spiders predominate, which have pects are compared with patterns exhibited by other wander­ h s i specialized morphological and behavioural traits that enable ing arachnids in deserts and in more mesic zones. With these l b them to burrow and hunt in and on sand (Main, 1957; Chew, data, I re-examine the hypotheSiS that biotic interactions may u

P 1961; Clouds ley-Thompson, 1983). not be as important to many desert invertebrates as physical e factors (Noy-Meir, Further details relating to diet, repro­

h Spiders are an important but little studied component of the 1973). t Namib dune fauna. Knowledge is limited to faunistic data and duction and population ecology of L arenicola are being y b

anecdotal descriptions of their biology (Lawrence, 1959, 1962, gathered for future presentation. d e 1965a, b, 1966; Holm and Scholtz, 1980; Robinson and Seely, t n 1980; Wharton, 1980). Lawrence (1959) first noted the impor­ STUDY AREA AND METHODS a r tance of large heteropodids in the Namib dunes and a discus­ g

e sion of distribution in different Namib dune habitats of some General study area c n heteropodids was presented by Holm and Scholtz (1980). Leucorchestris arenicola were studied from October 1986 to e c i One Namib heteropodid, the dancing white lady spider June 1988 at 26 sites within the area 23-24° S, 14-16° E of l

r Leucorchestris arenicola Lawrence, 1962, is particularly well the central Namib Desert. During a general survey, 143 e d suited for ecological and phYSiological study owing to its large L arenicola were collected from burrows and 108 empty bur­ n

u size, abundance, ease of handling and detectability in the field. rows were excavated at 15 sites. The habitat classification of

y The ability to track the movements of these spiders by their Robinson and Seely (1980) was adopted. a w footprints on the smooth sand surface and to regularly inspect e t and recognize known individuals in their burrows without dis­ Burrow and spider measurements a

G At all sites, tracks left by active arenico/a on the smooth turbing them provided a rare opportunity to investigate such 26 L t e aspects of their ecology as have seldom been recorded for sand surface, which were especially visible after sunrise, n i desert spiders. These aspects include their spatial relationship facilitated the discovery of burrows. Burrows located in this b a to the environment, their foraging and reproductive behaviours way were used for density estimates in some areas on S y b d

e Henschel, J. R., 1990. The biology of Leucorchestris arenicola (Araneae: Heteropodidae), a burrowing spider c ofthe Namib dunes. In: Seely, M. K., ed., Namib ecology: 25 years of Namib research, pp. 115-127. Transvaal u

d Museum Monograph No.7, Transvaal Museum, Pretoria. o r p e R 116 NAMIB DUNE SPIDER BIOLOGY

A .-< \ BD

\ . \ >t.:.

B

Fig. 1 Schematic representation of a transverse section through an occupied Leucorchestris arenico/a burrow showing a female

. spider, her prey remains and egg cocoon in their typical position, and (B) a cross section at A-A'. S = sand surface; ) = 9 H = horizontal orientation; SS = surface slope; BS =burrow slope; TD trapdoor diameter; EC egg cocoon; BL =burrow

0 length; BD = burrow depth; PR prey remains. 0 2 d e t a

d 6 m-wide line transects according to the method of Burnham, stretch of interdune sand 1 km S of Gobabeb (23° 34' S, 15° (

r Anderson and Laake (1980). 02' E) for an intensive study of reproduction, development and e

h The following measurements were made on excavated bur­ population ecology. The vegetation at Visnara consisted of a s i l rows (Fig. 1): slope (0; SS) of sand surface at burrow entrance, sparse covering of grasses, Stipagrostis ciliata, S. gonato­ b

u diameter of trapdoor (mm; TD), burrow circumference esti­ stachys and Centrapodium glaucum, and a cucurbit, P

mated as twice the measured width of the collapsed burrow Acanthosicyos horridus (!Nara). On the northern side, this area e h (mm; BC), burrow slope (0; BS) for the first 30 cm of its length, was bordered by riparian vegetation: Acacia erioloba, Euclea t

y burrow length (cm; BL) and vertical depth of burrow (cm; BD). pseudebenus, Tamarix usneoides and Salvadora persica. Me­ b Prey remains were collected from all burrows for identification teorological conditions were monitored at the first order d e t by comparison with voucher specimens. weather station at Gobabeb. n

a Spiders were captured from excavated burrows or in a The herbivorous and detritivorous invertebrate fauna at r g specially designed trap (Henschel, in press). The age, sex and Visnara comprised a diverse community of dune and riverine e

c live mass (± 0,1 mg) were determined and the following mea­ origin, dominated by tenebrionid, scarabid and curculionid n e surements (± 0,1 mm) were taken: maximum width of cara­ beetles, moth larvae and adults, termites and ants. In addition c i l pace; prosoma (cephalothorax) length from the dorsal centre to arenicola, the spider fauna consisted of a gnaphosid,

L. r e of the clypeus edge to the posterior end of the sternum Asemesthes lineatus; two eresids, Seothyra sp. and d

n ventrally; ventral width of opisthosoma (abdomen); dorsal Gandanomeno echinatus; three salticids; one philodromid; u length of opisthosoma; body length from clypeus to base of and one dysderid. Other predators of possible significance to y a anal tubercle; and length of the sclerotized shaft of left femur L. arenicola were geckos, Palmatogecko rangei and Ptenopus w e I antero-Iaterally. Regression equations interrelating body di­ spp.; a I~gless lizard, Typhlosaurus braini; scorpions, Op­ t a mensions were calculated and coefficients of determination isthophthalmus flavescens and Parabuthus vii/os us; a G

t obtained for their simplified formulas. For all mean values pompilid wasp, Schistonyx aterrimus; solifugids, Metaso/puga e

n throughout this paper, ± one standard deviation is indicated. picta, Prosolpuga schultzei, Solpugista bicolor and Un­ i b guib/ossa cauduliger; gerbils, Gerbillurus paeba, G. tytonis a S

Intensive Study Area and Desmodillus auricularis; jackals, Canis mesomelas; mon­ y

b A fenced site of 0,75 ha (Visnara) was established on a flat gooses, Galerella sanguinea; genets, Genetta genetta; owls, d e c u d o r p e R NAMIB DUNE SPIDER BIOLOGY 117

Bubo africanus and Tyto alba; and various passerines, espe­ Table 1 cially Mirafra erythrochiamys, Gercomela familiaris and Pic­ Equations that can be used to estimate carapace width (mm) and nonotus nigricans. mass (mg) of Leucorchestris arenicola using functions of body or burrow dimensions (mm). Activity, life history and demography During the course of intensive observations at Visnara from October 1986 to April 1987, all burrows of L. arenicola in a 0,5 ha area were marked with numbered flags placed 20 cm from the burrows. Spider activity was monitored by observa­ Carapace Width 1,01 x Femur I tions at night and by following tracks in the early morning, 0,95 292 0,59 x Trapdoor diameter 0,93 0,85 289 which provided detailed records of the spiders' activities on the 0,22 x Burrow circumference 0,85 0,79 301 sand surface. Burrow occupation status was ascertained by signs of activity at entrances. Data were collected for 79 nights Mass Body length x Carapace width x during which 8836 observations were made of 312 burrows. (Abdomen width + length)/3 0,98 58 During further intensive studies from June 1987 to June 0,052 x (Carapace width + Body length) 3 0,96 58 1988 at Visnara, activity data for L. arenicola were recorded 0,229 x Body length 2,878 0,95 58 as above for 7241 observations of 319 burrows on 122 nights 1,495 x Carapace width 3,107 0,90 58 0,393 x Trapdoor diameter 2,741 0,73 312 in a 0,33 ha area. Nearly all individuals were captured and later recaptured after moulting (n = 309). Individuals were identi­ fied, measured, and marked using five colours of water-soluble fluorescent paint (Plaka) in various combinations. Paint was applied to the central dorsal surfaces of leg patellae, areas on Identification of sexes the legs apparently without important sensory functions. Spi­ Live adults, subadults and some pre-subadults could be ders from excavated burrows were released at the capture sexed in the field on the basis of epigyna and pedipalps. These sites and protected in enclosures until they constructed new individuals of known sex were used to define other sex-related burrows. Trapped spiders were returned to their burrows. At attributes. Of all other external characteristics examined (eye approximately fortnightly intervals, optometrist's and dentist's arrangement, leg spination. pedipalp structure, tibial claws, mirrors were used to look into burrows to verify the identity and dimensions of prosoma, opisthosoma and legs), only leg reproductive status of their occupants. spination was found to be a useful guide to sex. A median

. Burrows of reproducing females were examined at shorter dorsal spine on the tibia was found on most (5-8) legs of adult )

9 intervals (usually < 7 days) to determine nymphal stages and and subadult males. This spine was rarely found on legs of 0

0 their duration, litter size, development rate and duration of adult and subadult females and then only on a maximum of 4 2

d maternal care. After the nursery burrow was abandoned, it was legs. The presence of this feature on five or more legs thus e t excavated and all exuviae of nymphS were counted. provided a guide for sexing juvenile males older than nymph a d The minimum duration of each post-nursery instar was stage III (carapace width> 7 mm). The sex of younger individ­ (

r determined from the interval between two observed moults uals could not be determined. e h (n = 85). Half the interval between sightings was added to the s i l minimum period to estimate the duration of instars. The total Morphology b u number of instars was calculated from successions of re­ Desert spiders are often larger than close congeners in less P

e corded nymphal stages of individuals that were first marked at arid habitats, and as a result have low surface-to-volume ratios h t a very young age or finally recaptured as subadults or adults. (Remmert, 1981; Cloudsley-Thom"'pson, 1983). Leucorchestris

y arenicola is large, up to 5,0 g (X = 1.7 ± 0,9 g) and 32 mm b

d RESULTS AND DISCUSSION (X = 20,9 ± 4,8 mm) body length; adult females are usually e t heavier than adult males (2,6 ± 0,9 g vs. 2,0 ± 0,5 g; t = 5,43; n a Taxonomy d.f. = 120; P < 0,05). Legs of males grow allometrically during r g

The first Namib dune heteropodid described was an imma­ the moult to adulthood, attaining a final standing leg-span of e c ture female Leucorchestris arenicola Lawrence, 1962, from the 10-14 cm (6-9 cm for adult females). n e vicinity of Gobabeb. Later collecting yielded an adult female Hagstrum (1971) found that carapace width corresponded c i l

from the type locality (Lawrence, 1966). An adult male L. kochi closely with instar stages of 13 spider species. As carapace r e Lawrence, 1965, described from the same locality, is probably width correlated with mass, he concluded that it can be used d n a male L. arenicola. Leucorchestris kochi is the only male in as a field guide to size and development stages of many u the genus sharing the same distribution as L. arenicola and s..eecies. In L. arenicoia, carapace width was 2,1-14,0 mm y a has been recorded mating with L. arenicola on 34 occasions. (X = 9,4 ± 2,1 mm; n = 386). Where it could not be measured w e without injuring a spider, it was estimated for nymphs and adult t This species and another possible junior synonym, L. sabuiosa a Lawrence, 1966, will be included in a taxonomic revision of the females from the length of the sci erotized shaft of left femur I G

t genus (Croeser, personal communication). Voucher speci­ (Table 1). A simple linear relationship could not be established e n mens from the present study will be kept at the State Museum, for adult males because of the allometry described above i b Windhoek. Throughout this paper the family name ((Z = 0,45; n = 36). a S

Heteropodidae is used in preference to Sparassidae (Croeser, Live mass was difficult to measure accurately in the field, but y b could be estimated on the basis of body length, carapace

1986). d e c u d o r p e R 118 NAMIB DUNE SPIDER BIOLOGY

KEY x • L.arenicola found x L.arenicola not found - Riverbed Dunes

Fig. 2 Distribution of Leucorchestris arenicola (n =452) in the central Namib Desert. . ) 9 0

0 width, abdomen width and abdomen length (Table 1). preferences for different dune habitats. During the present 2 survey, other heteropodid species were found in habitat adja­ d e t Distribution and habitat cent to that dominated by L. arenicola: Carparachne au­ a

d Leucorchestris arenicola appears to be confined to the reoflava and C. alba on the upper dune plinth, slipface and (

r southern Namib dune sea and adjacent sandy areas (Fig. 2). dune crest; Orchestrella browni and 0. longipes on the inter­ e

h It lives in the central, warm, foggy zone and the warm, inland dune gravel plains. s i l zone where high daily fluctuations of temperature and humidity b

u occur (Besler, 1972; Lancaster, Lancaster and Seely, 1984). Burrow structure P

It is rare or absent in the cold, foggy, coastal, crescentic dunes Leucorchestris arenicola lives in straight, silk-lined burrows e h that extend approximately 20 km inland from the coast. This dug at an angle into sand (Fig. 1; Table 2). Spiders excavate t

y may be due to an unsuitable climate dominated by strong burrows by removing sand from the base of a circular depres­ b winds, or a scarcity of adequate habitat. Their distribution is sion as described by Lawrence (1965a). The leg coxae, and d e t limited in the east by the edge of the dunes and in the north by curled pedipalps bearing stiff interlocking setae, are used to n

a gravel plains abutting the Kuiseb River. The southern limit is push loose sand sideways up to the entrance. Sand is dis­ r g unknown. Leucorchestris sabulosa, which may be synony­ persed from the entrance by flinging it sideways with the e

c mous with L. arenicola, is the southemmost Leucorchestris brush-like tarsal scapulae. The lower end of the depression is n e captured at 27° 30' S, 15° 45' E (Lawrence, 1966). secured by lifting and interweaving loose sand with adhesive c i l Relatively open stretches of firm, gently sloping dune sand silk from the spinnerets and pressing the sand- silk mixture into r e with sparse vegetation were favoured by L. arenicola. Burrows the substratum. This forms a nodule of silk and sand embed­ d

n were usually found within 1 to 10m of plants (96 %). The ded in the surrounding sand. The burrow end is secured with u abundance of L. arei1icola decreased up dune slopes: 91,0 % 25-35 adjacent nodules in an arc of about 330° (Fig. 1). A y a were collected on dune bases and interdune sand accumula­ 3-10 mm wide floor remains free of silk along which sand is w e tions surrounding vegetation mounds, 7,4 % on the lower pushed to the entrance. The spider lengthens the burrow by 3 t a plinth, 1,4 % on the upper plinth and only 0,2 % on the slipface. to 6 mm before securing another arc of silk nodules; additional G

t These proportions differed from the proportion of surface area arcs may be added at any time. The entrance is closed with a e

n covered by these habitats (Fig. 3), indicating selection reinforced curtain of silk and sand. The rim is later severed to i b (>f- = 73,6; d.t. = 4; P < 0,05) and contradicting Holm and form a thin (c. 1 mm) circular trapdoor, flush with the sand a S

Scholtz's (1980) conclusion that the Heteropodidae (n =16 surface (Fig. 1). A moderate wind suffices to obscure signs of y

b individuals of three species) did not appear to have strong the burrow. d e c u d o r p e R NAMIB DUNE SPIDER BIOLOGY 119

Table 2 Measurements of 560 Leucorchestris arenicola burrows from 15 50 Leucorchestrls Total • sites in the central Namib Desert (Fig. 1). o Measurement Unit Mean ± S.D. n Range ri 50 Leucorchestrls Khommabes

~ o~----~~~~~~~------~ Surface slope (55) 8,7 ±6,8 358 0-33 III Burrow slope (BS) 28,4 ± 3,4 314 14-40 Q. 50 Area Coyer Khommabes Trapdoor diameter (TO) mm 19,6±4,1 433 7-30 Burrow circumference (Be) mm BO,6± 18,0 415 26-48 Burrow length (Bl) cm 33,4 ± 13,4 437 7-125 Burrow depth (80) cm 23,5± 7,0 216 4-48

Interdune Dune Base Lowe.Pllnth UpperPlinth Slipface Foraging behaviour and resources DUNE REGION Similar to large desert spiders elsewhere (Cloudsley­ Thompson, 1983), Namib heteropodids are predominantly nocturnal. Diurnal activity in L. arenicola was observed in April Fig. 3 1987 during termite eruptions following rainfall of 14 mm. It Occurrence of Leucorchestris arenico/a in various dune habitats in the was apparent from the behaviour of captured spiders which central Namib Desert. The relative sizes of areas of each habitat at the 27 ha site at Khommabes are indicated. were released in the heat of the day, that diurnal conditions were stressful; moreover, their light colour against the reddish­ • = sample of spiders (n =94) from all regions besides Khommabes (n 33) and Visnara (n = 309), which were analysed separately. brown sand rendered them conspicuous to predators. When released during the day, spiders commenced burrow construc­ tion immediately and sealed the entrance within 15 minutes when the burrow was about 10 cm long. Spiders sought plant Spiders position themselves anywhere along the length of cover when the sand was too hot for burrowing. Prey was the burrow in an upside-down posture facing the side of the usually not captured by day, but diurnally active prey species

. burrow (Fig. 1). The feet are in contact with the roof and appear were sometimes caught at dusk. )

9 to be sensitive to vibrations on the sand surface. Other limitations to activity appeared to be cold nights (am­ 0

0 Trapdoor diameter and burrow circumference varied with bient temperature < 15°C), bright moonlight (;:: 3/4), strong 2

d body size and could be used to estimate the occupant's size wind (> 5 mls) and condensing fog, which made the trapdoors e t (Table 1). Burrows were usually constructed in firm sand of a clammy. Reduced activity during strong wind could be due to a d gentle slope (Table 2). They rarely occurred on steeper slopes sand abrasion or the high noise level of moving sand particles (

r associated with vegetation mounds or slipfaces. Burrow slope which would mask prey and predator vibrations. e h was 28 ± 3° from the horizontal, slightly less than the angle of The eight eyes of L. arenicola are relatively small and s i l repose (33°), the maximum slope at which dry sand can be probably are not important in locating prey. It most likely b u swept upwards. If surface slopes were> 15°, some burrows detects potential prey through the vibrations the latter makes P

e (n =7) were at angles of < 20°. Burrows with slopes of 33-40° when moving on the sand surface. Such surface vibrations are h t were sometimes (n = 6) built in moist sand. transmitted over long distances (Reichmann, personal com­

y Leucorchestris arenicola often failed to make burrows if kept munication) and are probably detected by sensitive mechano­ b

d in darkness in the laboratory, but burrowed when exposed to receptors (tactile hairs, trichobothria and slit sensilla; Foelix, e t light, suggesting that light may elicit burrowing behaviour. 1982; Barth, 1982) on the legs of L. arenicola, which would n a Similar responses have been reported for other nocturnal enable it to orientate accurately towards the prey, as is the r g

desert arachnids (Polis, Myers and Quinlan, 1986). case in the Central American ctenid Cupiennius salei e c The use of burrows for protection from extreme desert (HergenrOder and Barth, 1983). n e climates is well known for arachnids (Polis et al., 1986). In the Two methods of foraging were observed in L. arenicola. c i l

case of L. arenicola, the vertical depth of the burrow end They generally rushed out of their burrows to intercept ap­ r e (Fig. 1), usually approximately 25 cm, provided a suitable proaching prey at distances of up to 3 m. On occaSion, spiders d

n microclimate, which differed considerably from that at the hunted actively on the sand surface within 3 m of their burrow. u surface (Lancaster et al., 1984; Seely and Mitchell, 1987). At Visnara, only 47 % of the population ventured from their y a Burrow depth did not relate to surface slope (r= 0,06), but was burrows on a given night. Of these, 75 % remained within 1 m w e primarily a function of burrow length (depth = length x 0.55; and 92 % within 3 m of the burrow. Nearly all of those that t a ~=0,81; n=210). travelled further were adult males apparently in search of G

t Of 631 burrows monitored at Visnara, 5,3 % were temporary mates, or, on rare occaSions, any other spider that immigrated. e n shelters used for one day only. All others were occupied for a An attack on prey involved a short jump, re-orientation, i b mean of 68 ± 53 days up to a maximum of 460 days. With one seizing the victim with the front legs and pedipalps, and biting a S

exception, re-use of an existing, vacant burrow by a second immediately. The fangs penetrated exoskeletons of tene­ y

b individual was not observed. brionid and scarabid beetles ventrally, contrary to earlier d e c u d o r p e R 120 NAMIB DUNE SPIDER BIOLOGY

reports that they could not do so (Lawrence, 1962, 1965b). Table 3 Captured prey was held up with chelicerae and dragged about Diet of Leucorchestris arenicola determined from the contents of 145 burrows from 3 sites, which contained 377 prey remains. until it ceased to move. The spider then usually dragged the Unidentified parts (n = 24) were omitted. prey into its burrow where it was consumed. Very small prey, such as termites, were eaten on the surface. Silk was not used during the capture and handling of prey. Carrion was not taken. Number of Percent Although the fangs injected venom (observed when biting Taxa species items into plastic), its potency appeared to be low. For example, a Namib tenebrionid beetle which was bitten, initially became Insecta Coleoptera Tenebrionidae 30 48 paralysed, but recovered completely three days later. A bite Curculionidae 4 22 by L. arenicola merely caused local irritation in a person. Scarabaeidae 3 8 Another large heteropodid, Palystes natalius, has very low Other 3 1 toxicity to larger vertebrates (Newlands and Martindale, 1981), Lepidoptera > 1 8 Hymenoptera 1 3 even though it occasionally captures small vertebrates (War­ Orthoptera 5 2 ren, 1923). Other 3 1 A distinctive record of prey capture in the form of inter­ Arachnida Araneae L. arenico/a 1 4 mingled tracks of predator and prey was left on the sand near Araneae >2 Scorpionida 1 ~ 3 the burrow; drag marks often led away from the capture site to Solifugae 3 the spider's burrow. Although small prey items « 0,2 g), such Reptilia Squamata Gekkonidae 1 0,3 as termites, did not leave distinctive traces in the sand, most Total > 58 other prey could be identified on the baSis of their tracks. Prey capture was recorded 186 times on 122 nights at Visnara during 1987 and 1988 and the estimated annual predation rate was 1670 prey captureslha. spiders' diet items (Table 3). Two of these were excavated In summer, September to March, the average interval be­ after evidence from tracks and drag marks suggested that tween prey captures for a spider was 31 nights and on average geckos had been captured. Based on a set of tracks, Lawrence 3,2 % of the spiders caught prey on a given night, with a (1959, 1962, 1965b, 1966) noted that L. arenicola preyed on maximum of 14 % in December. In winter, the capture rate P. range; , but did not mention observing drag marks from the dropped to 1,5 % per night and most L. arenicola became attack site to the spider's burrow. This original observation has

. quiescent, some even torpid, possibly in response to adverse been construed as evidence that all major genera of Namib )

9 climate as is the case in other desert spiders (Riechert and dune heteropodids frequently prey on geckos (Lamoral, 1971; 0

0 Luczak, 1982). The ability of spiders to vary their resting McCormick and Polis, 1982; Newlands, 1987). 2 metabolic rate, which is usually very low (Greenstone and Although vertebrates may be minor diet items of arachnid d e t Bennett, 1980), may obviate the need for emigration during predators that have low toxicity venom, McCormick and Polis a

d periods of temporary food scarcity (Anderson, 1974). (1982) found that their impact on certain vertebrates that are (

r The soft parts of prey were chewed into tight balls, whereas smaller than them can be substantial. The size of the Namib e

h harder cuticle, such as elytra or legs, often remained intact palmatogeckos taken by L. arenicola could not be established, s i l Prey remains were stored at the base of the burrow. Analysis but judging by skeleton fragments they were probably lighter b

u of 377 prey items collected from 145 burrows (Table 3) showed than the spider. P that the diet reflected prey availability. Approximately 80 % of Leucorchestris arenico/a must have water to survive. In the e h their prey consisted of Coleoptera, half of which were tene­ laboratory, they died within three months if kept with food but t

y brionid beetles, thus confirming previous incidental observa­ without water. Water was imbibed directly when it was offered b tions (Holm, 1970; Seely, 1985). The curculionid Leptostethus on wet cotton wool. Possible sources of water in the field d e t waltoni was the most frequently captured single species include metabolic water and free water content of prey and n

a (14 %). Lepidopteran larvae and adults represented 8 % of the condensed fog or dew water, which might be obtained by r g

items and were also important in terms of biomass. Termites drinking from drenched trapdoors in a manner similar to that e c were sometimes captured and eaten on the surface, as evi­ observed in the laboratory with wet cotton wool. n e denced by spider tracks leading to Hodotermes mosambicus c i l

exit mounds where chewed termite remains were seen, but Reproduction and development r e their importance in the diet appeared to be relatively low. Reproduction was strongly seasonal in the population stud­ d

n Chewed remains of L. arenicola were found in 21 of 214 ied at Visnara. Adult females were present throughout the u burrows and cannibalism was confirmed by direct observa­ year, forming 10 to 30 % of the population. In contrast, adult y a tions. Conspecifics formed 4 % of the prey items, but could be males were absent in winter (May to August of 1987 and 1988). w e more important in terms of biomass. Occasionally, remains of Many males moulted to adulthood in September, reached t a other unidentified spider species, dune scorpions (Op­ peak abundance (8-12 % of the total population) in October G

t isthophthalmus flavescens) and solifugids (Metasolpuga picta and declined in abundance until May « 5 %). The frequency e

n and Prosolpuga schultzel) were found (n 12). Some of these of mating and the number and size of egg clutches and litters i b were bigger than their captors. all peaked in December. No mating or egg clutches were a S

The remains of small geckos, Palmatogecko rangei, were observed from May to August. y

b found only three times and represented about 0,3 % of the Mature L arenicola males frequently (on average every 4th d e c u d o r p e R NAMIB DUNE SPIDER BIOLOGY 121

Table 4 Table 5 Litter size of juvenile stages of Leucorchestris arenicola at Visnara Duration (days) of various developmental stages of Leucorchestris determined by counting live young seen within burrows, or by arenicola determined by inspecting nursery burrows at intervals of a counting excavated exuviae. few days, or by recapturing marked post-nursery spiders at Visnara. The period for immatures represents the interval between observed moults; for adults it represents longevity. Exuviae

Mean ± S.D. Range n Mean ± S.D. Range n Instar Development stage Mean Range n

Eggs 24 75,8 ± 59,0_ 25-161 4 Nursery Larvae 26,3 ± 19,6 1-74 24 41,5±43,9 5-161 15 1 Egg and pre-larva 15±3 9--19 13 Nymphs I 13,0 ± 7,2 1-23 15 45,1 ± 31,5 1-95 18 2 Larva 13± 3 10--16 8 Nymphs II 8,8 ± 9,2 1-35 24 8,0 ± 9 ,2 2-25 6 3 Nymphal stage I 23± 10 12-38 10 4 Nymphal stage II 33±9 22-45 14 Female with nymphal stage II 14± 9 4-34 13 'Live juveniles were difficult to count accurately until the female Female with brood 75± 19 59--115 8 .. abandoned them at nymphal stage II. Between broods 44± 21 14-77 12 This was probably underestimated, as the exuviae of larvae were fragile and difficult to count. Post-nursery 4 Nymphal stage II 52 1 5 Nymphal stage III 84±43 38--128 4 night; n = 233) wandered far from their own burrows 6 Nymphal stage IV 98±4 95--101 2 (X = 38 ± 71 m; maximum = 450 m). Silk drag-lines were often 7 Nymphal stage V 95±66 38--278 14 8 Nymphal stage VI 114 ± 61 32-249 19 observed along their tracks. They usually returned to their 9 Nymphal stage VII 118 ± 42 37-204 31 burrows on direct routes, navigating at night over more than 10 Adult male 47±29 17-98 22 100 m without using the drag-lines. Drag-lines were only oc­ 10 Adult female 130 ± 79 31-463 47 casionally retraced over short stretches, especially when the spiders were disturbed. It is likely that cues other than those associated with the drag-line enable L. arenicola to find their burrows, but the method of orientation is not known. It has been larvae (Tables 4 & 5; terminology for juveniles according to shown that other wandering spiders use landmark and kines­ Vachon, 1957, in Foelix, 1982). The larvae remained in the thetic orientation, which depend on the spiders' previous mo­ maternal burrow, or nursery, and began to feed upon moulting . )

9 tility patterns (Seyfarth and Barth, 1972; Barth, 1982; Foelix, into nymph stage I, as evidenced by their growth. Approxi­ 0

0 1982). mately 14 days after nymph stage II was reached (Tables 4 & 2 On outgOing trips, males often changed direction and 5), the female abandoned the burrow and usually established d e

t stopped every 0,5 m to 5 m to drum the ground with all eight a new territory. The new burrows were 0,4 to 16,0 m a

d legs in succession. This left an imprint of the outline of the (6,3 ± 5,1 m; n = 17) away from the old ones. (

r legspan, pedipalps, leg coxae and abdomen. Drumming While in the nursery, adult females hunted frequently and e

h behaviour may inhibit attack by territorial residents and attract carried prey into the burrow. This food source may have been s i l mates. This behaviour appears similar to leg drumming by utilized by the nymphs since the total mass of litters increased b

u adult male Cupiennius salei, a ctenid wandering spider about five-fold during nymphal stages I to II, but no data are P

(Rovner and Barth, 1981). available to support this notion. Some social or periodic-social e h Males of L. arenicola mated on 14 % of their wanderings spiders exchange food by trophallaxis or indirectly via the prey t

y (n = 233) usually with different females. Mating occurred (Foelix, 1982; Lubin, 1982). This has not been studied in b 1,6 ± 1,0 m (n = 33) from a female's burrow, leaving a series asocial species such as L. arenicola that have extended brood d e t of long scrape marks on the surface. Females mated several care. Attempts in the laboratory to raise two litters of orphaned n

a times in a fortnight. In four observed cases, females laid eggs Leucorchestrisstage I nymphs, using crushed insects as food, r g 31 to 35 days after the last mating. Four other females mated failed. e

c while caring for young. One female that overwintered as an On one occasion, an adult male from a burrow 22 m away n

e adult, laid viable eggs early in the breeding season before the provided stage II nymphs with prey after the female had left c i l appearance of adult males and therefore must have stored the nursery. He caught a large moth, dragged it 1 m into the r

e sperm through the winter. nursery and returned to his burrow. The nymphs were feeding d Each egg clutch was laid in two layers and was enclosed in on the moth upon excavation. All other observations indicated n u

a flat silk pouch, or cocoon, which was about 5 mm thick. The that only the female provided food to her offspring. y a cocoon was spun against the burrow roof at a vertical depth of The reproductive cycle was estimated at about 120 days, w

e about 12 cm (Fig. 1). Average egg clutch size was 76 and based on observed durations of maternal care and periods t a incubation period 15 days (Tables 4 & 5). The mass of a single between successive broods (Table 5). In one year 58 adult G egg was approximately 4 mg. After producing a cocoon, a females produced 44 litters, or 0,76 litters/female/annum. t e female's mass dropped by about 15 %, or 350-700 mg. The Some females moved in from or to adjacent areas, giving rise n i

b female guarded the eggs. When disturbed, she ate the eggs to a slight underestimate in their reproductive rates. In one a

S and abandoned the burrow. breeding season 45 % of the adult females produced no y The female ate the empty cocoon after emergence of the offspring, 40 % produced one litter, 10 % two and 5 % three b d e c u d o r p e R 122 NAMIB DUNE SPIDER BIOLOGY

TableS Growth of Leucorchestris arenicoia determined from recaptures at Visnara.

Carapace width Body length Mass

Stage Mean ± S.D. Range Mean ± S.D. Range Mean± S.D. Range n mm mm mg

Egg & pre-larva 2,5 4 161 Larva 2,5 4 1 Nymph I 2,8 2,1-3,5 6,8 5,9-7,6 55 27--83 2 Nymph II 4,4±0,3 4,0-4,9 10,2 ± 0,9 9,2-12,5 169 ± 57 105-309 10 Nymph III 5,8±0,6 5,0-6,9 13,0±1,7 11,3-17,6 361 ± 172 173--846 16 Nymph IV 7,2 6,4-8,1 16,4 12,9-20,0 801 355-1247 2 Nymph V 7,6 ± 0,6 6,4-8,8 17,1±2,3 13,0-22,4 824 ±308 380-1603 14 Nymph VI 8,9 1,0 6,9-11,0 20,0±2,7 14,6-25,3 1371 ±585 500-2544 27 Nymph VII 10,0 ± 1,0 7,1-13,2 22,4± 2,8 16,7-28,1 1904 ± 666 731-3535 62 Adult male 11,4 ± 0,9 9,6-13,3 22,4± 1,9 18,9-27,1 1938 ± 448 999-2749 30 Adult female 11,1 ± 1,0 7,3-14,0 24,8 ± 3,1 14,8-31,8 2570 ±901 623-4981 71

litters. One female produced 5 litters in two breeding seasons. having fed. Sometimes a large decrease in mass (> 0,5 g) was Kessler (1971) noted that the size of eggs of female wolf caused by the loss of a limb or of haemolymph due to injury. spiders remains constant, but egg clutch size varies as a Adult males were no heavier than subadults (Table 6), and function of food supply. In L. arenicola, it appears that both egg they did not appearto feed in the field although theysOmetimes clutch size and the interval between clutches may vary, while accepted food in the laboratory. The size of adults, especially some females fail to produce any clutches in a breeding females, varied considerably: by a factor of two for carapace season. Differences in egg sizes were not detected. width and body length, and by a factor of eight for mass (Table Within a month of reaching nymphal stage II, the number of 6). Their mass could increase rapidly by as much as 50 % after spiderlings in a litter decreased to about 9 (Table 4). No eating a large prey item, or drop by 15 % when they laid eggs. dried-up carcasses of nymphs and no evidence of early dis­ Similarly, adult size of some other spiders varies by a factor of . )

9 persal or interspecific predation were found, thus suggesting two for carapace width and by a factor of 12 for mass (Jocque, 0

0 that nymphs cannibalized siblings. However, cannibalism can­ 1981; Vollrath, 1987). Because of this, it is impossible to 2 not account solely for the 42-fold increase in the mass of estimate the age or nymphal stage from size for L. arenicola d e juveniles while in the nursery. Sibling cannibalism is not un­ beyond nymphal stage III. t a common among communal spiderlings of non-social species Pronounced seasonality in activity was evident. According d ( (Turnbull, 1973; Polis, 1981). Resorption or consumption of to the classification of Schaefer (1977, in Foelix, 1982), L. r e eggs by a female as well as sibling cannibalism can be arenicola can be classified as a eurychronous species that h s i considered bet-hedging by desert arachnids and would enable overwinters in various stages of development. All arenicola l L. b rapid population response to favourable conditions (Polis, instars lasted significantly longer in winter, between April and u P

1988). August, than in summer, September to March (156 ± 42 vs. e h At approximately 70 days and 169 g (Table 6), remaining 85 ± 41 days; t = 8,99; d.f. = 59; P < 0,05). In winter, the rate t

y stage II nymphs constructed a small exit next to the trapdoor of prey capture dropped to less than half the summer rate (see b of the nursery and built individual burrows close by. They above). Adult males were absent and females were reproduc­ d e moulted to nymphal stage III about 52 days later. Each subse­ tively inactive. Some spiders were in a torpid condition. It is not t n quent moult occurred at a mean interval of 110 ± 54 days unusual for spiders to reduce activity and prolong development a r

g (32-278 days; n = 85). The moulting interval did not differ in winter (Almquist, 1969).

e significantly between nymphal stages (Table 5; < 0,94), nor Successive recaptures indicated that arenicola of both c t L. n

e with size or magnitude of size change (carapace width; r sexes have a total of 10 instars. No evidence was found that c i

l < 0,16), but varied seasonally (see below). this may vary as it does in some other spiders (Vollrath, 1987). r Between successive nymphal stages, carapace width in­ arenicola e From pre-larva and larva, L. went through seven d creased by 1,1 ± 0,7 mm, body length by 2,3 ± 1,9 mm and nymphal stages, reaching adulthood at 24 months of age. This n u

mass by 144 ± 44 % of initial mass (n = 55; Table 6). This was age was calculated by adding the nursery and the post-nursery y

a similar for both sexes. Measurements taken one day before nymph II periods (70 and 52 days respectively) to two winter w

e and after ecdysis from nymph stage IV to V indicated that and three summer instar periods (156 and 94 days each t a carapace width increased by about 20 % at the expense of a respectively) to give a total of 716 days. Adults were not G

20 % decreases in abdomen length and width. For nymphs of observed to moult. Upon reaching adulthood, males survived t e stages III to VII, the average rate of mass gain was for only 47 ± 29 days. In contrast, adult females survived for n i

b 7,4 ± 10,8 mglday (n = 53). However, some spiders (28 %) at least 130 ± 79 days (maximum 463). Discounting mortality a

S lost mass between instars, although they usually increased in by predation, the life expectancy of females was 30-40

y carapace width. Normally this occurred if they moulted without months. b d e c u d o r p e R NAMIB DUNE SPIDER BIOLOGY 123

Table 7 Density of Leucorchestris arenico/a burrows in the central Namib Desert dunes determined by various methods: 1 =counts along 6 m wide transects across suitable habitat; 2 monitor all burrows of unmarked spiders for 6 months; 3 =monitor all burrows of marked spiders for 12 months.

Census Census Sample Area Number Population Site method period days (ha) counted numberlha

Khommabes Dune Base 1 May 87 1 1,00 16 32 Khommabes Dune Plinth 1 May 87 1 1,56 7 9 Far East Dune 1 Nov 86 1 1,50 23 31 Noctivaga 1 Oct 86-Jan 87 2 1,00 10 20 Visnara I 2 Oct 86-Apr87 73 0,50 120± 34 239 (64-302) Visnara II 3 Jun 87-Jun 88 122 0,33 59± 14 176 (75-243)

Old females sometimes (n = 4) displayed aberrant behav­ mum of 0,11 burrows/m2 in one patch of 0,01 ha. iour. Becoming very aggressive, they abandoned territories Females and nymphs of other species of wandering spiders and moved burrows frequently. They eventually failed to con­ have relatively small home ranges (Turnbull, 1973). In L. struct proper burrows (burrow length < 5 cm), which became arenico/a, an area described by a 1-3 m radius around a too hot in the day; these were abandoned in favour of shelter burrow was usually defended against intrusion by smaller amongst plants. At night, these females moved into other conspecifics. The presence and size of the defended area was territories, where they wounded or killed the residents if chal­ confirmed by releasing spiders in small enclosures close to lenged. Eventually, they died of heat stress in shallow burrows, known burrows and monitoring the response of the resident. or were wounded or captured by predators. According to Kaufmann's (1983) definition, L. arenico/a is In general, the phenology of L. arenico/a appears to be territorial: at a given time an individual has priority of access similar to that of a Sonoran Desert wolf spider, Lycosa car­ to resources in a fixed area within its home range. Territory o/inensis, which has a period of quiescence in winter and is boundaries were not clearly delimited by natural boundaries or

. most active and has the largest proportions of adults in mid­ marks and occupation was advertised physically. )

9 summer. Females of this wolf spider produce two litters per Energy-based territoriality is widespread in desert spider 0

0 season, development is relatively slow, and sexual maturity is families (Cloudsley-Thompson, 1983). Territorial agelenids 2

d achieved only in the third year of life (Shook, 1978). (Age/enopsis aperta) adjust the cost of physical defence ac­ e t cording to resource quality at a site (Riechert, 1979). Although a d Population ecology the contest situation favours the territory owner, its intensity (

r Within a suitable habitat, densities of L. arenico/a varied may serve as a cue of site quality to the intruder. More energy e h spatially and temporally. In addition to seasonal fluctuations of is required to gain or maintain territories in regions where s i l mature males and of reproducing females, a major factor spider densities are high than where they are low. Territory b u influencing population density appeared to be predation (see sites should thus be selected optimally (Riechert, 1979). In this P

e below). The estimated abundances ranged from 9/ha in winter respect, the uneven distribution of L. arenico/a burrows in an h t on a dune plinth at Khommabes, 5 km from Visnara, to a peak area warrants further investigation.

y of 302lha in summer at Visnara (Table 7); the wet biomass The average nearest-neighbour distances of 997 different b

d ranged from 15-513 giha. pairs of L. arenico/a burrows measured in one year was e t Between June 1987 and June 1988, 58 adult females and 3,90 ± 2,10 m. Close neighbours « 2 m apart) remained in n a 22 adult males were resident in 0,33 ha at Visnara. Another 12 their relative positions for shorter periods than more distant r g

males wandered through the area. During that year, 44 litters pairs. Distant neighbours (91 %) with burrows > 2 m apart e c were produced with approximately 9 stage II nymphs entering remained in position for 20 ± 28 days, 14 % of them for> 1 n e the population from each litter (Table 4). The potential recruit­ month. In contrast, closer neighbours remained in position for c i l

ment rate was thus 1188ihalannum, or 6,8/female/annum. only 9 ± 16 days (t= 3,01; d.t. = 159; P< 0,05), with only 6 % r e Assuming that emigration balanced immigration for all instars lasting> 1 month, which is indicative of instability among close d n and that recruitment and development rates were similar in pairs. u previous generations, the probability of post-nursery nymphs Juvenile and adult sex ratio was uneven. Owing to their short y a surviving to adulthood was calculated as 0,134 (egg to adult: natural life expectancy upon reaching adulthood (7 weeks), the w e number of adult males in an area was usually low. During the t 0,016). a Visnara had a peak of 81 burrows (0,025/m2) on 8 January breeding season at Visnara, there were usually only one or G

t 1988. These were occupied by 24 adult females (4 of which two adult males to 19-32 adult females, except in early Octo­ e n had a total of 55 nymphs), 1 adult male, 14 subadult females, ber 1987, when there was a peak of 8 adult males to 19 adult i b 6 subadult males and 33 immature spiders, with a total mass females (1 : 2,4). In the course of a year, the proportion of a S

of 111 g. The population was distributed unevenly: 52 % of the resident males to females was 1 : 2,6 for adults (n = 80) and y b burrows occurred in 24 % of the area (0,05/m2) with a maxi- 1 : 1,6 for nymphs V to VII (n 105). These ratios differed d e c u d o r p e R 124 NAMIB DUNE SPIDER BIOLOGY

significantly from parity (X2 > 6,5; d.t. = 1; P < 0,01). but not Cannibalism from each other (X2 = 2,8; dJ. = 1; P> 0,05).

Cannibalism Cannibalism was observed among marked individuals at Visnara on 14 occasions. Additional remains of conspecifics were found in 21 of 214 excavated burrows (see above). Palmatogecko Although conspecifics formed a minor proportion (4 %) of the Scorpion .... , ...i!!§::....:::....:=iDune lark total diet of post-nursery spiders, cannibalism was an impor­ ~ Wasp tant mortality factor, contributing 19,4 % of the known causes of death. All post-nursery cannibals, but one, were subadult or adult females larger than their victims (difference in carapace width 1.5 ± 1,2 mm; n 14). Victims were captured at dis­ tances of 5,4 ± 4,2 m from their own burrows by cannibals that were 1,4 ± 1,9 m from their burrows. In 13 cases, smaller spiders survived the attack of larger ones by retreating into their burrows 0,2 ± 0,4 m away. When interactions with close Fig. 4 nearest-neighbours (mean distance = 2,3 ± 2,3 m) were re­ Cause of death of Leucorchestris arenico/a in 72 incidents at Visnara. peated. smaller individuals were prevented from foraging and Percentages are indicated. relocated their burrows (n = 13). Hallander (1970) described similar behaviour in two species of wolf spider, both in the field and in the laboratory. Conspe­ ricularis. This density of gerbils was high compared with a cific Iycosids made up 20 % of their diet. In particular, adult maximum of 18,2 gerbils/ha found elsewhere in the dunes females caught nymphs and adult males, and spiderling sib­ (Boyer, 1988). Namib dune gerbils are omnivores with 53- lings ate each other. These Iycosids, like L arenicola, coun­ 59 % of their diet, determined from stomach contents, consist­ tered cannibalism by having very distinctive courtShip signals ing of invertebrate matter, including spiders (Boyer, 1988). and by juveniles that remain concealed beyond the reach of Between June 1987 and June 1988 at Visnara, 253 attacks adults. by gerbils on spiders on the surface or in their burrow en­

. In a review of the occurrence of intraspecific predation trances were recorded. In one extreme night, 51 % of the )

9 among animals, Polis (1981) found that generally the more burrows were disturbed and 18 % of the post-nursery popula­ 0

0 cannibalistic individuals were large, female and hungry. Star­ tion was killed. Attacks were concentrated during January and 2 vation increased the likelihood of choosing con specifics as June when the spider population abruptly decreased by 27 % d e t prey and the vulnerability of juveniles. Cannibalism increased and 51 % respectively. Many attacks destroyed the burrow a

d as other resources decreased, forming a type of reserve for entrances but did not harm the spiders. Spiders were often (

r times of stress (Hallander, 1970; Polis, 1988). Furthermore, attacked again when they were repairing the damage. Those e

h the rate of cannibalism was density- dependent and increased attempting to build new burrows were sometimes captured by s i l when nearest neighbours were close (Polis, 1981). This gerbils or by conspecifics. b

u makes cannibalism a potentially sensitive regulator of popula­ Infrequently, pompilid wasps opened L arenicola trapdoors P tion densities. and entered burrows. Usually they were evicted by the spider. e h Once, a wasp did not emerge from a burrow in 15 minutes and t

y Other causes of mortality the unharmed wasp was excavated together with a live immo­ b Cannibalism was one of eight identified causes of death of bilized spider (wasp body length 13,6 mm; spider nymph stage d e t L arenicola that were observed in 72 incidents at Visnara III, body length 11,9 mm. mass 240 mg). This behaviour re­ n

a (Fig. 4). Four old females died after exposure to adverse sembles the pattern of hunting, paralysis and oviposition by r g Anoplius spp. pompilid wasps on Geolycosa wolf spiders

climate or injury. Non-feeding adult males became inactive e c and died after several periods of intense activity in pursuit of (Gwynne, 1979) and several other wasp-spider pairs (Grout n e mating opportunities. Some spiders, unable to free their legs, and Brothers, 1982). It is not known whether Namib pompilids c i l

died during ecdysis. This was recorded four times in the field specialize in hunting only one family of spiders, but it is likely r e and often in the laboratory. Once, a coleopteran larva killed that the finding and excavation of heteropodid burrows re­ d

n and partially consumed a L arenicola during ecdysis in the quires particular skills. Gess and Gess (1980) suggest that one u field. South African pompilid species is a specialist hunter of y a The most important predators of L arenicola were gerbils, Heteropodidae, although other species capture several other w e Gerbillurus paeba, G. tytonis and DesmodUlus auricularis, spider families as well (Gess and Gess, 1974). t a accounting for 64 % of known deaths. The density of gerbils In the present study, two L arenicola were caught on the G

t at Visnara was determined in February 1988 by Dednam surface at dawn by a dune lark, Mirafra erythrochlamys. The e

n (1988) over a 1 ha area containing about 207 L arenicola scorpion Opisthophthalmus flavescens was recorded once as i b burrows. Using a mark-recapture technique for 8 trapping predator and twice as prey, an example of cross-predation a S

nights, the gerbil density was calculated as 66/ha, with (McCormick and Polis, 1982). Similarly, a large Palmatogecko y

b 47± 7,3 G. paeba, 13 ± 5,1 G. tytonis and 6 ± 1,3 D. au- rangei once caught an L arenicola on the surface and three d e c u d o r p e R NAMIB DUNE SPIDER BIOLOGY 125

small geckos were consumed by spiders. The L. arenicola population at Visnara underwent five and Cross-predation has previously been recorded for several three-fold density fluctuations in two successive years. Based arachnids, when they prey on juveniles of a species, but on measurements of minimum territory size, the theoretical themselves fall prey to larger individuals of the same species maximum population density is 1000 spiderslha, a density that (McCormick and Polis, 1982). Besides the acquisition of food, was actually achieved only in small patches of 0,01 ha. the benefits of cross-predation include reductions in future Variable predation pressure, which reached catastrophic pro­ risks of predation for the predator or its offspring. portions in some patches, produced uneven density patterns Leucorchestris arenicola display several anti-predator of spiders in suitable habitat with abundant prey. behaviour patterns. Spiders at the bottom of their burrows It has been proposed that populations of many other desert appear to be safe from most predators. Risk-sensitive forag­ organisms are not affected by biotic interactions, but that ing, as seen by Polis (1988) for the scorpion Paruroctonus climate plays a fundamental role in producing autecological mesaensis, may enable L. arenicola to avoid the time and effects (Noy-Meir, 1973). For example, although Namib tene­ region where their predators forage. The suggestion that brionids have many predators, these do not appear to limit their L. arenicola has the capability to flee by cartwheeling (New­ populations, but densities probably depend on environmental lands, 1987) in the manner of Carparachne aureoflava conditions (Seely, 1985). In contrast, L. arenicola and many (Henschel, 1990) was not confirmed. In a series of trials, no other desert arachnids appear to be affected to a large degree L. arenicola of any size showed an ability to wheel on a slope. by biotic interactions, especially intra- and inter-specific pre­ Many spiders of sandy deserts, like L. arenicola, are very dation (Polis and McCormick, 1986b). pale and lack distinct markings, which may render them less Several common tenebrionid beetles, prey species of visible to predators at night (Cloudsley-Thompson, 1983). L. arenicola, foraged on detritus and reproduced throughout When approached by potential predators on the surface, L. the year (Seely, 1983, 1989). Soil temperatures at a depth of arenicolascuttled fortheir burrows, sheltered in a nearby plant, 20-30 cm, where L. arenicola reSide, fluctuate little on a daily or froze motionless if shelter was remote, as in the case of basis, but vary seasonally, dropping from approximately 30 °C wandering males. When confronted closely, they showed in summer to approximately 20°C in winter (Lancaster et al., overt aggression, jumping forwards, then standing threaten­ 1984). Cooler soil temperatures during winter may decrease ingly with body up, some legs raised and spines erect. Ap­ the metabolic rate of L. arenicola, suppressing activity and proach and threat posture alternated to give the appearance eliciting torpor in some cases. Climate, rather than prey avail­ of a dance (Lawrence, 1962). Upon contact with the foe, the ability, could thus explain seasonal patterns in the foraging spider vigorously embraced it with the legs and chelicerae and behaviour, reproduction and development rate of this chthonic

. then tried to flee. nocturnal spider. )

9 Aggression can account for dominance of temporal zones Wandering spiders that live in burrows are generally longer­ 0

0 by one arachnid species over others in an area (Polis and lived in deserts (Cloudsley-Thompson, 1983) than similar spi­ 2

d McCormick, 1986a). Polis and McCormick (1986b) found no ders in more mesic regions (Foelix, 1982). In a few known e t evidence of exploitation competition among sympatric desert cases, the longevity of large aranaeomorph spiders was found a d arachnids of a guild, but suggested that intraguild predation to be more than two years in deserts, whereas elsewhere such (

r directly inffuenced their behaviour, distribution and abun­ spiders usually have annual life-cycles. This may be related to e h

s dance. Similar circumstances may apply to L. arenicola in the food availability, which appears to be less predictable on a i l Namib Desert. daily basis in a desert than it is in wetter temperate and tropical b u regions. P

e Unpredictable environmental extremes may affect food h t GENERAL DISCUSSION availability and offspring survival of many desert invertebrates y (Louw and Seely, 1982). Under such Circumstances, a pattern b

d The biomass of Namib spiders appears to be at least an order of bet-hedging (Murphy, 1968) is often adopted, indicated by e t of magnitude lower than that of wandering spiders in other the long life of females, iteroparity and small broods as seen n a temperate regions (Chew, 1961; Turnbull, 1973). In the pres­ in many Namib tenebrionids (Seely, 1983). Leucorchestris r g ent study, most of the surveyed areas were selected for their arenicola is iteroparous and produces relatively small egg e c high densities of L. arenicola. On visits to other sparsely clutches compared with semelparous spiders, for example n e many orb-weavers (Foelix, 1982). Although reproductive effort c vegetated firm dune slopes in the central Namib that were not i l

surveyed systematically, searches on calm mornings usually of L. arenicola females in the form of brood care is high (62 % r e revealed the presence of L. arenicola burrows. The general of reproductive cycle) in contrast to some other Namib inver­ d n impression was gained that within suitable habitat, L. arenicola tebrates (Seely, 1983), the reproduction of L. arenicola is u

y densities were approximately 5-10/ha and that this species consistent with a pattern of bet-hedging. This may enable a a appears to be the dominant spider in terms of biomass (8- rapid population response in the form of increased survival of w e

t 17 g/ha) although small gnaphosids, Asemesthes spp. offspring if conditions should suddenly improve, such as after a « 0,1 g), and an eresid, Seothyra sp. « 0,3 g) appear to be rare events of rainfall (Seely and Louw, 1980). G

t locally more abundant (Henschel, unpublished data). Accept­ It is concluded that the phenology of L. arenicola closely fits e n

i ing Seely and Louw's (1980) estimate of 55 g/ha biomass of a K-selected life history strategy (Pianka, 1970). On an annual b basis, this spider has a fairly predictable supply of food, but its

a all invertebrate carnivores on a dune plinth during a dry year, S L. arenicola would constitute 15-30 % of the carnivore bio­ foraging activity and spatial organization are influenced by the y b

mass. risk of predation, whereas its metabolism and development d e c u d o r p e R 126 NAMIB DUNE SPIDER BIOLOGY

rate appear to be subject to climatic seasonality. Under these longevity, produces relatively small clutches of eggs. is conditions, L. arenicofagrows and develops slowly, has a high iteroparous and has an extended period of brood care.

ACKNOWLEDGEMENTS Field and laboratory work were conducted with assistance Beverley Marsh and Mary Seely. The Foundation for Research from Diane Grant, Inge Henschel and Allison Hornby. I wish to Development granted research funds and the Directorate of thank Eryn Griffin, Robert Pietruszka and Gary Polis for valu­ Nature Conservation and Recreation Resorts of Namibia pro­ able consultation. Drafts of this manuscript were kindly re­ vided facilities and permission to work in the Namib-Naukluft viewed by Peter Croeser, Ansie Dippenaar, Yael Lubin, Park.

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B. A. Curtis State Museum, P. 0. Box 1203, Windhoek, 9000 Namibia

Camponotus detritus is a large formicine ant which occurs in the Kuiseb River and dunes of the central Namib Desert. Aspects of its biology, ecophysiology and behaviour are discussed in relation to the dune ecosystem, and possible areas of future research are considered. It is a diurnal species, avoiding the most stressful periods of the day by returning to the nest or staying in the shade of the vegetation. For and ant, it has a high resistance to temperature and desiccation. It is the major consumer of honeydew, limited chiefly by intraspecific competition for food and the availability of honeydew.

INTRODUCTION Colonies comprise from one to four nests that are excavated among the roots of the perennial dune vegetation (Curtis, Camponotus detritus Emery is a large and conspicuous 1985b). Nests consist of a series of tunnels and chambers formicine ant occurring in the central Namib Desert, where it extending to a maximum depth of about 0,4 m, often lined with is confined to the sand dunes south of the Kuiseb River and to detritus. The comparatively simple nest construction enables the dry river-bed (Curtis, 1985a). Morphologically and the ants to exploit a fairly wide range of nesting sites as well behaviourally C. detritus is very similar to Camponotus as allowing them to excavate new nests with relative ease, fulvopi/osus De Geer, which occurs in the arid and savannah either when old nests become too small or when environmen­ areas of southern Africa. The latter species is present on the tal factors such as the encroachment of a dune or strong winds . ) gravel plains of the central Namib Desert and the dunes of the destroy the nest (Curtis, 1985b). Multi-nest colonies may have 9

0 Skeleton Coast, but is not sympatric with C. detritus. disadvantages since workers appear to spend considerable 0 2

Apart from a few paSSing mentions (Robinson and Cunning­ time and energy walking between nests. Nevertheless, should d e ham, 1978; Seely, 1978; Holm and Scholtz, 1980), no study one nest be destroyed suddenly, the entire colony is not lost, t a had been made of C. detritus before 1980. In 1980 I started a and workers can rapidly transport surviving brood and other d (

two-year study of the natural history and general ecology of colony members to an established nest. C. detritus colonies r e this species. I examined its physiological tolerances and com­ generally only have one queen (or occasionally a few queens) h s i pared them with data on other ant species. I also determined in one of the nests and brood is transported by the workers to l b the effect of an environmental gradient across the dune-field other nests. This seems a hazardous task for a diurnal desert u

P on the distribution and abundance of workers. species exposed to desiccating external conditions and e

h This paper provides a synthesis of my published findings predators. For example, Murray and Schramm (1987) ob­ t and other information on C. detritus. The ant is discussed in served the lizard, Meroles cuneirostris, robbing C. detritus y b

relation to the dune ecosystem as a whole and future research 'carrying objects in their jaws', some of which may have been d e possibilities are considered. brood. However, queenless nests containing brood occur in t n both C. maculatus and C. werthi (Skaife, 1961), suggesting a r BIOLOGY that this phenomenon is not unusual for the genus. g

e One factor contributing to the success of C. detritus appears c n The genus Camponotus has a worldwide distribution with over to be its ability to reproduce all year round as a result of fairly e c i 1500 species and almost as many variations in behaviour and high temperatures throughout the year, combined with a con­ l

r ecology (Wilson, 1971). In southern Africa alone there are stant food supply. Although brood is present throughout the e d more than 70 species (Prins, 1978), of which four have been year, there is a great variability in the ratio of brood to workers n

u recorded in the Namib Desert (MarSh, 1986). and number of callows in different nests (Curtis, 1985a). The

y Like many Camponotus species, C. detritus workers are highest numbers of pupae were found in summer. a

w polymorphic, with a continuous, non isometric size distribution Alate reproductives were present in the nests throughout e t from 7-16 mm. This polymorphism results in a certain amount most of the year with a maximum in December (summer) a

G (Curtis, 1985a). Since only one nuptial flight was actually of task specialization, but behavioural flexibility is retained. For t e example, although the majority of foraging is performed by witnessed, it could not be established exactly what factors n i minor workers, majors assist in retrieving food items too large trigger the flight. Rainfall is presumably important, since that b a for the minors (Curtis, 1985a). flight occurred after 3 mm of rain. However, not all rainfall S y b d

e Curtis, 8. A.. 1990. Behaviour and ecophysiology of the Namib dune ant. Camponotus detritus (Hymenoptera: c Formicidae).ln:Seely, M. K., ed .. Namib ecology: 25 years ofNamib research, pp. 129-133. Transvaal Museum u

d Monograph No.7, Transvaal Museum. Pretoria. o r p e R 130 DUNE ANT BEHAVIOUR & ECOPHYSIOLOGY

events trigger nuptial flights. Rainfall in the Namib is unpredict­ back to nests, 22 % were bird and lizard faeces (Curtis, able and may not occur for extended periods, particularly in 1985c). These may have been used as building material, but the west (along the coast). What, then, happens to the alates unlike gravel and detritus, which were used for external as well which are present in the nest if there is no nuptial flight? Alates as internal nest construction, the faeces were only found inside may often be seen just outside the nest entrance, especially the nests, often in piles at the end of tunnels. Workers were after a light rain. On one occasion a worker was seen laying a also seen breaking sand hardened by oryx urine into small pheromone trail, followed by several alate females. On a few pieces and carrying them to the nest (Curtis, 1983). Again, other occasions alate males and females were observed with these may simply be used for nest construction. their heads in Trianthema hereroensis flowers, collecting pol­ Rat and bat droppings have been found in the nests of len. Dealate females were also seen collecting honeydew with Cataglyphis halophila and were thought by Bernard (1960) to the workers. Although deal ate females acting as workers have be utilized as a food source. The rumen of cattle contain been recorded for other advanced genera such as bacteria which can synthesize amino acids from urea Formicoxenus (see Buschinger, 1987), this behaviour is un­ (Schmidt-Nielsen, 1975) and it is known that various Campo­ usual among ants with a definite queen caste, as is foraging notus species possess intracellular bacteria in their guts byalates. (Hecht, 1924; Ulienstern, 1932; Kolb, 1959). Thus the possi­ Opportunism and flexibility are characteristics of many of the bility exists that C. detritus also has amino acid-synthesizing larger, longer-lived species inhabiting areas where environ­ bacteria. mental conditions are variable and unpredictable and produc­ Availability of honeydew appears to be the critical factor that tivity is low (Louw and Seely, 1982). An ant colony, regarded limits the number and distribution of ant colonies. This was as a whole, is large and long-lived. Most ant species are shown when distribution and abundance of workers was cor­ omnivorous opportunists, combining predation and scaveng­ related with various biotic and abiotic factors across an envi­ ing with collection of plant foods (Carroll and Janzen, 1973; ronmental gradient (Curtis and Seely, 1987). As with other Stradling, 1978). This is largely true of C. detritus. Although the Camponotusspecies (Wilson, 1971; H611dobler and Lumsden, majority of its diet at all times of the year consists of the 1980), workers are aggressive and highly territorial, maintain­ honeydew excretions of scale insects and aphids, it is oppor­ ing discrete foraging territories. In this way the number of tunistic in its honeydew collection, tending any homopteran. colonies which can become established is limited. The depen­ The succulent, Trianthema hereroensis, is infested with the dence of ant colonies on their honeydew food source has been mealy bug, Eriococcus sp., which is always tended by numer­ demonstrated at a site in the dunes near Gobabeb. In October ous C. detritus workers. Aclerda namibensis is another impor­ 1980 all nests and plants hosting scale insects were mapped

. tant species for the ants and infests grass of the species in an area of 9,4 ha. At that time there was a total of 14 nests )

9 Stipagrostis sabulicola, S. lutescens, S. cf. namaquensis and and 30 scale-hosting plants. Gradually the plants began to die 0

0 Cladoraphis spinosa. A third species, Membranaria sp., oc­ off and along with them the scale insects and nests, until, in 2 curs quite frequently on S. ct. namaquensis and less frequently January 1984, there were only nine plants and 4 nests. By the d e t on S. lutescens. This was not utilized very often by C. detritus. end of 1987 not a single scale insect or nest remained (Curtis, a

d Unidentified aphids on Acanthosicyos horridus and leaf hop­ personal observation). (

r pers on Acacia spp. were tended by ants living nearby but e

h were not widespread. ECOPHYSIOLOGY s i l C. detritus is also opportunistic in its scavenging behaviour, b

u taking dead and defenceless animals and consuming pollen It has been suggested that ants are not physiologically well­ P and nectar when this is available. It rapidly exploits new and adapted to arid environments, but that they use behavioural e h transient food sources. For example, when an experiment was means of escaping the extreme climatic conditions (Delye, t

y being carried out in the dunes on the decomposition of an oryx 1968; Schumacher and Whitford, 1974; Kay, 1978). Labora­ b carcass by maggots, C. detritusworkers collected the maggots tory studies of preferred temperatures show that, compared d e t as they moved across the sand and transported them back to with ant species from mesic areas, and with various species n

a the nest (Curtis, 1985c). from the Sahara and New Mexico, C. detritus has a rather high r g The suggestion that C. detritus is a detritivore (Holm and preferred temperature of 31°C (Curtis, 19850'). This is only e c Scholtz, 1980) is not supported by evidence from stomach exceeded by various Cataglyphis species from the Sahara n e content analyses (Curtis, 1985c). The workers use detritus (Delye, 1968) and possibly Ocymyrmex spp. (Marsh. 1988). c i l extensively for nest construction, however (Curtis, 1985b), and The laboratory-determined critical thermal limits for C. detritus r e may often be seen carrying it back to the nest. are CTmin of 4,6 °C and CTmax of 53,8 °C (Curtis, 1985d), d

n Unlike various other Camponotus species (Prins, 1978; which give it a very wide range of temperatures over which u Wilson, 1971), C. detritus maintains no food stores in the nest, activity can take place. The CTmax is comparable with that of y a nor does it have a replete caste. Only 10 % of 200 workers various Pogonomyrmexspecies from North America (Whitford w e dissected contained fat deposits (Curtis, 1985c). This sug­ and Ettershank, 1975), but higher than that for other desert t a gests that all food returned to the nest is consumed immedi­ species, both in North America and the Namib (Schumacher G

t ately and that availability of food for most of the year, at least, and Whitford, 1974; Kay and Whitford, 1978; Marsh, 1988). e

n is sufficient to meet the colonies' needs. Thus, compared with other ants, C. detritus is rather well i b A puzzling phenomenon is the ants' attraction to nitrogenous adapted physiologically to withstand the high temperatures of a S

compounds such as bird and lizard faeces and mammalian the desert. It cannot tolerate the extremely high temperatures y

b urine. Of the 346 identifiable objects which were transported tolerated by other desert arthropods (Cloudsley-Thompson, d e c u d o r p e R DUNE ANT BEHAVIOUR & ECOPHYSIOLOGY 131

1975; Hamilton, 1975), however. (Robinson, 1976) as well as on the abundance and distribution Although high by comparison with other desert arthropods, of various arthropod species (personal observation). When the C. detritus has a low rate of water loss for an ant (Curtis, 1983). density of C. detritus nests and the abundance of individual In addition, it is able to tolerate high losses of tissue water, up ants were examined at six different sites across the dune-field, to 60 % of initial body weight. This, combined with its predom­ it was found that although there was great variability in the inantly liquid diet, allows it to be present on the vegetation number of workers per nest (standard errors of up to 2700 throughout the most desiccating periods of the day. around a mean of 5076), the largest nests occurred in the Gobabeb area (56 km inland) (Curtis and Seely, 1987). This BEHAVIOUR corresponded with the size of the mounds which accumUlate around the plants. However, the greatest nest density and the Camponotus detritus can be regarded as a diurnal ant. Al­ overall abundance of workers per hectare was at Noctivaga, though workers may be present on scale-infested plants midway across the dune-field. This corresponded with the throughout the night, there is no transit activity (movement apparent abundance of scale insects. between nests or between the nest and the foraging grounds) Ant density was not significantly correlated with any environ­ at night (Curtis, 1985e). Briese and Macauley (1980) divided mental factor, but did show a similar trend to mean annual the determinants of activity into those that are stimulatory-in­ temperature and mean temperature amplitude. Nearthecoast, hibitory and those that are regulatory. Light can be regarded nests were small and had little brood. The lower temperatures as a former determinant of transit activity for C. detritus, but may have affected brood production and therefore ant density. has no effect on the collection of honeydew (Curtis, 1985e). Activity and honeydew production may also be affected by Temperature is a regulatory factor, determining the intensity of lower temperatures. transit activity, which is bimodal in summer, with a total cessa­ tion during the hottest part of the day. Workers avoid exces­ CAMPONOTUS DETRITUS AS PART OF THE sively high temperatures by remaining either in the nest or on DUNE ECOSYSTEM plants, where air temperatures seldom exceed about 45 °C.ln summer, transit activity starts with first light and ceases when Being a member of the dune base and plinth communities surface temperatures reach about 56°C. Owing to the steep described by Robinson and Seely (1980), C. detritus occupies thermal gradient above the sand during the heat of the day, a unique position as the major exploiter of honeydew. From and to the relatively long legs of the workers, temperatures to the coast to about midway across the dune-field, C. detritus is which the ants' bodies are exposed are about 10°C less than the only honeydew-collecting species of ant found on the dune

. surface temperatures. Thus their activity range is well within plinth. In the eastern dune-field, where productivity and habit )

9 their physiological limits. Peak activity occurs when air temper­ heterogeneity are greater, and other ant species are present, 0

0 atures at ant level are 25-38 °C. In winter, transit activity only C. detritus shows the same behaviour as at Gobabeb. Its chief 2

d begins when surface temperatures are 10°C and continues potential diurnal competitor, Crematogaster sp., appears to e t throughout the day until just before sunset. avoid competition by predominantly tending the gall-forming a d Since C. detritus is the dominant ant speCies in the southern scale insect, Membranariasp., on Stipagrostis namaquen­

( ct.

r dunes, it is able to utilize the full thermal range available to it, sis, which C. detritus only rarely tended. The nocturnal e h Camponotus maeulatus mystaeeus, s unlike ant speCies of the North American deserts, where species, and C. are i l interspecific competition has led to temporal partitioning of honeydew feeders (Skaife, 1961) but were never seen collect­ b u food resources (Bernstein, 1974; Schumacher and Whitford, ing honeydew. Thus, as the dominant species in the honey­ P

e 1974; Whitford and Ettershank, 1975; Chew, 1977; Davidson, dew-feeding guild, C. detritus appears to be relatively free from h t 1977). Nor detritus the effects of direct interspecific competition.

is the activity of C. restricted by seasonality y in food production, as occurs among seed-harvesting species It is not possible to determine the exact contribution of C. b

d (Briese and Macauley, 1980; Whitford, Depree and Johnson, detritus to the total biomass of the dune fauna since no data e t 1980; Whitford, Depree, Hamilton and Ettershank, 1981). on other animals are available. The study by Seely, De Vos n a Trophallaxis is particularly adaptive in an arid environment and Louw (1977) of the satellite fauna of Trianthema hereroen­ r g where the rate of desiccation is high. One individual can obtain sis showed that these ants contributed about 11 % of the e c liquid from another without having access to the source of biomass while the biomass of the honeydew-producing coc­ n e moisture. This was observed frequently among C. detritus. coids represented 46 'Yo. Seely and Louw (1980) also showed c i l

Furthermore, groups of ants had lower rates of desiccation that biomass varies considerably from dry to wet periods. They r e than solitary individuals (Curtis, 1983). These two factors found that total faunal biomass (dry weight) at Gobabeb in­ d 1 n appeared to increase the survival time of C. detritus workers creased from 100 to 600 g.ha- while that on the dune slopes, u

1 where C. detritus occurs, increased from 80 to 990 g.ha· . y at extreme temperatures and humidities. a Biomass of C. detritus measured at Gobabeb during 1981 was w 1 e

t CAMPONOTUS DETRITUS AND THE ENVIRONMENTAL 96 g.ha· (Curtis, 1983). As 1981 may be regarded as a fairly a GRADIENT dry period, the contribution of C. detritus to the total dune fauna G

t is therefore likely to be fairly high. e n Camponotus detritus does not appear to have many major

i Despite the narrowness of the Namib Desert dune-field (ap­ b proximately 130 km), there is a distinct climatic gradient from predators. It is probably avoided by vertebrates because of the a S the coast inland (Besler, 1972; Curtis and Seely, 1987). This unpalatable formic acid which it secretes. The dune lark, y b

has an effect on the species diversity and abundance of plants Mirafra erythrochlamys, takes the occasional worker d e c u d o r p e R 132 DUNE ANT BEHAVIOUR & ECOPHYSIOLOGY

(Willoughby, 1971; personal observation) and Ludwig's bus­ allaxis. The simple nest construction, using the roots of peren­ tard, Neotis /udwigii, will dig open a nest to get at the brood nial vegetation to provide the structure, enables it to exploit the (personal observation). Both species of dune lizard, Mero/es unstable substrate of the dunes. Multi-nest colonies allow for cuneirostris and Aporosaura anchietae, prey on C. detritus rapid movement of ants and brood to other nests if one of the (Louwand Holm, 1972; Robinson and Cunningham, 1978; nests becomes uninhabitable. Murray and Schramm, 1987; R. Pietruszka, personal commu­ With few competitors and predators to limit its distribution nication). However, none of these predators seems to take the and activity periods, C. detritus has become a very successful ants in large numbers. species, forming a dominant part of the dune fauna and On only one occasion did I find indirect evidence of mam­ utilizing a food resource not utilized by other animals - honey­ malian predation. A nest had been opened and was sur­ dew. Although its opportunistic use of various other food rounded by small canine footprints of either the Cape fox, sources allows it to supplement its diet, the most limiting factor Vu/pes chama, or the bat-eared fox, Otoeyon mega/otis. The to C. detritus is probably the availability of honeydew and predator did not appear to have done much damage to the intraspecific competition for food, as shown at the study site nest. Near the river, baboons tear open nests to eat the brood near Gobabeb. It thus plays an important role in the dune and workers (M. K. Seely, personal communication). ecosystem as secondary consumer. Invertebrate predators in the form of spiders and solifuges There are still many unanswered questions about the biol­ probably account for most of the predation on C. detritus, ogy of C. detritus, which may profitably be pursued in the although this aspect has not been quantified. The ant­ future. This ant is a very suitable species for research for mimicking spider, Cosmophasis sp., which closely resembles various reasons. Nests are easy to excavate. The workers are C. detritus in colouration, was seen on two occasions with a large and conspicuous, which makes behavioural studies pos­ dead ant in its jaws, but was never observed to capture the sible without interfering in the ants' activities. They inhabit a ants (Curtis, 1988). The buckspoor spider, Seothyra sp., is a relatively simple, open environment. sand-dwelling species which constructs a web on the sand More detailed studies on the reproductive biology of this surface. C. detritus workers, and other arthropods, which walk species are needed. The following questions could profitably across the web become entangled in the silk and can be bitten be addressed: What is the rate of egg production and how is by the spider. This has been observed on numerous occasions this influenced by biotic and abiotic factors? What is the rate (J. Henschel, personal communication). Asemesthes sp. is a of development of the brood? How does brood transport from tiny, extremely fast moving spider with sandy colouration. It one nest to another affect the survival of the brood? What has been seen to rush up and run a thread of silk around an factors determine whether a nest has one or more queens and

. ant before moving in to kill it (J. Henschel, personal communi­ how are new queens recruited? What factors trigger nuptial )

9 cation). Eber/anzia flava, a small solifuge, can be seen climb­ flights and what happens to alates if there is no nuptial flight? 0

0 ing up and down grass stalks at night and on two occasions An interesting field for future research would be to consider the 2 they were observed catching and consuming C. detritus possibility of dealate females acting either as a fat storage d e t (J. Henschel, personal communication). depot or as foragers when nuptial flights do not occur. a

d A study of the composition of the honeydew and the amount (

r assimilated by a colony would be beneficial. This could be e

h CONCLUSION linked to a search for any evidence of amino acid-synthesizing s i l bacteria in the guts of C. detritus, or some other reasons for b

u Although ants in general may not be as well-adapted physio­ the transporting of faeces to the nest by the workers. The P logically to arid environments as many other desert arthro­ association between these ants and the honeydew-producing e h pods, for an ant, C. detritus shows a comparatively strong homopterans appears to be very close, but more detailed t

y resistance to high temperatures and desiccation, and studies are needed to establish the exact extent of this asso­ b behaviourally avoids the most stressful periods of the day. It Ciation, and the extent to which colonies are regulated by d e t exhibits social behaviour common to formicines, much of intraspecific competition for food. Further studies on the inter­ n

a which pre-adapts it to life in a harsh environment, for example actions of this species with others would help to elucidate the r g its ability to share food and moisture with nestmates by troph-

position of this species in the dune ecosystem. e c n e c i ACKNOWLEDGEMENTS l

r This research was carried out while I was a post-graduate also like to thank Bob Pietruszka, Johannes Henschel and e d student stationed at Gobabeb. I thank the C.S.I.A. and the Mary Seely of DERU fortheir observations on these ants; Eryn n u

Transvaal Museum for financial and logistiC support; the Direc­ Griffin, John Burke, Mary Seely and Alan Marsh for their y

a torate of Nature Conservation and Recreational Resorts for comments on the manuscript and the Department of National w permission to work in the Namib-Naukluft Park and Prof. G. N. Education (Namibia) for permission to publish. e t

a Louw and Dr M. K. Seely for supervision of the project. I should G t e n

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( I'ethologie des Fourmis du Sahara. Unpublished Ph.D. thesis, effects of rainfall on the ecology and energetics of a Namib Desert

r University of Aix-Marseille, France. Dune ecosystem. Joumal of Arid Environments 3: 25-54. e

h HAMILTON, W. J.III, 1975. Coloration and its thermal consequences SKAIFE, S. H., 1961. The study of ants. Longmans, Cape Town. s i for diurnal desert insects. In: HADLEY, N. F., ed., Environmental STRADLING, D. J., 1978. Food and feeding habits of ants. In: BRIAN, l b physiology of desert organisms, pp. 67-89. Dowden, Hutchinson & M. V., ed., Production ecology of ants and termites. I.B.P. 13: u Ross, Stroudsburg, Pa. 81-106. Cambridge University Press, Cambridge. P HECHT, 0., Embryonalentwicklung und Symbiose bei WHITFORD, W. G., DEPREE, D. J., HAMILTON, P. and e 1924. h Camponotus ligniperda. Zeitschrift fiJr wissenschaftliche Zoologie, ETTERSHANK, G., 1981. Foraging ecology of seed-harvesting t Leipzig 22: 173-204. ants, Pheidolespp., in a Chihuahuan Desert ecosystem. American y b Midland Naturalist HOLLDOBLER, B. and LUMSDEN, C. J., 1980. Territorial strategies 105: 159-167. d in ants. Science 210: 732-739. WHITFORD, W. G., DEPREE, D. J. and JOHNSON, P., 1980. Forag­ e t HOLM, E. and SCHOLTZ, C. H., 1980. Structure and pattern of the ing ecology of two Chihuahuan Desert ant species: Novomessor n Namib Desert dune ecosystem at Gobabeb. Madoqua 3-39. cockerelli and Novomessor a/bisetosus. Insectes Sociaux 27: 148- a 12: r KAY, C. A. R., 1978. Preferred temperatures of desert honey ants 156. g

e (Hymenoptera, Formicidae). Journal of Thermal BiOlogy 3: 213- WHITFORD, W. G. and ETTERSHANK, G., 1975. Factors affecting c 218. foraging activity in Chihuahuan Desert harvester ants. Environmen­ n e KAY, C. A. R. and WHITFORD, W. G., 1978. Critical thermal limits of tal Entomology 4: 689-696. c i

l desert honey ants: possible ecological implications. Physiological WILLOUGHBY, E. J., 1971. Biology of larks (Aves: Alaudidae) in the

r Zoology 51: 206-213. central Namib Desert. Zoology of Africa 6: 133-176. e

d KOLB, G., 1959. Untersuchung Ober die Kernverhaltnisse und WILSON, E. 0., 1971. The insect societies. Belknap Press of Harvard n morphologischen Eigenschaften symbiotischer Mikroorganismen. University Press, Cambridge. Massachusetts. u y a w e t a G t e n i b a S y b d e c u d o r p e R Scale-related Habitat Use by Physadesmia globosa (Coleoptera: Tenebrionidae) in a Riparian Desert Environment

C. s. Crawford\ s. A. Hanrahan2 & M. K. Seely3 1Defrartment of Biology, University of New Mexico, Albuquerque, NM 87131, New Mexico, US.A. Department of Zoology, University of the Witwatersrand, Johannesburg 2050, South Africa 3Desert Ecological Research Unit of Namibia, P.o. Box 1592, Swakopmund, 9000 Namibia

Scale-related use of habitat by the tenebrionid beetle, Physadesmia globosa, was studied beneath tree canopies on the Kuiseb River's floodplain in the Namib Desert. Beetle activity and relative density were estimated using pitfall traps installed in soil-litter subcanopy substrates, which provide the insects with shelter and food. Considerable variability in both large-scale (ca. 0,5 km) use of subcanopy space among different trees, and in intermediate-scale (1-5 m) use of quadrants under the same trees, was recorded between late August and mid-November, suggesting that quality of substrate influences habitat use.

We tested beetle responses to substrate quality at two scales. At the intermediate scale we placed different substrates in circles around traps in field arenas and beneath canopies. No preferences were noted for 'good' or 'poor' litter from selected trees, or for Acacia erioloba leaf and flower litter, Salvadora persica leaf litter, or bare soil. However, in a separate experiment, crops of P. globosa confined for 48 h beneath acacias contained significantly more flowers than leaves, despite the obvious abundance of leaves. Therefore, we placed beetles in small laboratory arenas containing leaf litter on which flowers were positioned 0,5, 2,5, and 5,0 cm apart. Flowers were again chosen significantly over leaves. We conclude that nutritional components of the substrate are meaningful to P. globosa only as they are encountered, while non-nutritional compounds determine habitat use at larger scales. . ) 9 0 0 2 d e t a INTRODUCTION substrate depends on responses to the same resources at d (

different spatial scales. r e Terrestrial detritivores frequently inhabit substrates that pro­ h s

i vide both food and shelter. Therefore, the relative value of each METHODS l b of these resources to a detritivore should influence its spatial u

P use of a given amount of substrate. Two possibilities exist. The Study Area

e detritivore may respond to cues emanating from both re­ Study sites were located in riparian woodland on the north h t sources. Alternatively, space use may be based on information bank of the nonnally dry bed of the Kuiseb River 1 km from y b

from only one resource. Gobabeb, in the central Namib Desert. The Kuiseb at this point d

e How a mobile detritivore responds to food and shelter may separates the extensive dune fields to the south and west from t

n also be scale-related. Morris (1987) pOints to the need to study the gravel plain to the north and east. The woodland is irregular a r habitat preference at different scales. Versions of this ap­ in width, but seldom exceeds 0,5 km. Theron, Van Rooyen and g

e proach have been used for some time to study insects that first Van Rooyen (1980), Giess (1962), and Seely, Buskirk, Ham­ c

n search for a habitat in which certain resources may be present, ilton and Dixon (1980/81) describe its vegetation, which in­ e c cludes the following major woody species: Acacia albida Del., i and then, in a more restricted space, locate a specific re­ l

r source. Behaviour of this sort occurs in some herbivorous A. erioloba E. Meyer, and Salvadora persica L. We intensively e

d species seeking oviposition sites (e.g., Chew, 1977; studied habitats provided by the canopies of eight large acacia n

u Rauscher, 1979), and is common in parasitoids (Vinson, trees (five Acacia erioloba, three A. albida) and three S. persica

y 1976). We are unaware of reports dealing specifically with trees. Other special studies were conducted using other A. a

w scale-associated habitat use by detritivores. In this paper we erioloba trees and are described in the text. Figure 1 is a map e t investigate the use of a mixed soil-litter habitat by the tene­ of the woodland showing locations of all study trees in relation a

G brionid beetle Physadesmia globosa (Haag). This diurnal spe­ to each other and to the river bed and gravel plain. t e cies is endemic to dry riparian woodlands in the Namib Desert, Three of the A. erioloba trees (designated AeN, AeE and n i where it is locally abundant in litter and loose soil beneath AeW after their relative compass-direction locations) were b a canopies of acacia trees. We ask here whether its use of adjacent to smaller trees of S. persica having the correspond- S y b

d Crawford, C. S., Hanrahan, S. A. and Seely, M. K., 1990. Scale-related habitat use by Physadesmia globosa e c (Coleoptera: Tenebrionidae) in a riparian desert environment. In: Seely, M. K., ed., Namib ecology: 25 years u

d of Namib research, pp. 135-142. Transvaal Museum Monograph No.7, Transvaal Museum, Pretoria. o r p e R 136 SCALE-RELATED HABITAT USE BY PHYSADESMIA . ) 9

0 Fig. 1 0

2 Map of trees in the Kuiseb study area. Most large species are either Acacia a/bida or A. eri%ba. Many smaller trees,

d especially at the upper right, are Satvadora persica. The three S. persica associated with A. eri%ba as study pairs are e t shown in black. Tree symbols are indicated in the text. a d ( r e

h ing directional designations: SpN, SpE and SpW. The two the minimum depth recorded by pencil pushing). Cylinder s i l other A. erioloba (FAeE and FAeW) were not associated with contents were weighed, then poured through a Tyler Standard b

u S. persica, and had been enclosed for about five years by a Screen (0,991 mm mesh size). The residue (considered or­ P

fence to prevent access by domestic stock. Likewise, the th ree ganic matter only) was then weighed again. e h A. albida habitats were not studied in association with other An average of five readings of both depth and compressibil­ t

y vegetation. The tree termed AaS grew in the river bed but ity was taken from representative locations at each of the b shaded its bank, whereas AaE and AaW were in the main following sites in each directional quadrant of the two sub­ d e t woodland strip. We monitored habitat temperatures by placing canopy habitats: 1) one-third of the distance from the trunk to n

a maximum-minimum thermometers - one per tree - upright the canopy edge (a Single location), and 2) two-thirds of the r g against the south-facing (shaded) trunk bases of AeE, AeN, distance from the trunk to the canopy edge (two locations). e

c and AeW. Proportions of organic in relation to inorganic matter were n e Depth, compressibility and organic content of the relatively recorded from single measurements at each of these loca­ c i l loose litter and soils were measured, in early September, tions. r e beneath the canopies of AeN and AeE. We chose these trees Numbers of fallen flowers on the surface litter of the same d

n because although they were close to each other, early in the two trees were recorded on 10 September (shortly after peak u study the populations within their subcanopy habitats differed flowering) and 23 November Gust before the next flowering y a markedly in apparent density (see below). Depth of loose period) by photographing at the locations described above w e material was measured by pushing the blunt end of a pencil - units of relatively undisturbed surface confined within a t a into the litter. Relative compressibility (in kg m'2) was mea­ 23 x 25,5 cm metal rectangle. Flowers enclosed in the rectan­ G

t sured with a Farnell pocket penetrometer, fitted with a circular gle were counted later from projected slides. e

n plate of brass, that was pushed into the litter until the plate had i b sunk to a distance equal to its thickness. Proportions of organic Use of the substrate habitat a S

and inorganic materials were determined by inserting a metal Pitfall traps were established periodically beneath the can­ y

b cylinder, 7,2 cm in diameter, to a depth of 6 cm (approximately opies of all regular study trees. Traps were made of hemispher- d e c u d o r p e R SCALE-RELATED HABITAT USE BY PHYSADESMIA 137

ical metal bowls, 17 cm in diameter and 10 cm deep. In used 10 males, then 10 females, then five beetles of either sex. population studies, unless otherwise indicated, four traps, Results are expressed as adjusted G values from contingency each representing one of the four cardinal directions, were tests. each situated halfway between the canopy edge and the trunk Habitat manipulations beneath trees were performed in mid­ of an acacia, and about 0,5 m inside the canopy edge of an October following completion of the arena tests. Here we S. persica. Special uses of pitfall traps for other purposes are tested whether certain obvious litter components might influ­ discussed below. ence the choice of habitat when free-roaming beetles were Traps were used in population studies during the following presented with four substrate options within the subcanopy intervals: 26-29 August, 27-30 September, and 14-17 No­ habitat. We selected three previously unused A. erioloba trees, vember, 1985. During each interval, pitfall captures were re­ each having a subcanopy habitat with a deep layer of mainly corded in daylight within two hours of dawn or dusk. All leaf litter, within which we initially installed three sets of four captured specimens were released at the canopy edge (aca­ pitfall traps, each set arranged in a 1 m2 area. The trap sets cias) or several metres beyond in the case of S. persica. were roughly equidistant from each other and situated about Trapped beetles were colour-marked with a fine brush and halfway between the canopy edge and main tree trunk. For the poster paint, according to a code indicating the particular tree, first three days after trap installation we recorded the number trap and date. Densities of active P. g/obosa were estimated of P. g/obosa found in each trap by late afternoon. Beetles for each of the six main study trees by the Schnabel were then released at least 3 m distant. In the early morning mark-recapture method, a variant of the Lincoln Index, which of the fourth day, we removed litter from within a circle 1 min takes into account observations made on a series of occasions diameter around each trap in every set. Sets were then treated (Overton, 1971). so that, for each one, four new conditions existed following litter We documented the potential for inter-tree movements of P. removal: 1) replacement by newly fallen flowers from another g/obosa by designating trapped beetles as 1) 'residents' when A. eri%ba tree, 2) replacement by leaf litter from beneath an they bore only colour marks assigned to the tree where they S. persica tree, 3) replacement by the original substrate, and were trapped, 2) 'transients' when their marks were from that 4) nothing added so that nearly bare soil remained. Trapping tree and at least one other, and 3) 'immigrants' when they were and recording then proceeded for another three days. either unmarked or only bore marks with colours assigned to other trees. Trapped 'immigrants' were marked, and then Food choice: subcanopy enclosure tests became 'transients' or 'residents' if they were trapped again. We studied food choice in P. g/obosa using two approaches. One involved determining crop contents from beetles allowed

. Substrate choice to feed only at pitfall trap sites beneath particular acacias. This )

9 We tested potential substrate choice using 1) an arena, and was first done in early September, when freshly fallen A. 0

0 2) habitat manipulations beneath trees. The arena tests com­ eri%ba flowers were plentiful in the leaf litter, and later in 2

d pared choice of litter from presumably 'good' and 'poor' sites. mid-November, when those flowers had undergone some e t The former was adjacent to the east trap at AeE, where an physical disintegration. We caged specimens for 24 h with one a d average (± SE) of 12,5 (± 2,4) P. globosa was trapped over the of the following available items: 1) dry, fallen green or brown (

r previous two sampling periods, while the latter was next to the leaves of both Acacia species, 2) freshly fallen A. erioloba e h west trap at AeN, where an average of only 4,6 (± 0,6) beetles flowers and fallen A. a/bida flowers several weeks old, 3) local s i l was trapped simultaneously. The arena consisted of six pre­ green and brown Euclea pseudebenus E. Meyer ex A. D. C. b u constructed sheet metal enclosures, each 1 x 2 m in length leaves, and 4) dry leaves of S. persica. P

e and about 30 cm in height. The enclosures were partly shaded Next, we stocked subcanopy enclosures for 48 h with bee­ h t by overhanging branches. Preparation of the arena consisted tles, seven to an enclosure, obtained the day before from ca.

y of sifting out most of the accumulated litter, then adding about 7 km downriver. Enclosures of about 1,5 m2 apiece were made b

d 2 cm of clean river sand. of corrugated sheet metal strips driven into the ground adja­ e t Single pitfall traps were inserted 30 cm from the ends of each cent to trap sites beneath AeN, AeE, and AaS. In September, n a enclosure. Around each trap - in a circle 30 cm in diameter - we added beetles without regard to sex ratios; in November, r g

we spread experimental litter to a depth of ca. 5 cm. Circles of each enclosure received four males and three females. Bee­ e c 'good' and 'poor' litter were arranged at opposite ends of an tles were removed and frozen in mid-afternoon, 48-54 h after n e enclosure, and alternated at the same end of a row with circles stocking; thus they had ample time to feed. Crop contents from c i l

of the opposite litter type, so that six circles of each type were each beetle were removed by dissection and dispersed in 2 ml r e always at the east or west ends of enclosures. Cardboard cut water in a shallow circular container 30 mm in diameter. Ex­ d n to trap dimensions was used to prevent beetle entry when amination by one of us (S.A.H.) using a dissecting microscope u at x 200 magnification involved estimating proportions of flow­ y traps were not in use, and to shade traps during collection a periods. ers, leaves, insect parts, and mineral matter in three fields w e

t Ten beetles, collected from beneath acacias ca. 7 km down­ chosen at random. Each field was divided into four equal a river, were placed in each enclosure 24 h before traps were quadrants. The proportion of each food type in a quadrant was G

t uncovered for testing. During subsequent tests, which ran for estimated at a percentage of all material visible, and all 12 e n percentages of each food type were averaged for each crop.

i three days each, beetles in traps were counted and removed b to the centre of the arena twice a day. Traps were kept covered a S when P. g/obosa was inactive (late afternoon to mid-morning). Food choice: laboratory tests y b

In sequential choice tests involving each enclosure, we first To examine preferences between acacia flowers and d e c u d o r p e R 138 SCALE-RELATED HABITAT USE BY PHYSADESMIA

Table 1 Comparison of litter and soil characteristics (X ± SE) from beneath canopies of two intensively studied trees. See text for details.

Tree Depth Compressibility Mass/cylinder Organic matter

(cm) (kg. m'2) (g) (g) (%)

AeN 8,1 ± 0,50 0,04 ± 0,01 238 ± 23 38 ± 3,7 18 ± 4,3 NS NS NS P< 0.005 P< 0.05 AeE 8,5 ± 0,77 0,05 0,002 179 ± 20 55 ± 2,3 35 ± 5,7

leaves, we tested, in two experiments, the degree to which were recorded during a 3-month period (20 August - 28 flower selection is independent of flower availability. One November) and compared by F-test for differences in variance. experiment involved consumption of fresh A. erioloba flowers In no case were differences significant at P =0,05. (picked from trees in early December, since few had fallen at Litter and soil characteristics from beneath AeN and AeE are the time), while the other involved 'dry' flowers that had been contrasted in Table 1. Mean values of litter depth, litter com­ in A. erioJoba litter for at least two months. For both tests, we pressibility, soil-litter mass, and organic matter per cent were took leaf litter (with foreign elements removed) from beneath analysed using a simple t-test. Only organic matter per cent AeW. Plastic boxes, 15 x 30 cm in area and 10 cm in height, was significantly different (greater in AeE) between trees (t = served as choice chambers. Leaf litter was sprinkled to a depth 3,873,14 d.f., P< 0,005). of 3 cm in each box, and individual flowers were placed on the Mean subcanopy flower densities (per m2 ± SE), in Septem­ litter surface at the following distances from each other: 0,5 cm ber and November, respectively, were for AeN: 65,1 ± 10,9 (high concentration) 2,5 cm (medium concentration), and 5,0 and 50,8 ± 9,9, and for AeE: 77,2 ± 5,1 and 36,S ± 5,1. Fallen cm (low concentration). Four boxes were prepared for each flowers following 2,5 months of exposure, were noticeably flower concentration. reduced in size. One series of boxes contained no beetles and was used for

. controls. We added beetles collected downriver the same day Use of the substrate habitat )

9 to each box of the other series. One series contained 10 Estimated densities of P. globosa remained relatively stable 0

0 females per box. Beetles were allowed to feed for 48 h, then under all trees combined between late August and late Sep­ 2

d frozen before crop examination, which was done as described tember (Table 2). Summed means (± SE) for those periods e t in the previous section. Meanwhile, proportions of flowers to were 284 ± 38 and 309 ± 92, respectively. During that period a d leaves in the control feeding boxes were determined by first one tree (AeW) tended to have much greater populations, (

r grinding the entire contents of each box in a coffee grinder until whereas another (AaS) had lower populations. Six weeks later e h particle sizes of fragments resembled those found in crops. populations beneath all but AaS and AaW had declined con­ s i l Subsamples were then examined microscopically in the same siderably (summed mean ± SE = 124 ± 45), Mean (± SE) b u way as the crop contents. recapture percentages (Table 3) averaged 30,6 ± 5,2 (August), P 29,0 3,5 (September), and 29,2 4,8 (November), regard­ e ± ± h

t RESULTS less of apparent density fluctuations.

y Relative densities and activity of P. gJobosa beneath each b

d Characteristics of substrate habitat tree for any sampling period are reflected by the total number e t Temperatures taken within 10 cm of the soil-litter surface of captures. Table 3 gives total capture results from each n a r g e c Table 2 n Subcanopy densities (mean and 95 % confidence intervals) beneath study trees of Physadesmia g/obosa as e c

i estimated by the Schnabel method. l r e d n 26-29 August 27-30 September 14-17 November u y a Tree X 95%CI X 95%CI X 95%CI w e t a

G AeN

209 173-246 306 200-422 78 22-145 t

e AeE 307 170-461 206 133-286 70 40-104 n AeW i 410 308-518 756 554-973 60 8-124 b AaN 314 244-387 200 153-250 33 13-56 a

S AaS 144 95-198 229 130-341 320 0-788

y AaW 320 201-451 155 132-220 183 40-356 b d e c u d o r p e R SCALE-RELATED HABITAT USE BY PHYSADESMIA 139

sampling period for each tree. Changes in the tree-specific Table 3 values (Table 3) agree with the November population decline Tree-specific captures (and period-specific recapture percentages' (Table 2). In addition, they reveal a dramatic November decline of previously marked Physadesmia globosa) from main acacias. beneath S. persica, and show that while densities/activity also diminished beneath most of the main acacia trees, they in­ Sampling Periods creased under fenced trees (especially the isolated FAeW). Since only three specimens from beneath the fenced trees had Tree 26-29 August 27-30 September 14-17 November been marked as captures elsewhere, the increase must have been due to a relatively localized, nondispersing pool. AeN Percentages of beetles in the 'resident', 'transient', and 128 (18,9) 136 (29,3) 66 (38,8) AeE 305 (53,0) 170 (24,0) 41 (23,4) 'immigrant' categories for each sampling period (Table 4) show AeW 280 (24,7) 346 (20,6) 20 (31.3) that about one-fifth to nearly one-half of the beetles trapped AaN 278 (35,2) 206 (45,5) 32 (45.5) under acacias were residents, whereas about one-half to over AaS 119 (31.6) 106 (26,4) 19 (23.1) AaW 160 (20.2) 105 (28,4) 55 (13.3) three-fourths were immigrants. (Of the immigrants, 0-34 % FAeE 208 151 236 beneath acacias and 13-68 % beneath S. persica were FAeW 120 168 403 marked.) Transients were sometimes nonexistent and never SpN 42 47 13 exceeded 23 % of the captures. Percentages of beetles in SpE 40 40 9 SpW 105 114 4 each category varied over time. For example, P. globosa exhibited a high degree of 'residence' beneath AeE in August and September, but showed a consistently weaker tendency 'Calculated as the number of individuals marked one or more times to 'reside' beneath the nearby AeN during all three sampling from that sampling period divided by the number of originally marked and released individuals from the same period x 100. Note: values periods. include beetles caught more than once. Patterns of movement and habitat were less obvious for beetles beneath S. persica. Especially large changes in pro­ portions between sampling periods characterized SpE. canopy space characterized by either high or low populations. Net exchanges among the three Acacia-Salvadora pairs In the habitat manipulation tests. postmanipulation sub­ showed that within anyone sampling period generally fewer strate-specific means (± SE) of capture proportions ranged than 10 recaptured beetles travelled from one tree to another. between 0,20 (± 0,02) and 0,29 (± 0.03), as compared with Exceptions included exchanges during September in which 0,24 (± 0,03) and 0,28 (± 0.03) for premanipulation controls. .

) AeW gave up 22 to and gained 27 from SpW, while AeE gave Clearly there were no significant differences among the pro­ 9

0 up 23 to and gained 7 from AeN. portions of substrate-associated captures within and among 0

2 subcanopy habitats. d e t

a Substrate choice d

( In the arena test, males chose AeE litter at a significant level Food choice: subcanopy enclosure tests r

e (G = 11,93, P < 0,05) only once in six trials. To eliminate the Crop dissections from beetles allowed to feed initially on h s possible bias of male following behaviour, we then ran six tree-derived litter components indicated ready ingestion of i l

b choice tests on individual males. The resulting G value was only acacia flowers and leaves (Table 5). u not significant at P= 0,05, hence there is little reason to believe More complete results were obtained when average per­ P

e that P. globosa is able to distinguish between litter from sub- centages of all litter components eaten by P. globosa in habitat h t y b

d Table 4 e t Percentages of 'resident'. 'transient', and 'immigrant' Physadesmia globosa trapped per n study tree during indicated sampling periods. a r g e c

n Marked residents Marked transients Immigrants' e c i l

Tree Aug. Sept. Nov. Aug. Sept. Nov. Aug. Sept. Nov. r e d n u

AeN 19 18 24 0 7 14 81 75 62 y AeE 46 45 12 2 6 22 52 49 66 a AeW 26 21 0 2 0 74 w 25 n 75 e

t AaN 32 46 28 0 1 3 68 53 69 a AaS 29 28 21 0 0 0 71 72 79 G

AaW 28 30 16 0 4 0 72 66 84 t e SpN 3 5 11 0 3 11 97 92 78 n i SpE 0 34 23 2 11 23 98 55 54 b SpW 10 15 25 0 1 0 90 84 75 a S y b 'These may have included unmarked 'residents'. d e c u d o r p e R 140 SCALE-RELATED HABITAT USE BY PHYSADESMIA

Table 5 Seasonal and within-species comparison of Physadesmia g/obosa crop contents from individuals confined to subcanopy enclosures for 48 h.

Tree & n Per cent' of crop contents lx ± SE) Number with period empty crops" Males Females Flowers Leaves Insect parts Mica & sand

AeE a September 16 12 78,7 ± 3,7a 18,7 ± 3,4a 1,8 ± 0,6 1,5 ± 0,9 0 P< 0,05 P<0,05 NS NS a a a November 16 12 96,1 ± 1,1a 1,2 ± 0,6 1,2 ± 0,5 1,3 ± 0,5 0

AeN 8 b a September 13 6 86,1 ± 3,7a 13,3 ± 3,6 0,9 ± 0,5 0,04 ± O,04 3 NS P< 0,05 NS P< 0,05 a a November 16 9 95,6 ± 1,6ab 2,6 ± 0,7 1,2 ± O,7a 1,3 ± 0,5 2

AaN b ab b September 16 0 50,3 ± 7,1 44,3 ± 6,Ob 3,5 ± 2,3 2,1 ± 0,8 9 P< 0.05 b P< 0,05 NS P< 0,05 b bc November 15 11 91,5 ± 1,7 2,3 ± 0,6 0,5 ± O,4a 6,9 ± 2,2 2

AaS September 9 2 73,3 ± 8,1 a 18,7 ± 7,7a 6,7 ± 2.5a 1,1 ± O,7ab 17 P< 0,05 P< 0,05 P< 0,05 P< 0,05 a November 13 9 92,7 ± 2,1 ab 0 1,7 ± 0,8 5.7 ± 2,Oc 4

'Differences in proportions, if significant at P = 0,05, are indicated by different letters (a, b, c) in each column for each season, and by P-value or NS between seasons (Kruskal-Wallis H-test). "Not counted in calculations. All with empty crops were male, except for two females from AaS in November. . )

9 enclosures were compared using the Kruskal-Wallis rank test leaves and eaten at relatively constant rates when available. 0

0 (Table 5). In all but one case (AaN in September), the majority 2

d of ingested food items was flower material, largely anthers. DISCUSSION e t Leaf litter, insect parts, mica and sand, and fungal spores a d accounted for the obvious remaining contents. Food items In the soil-litter substrate inhabited by adults of Physadesmia (

r ingested did not differ significantly between male and female globosa, the distinction between food and shelter is not readily e h beetles. In November, the intake of flowers over leaves and apparent, because the accumulation of dead leaves and flow­ s i l other food items was significantly more pronounced than it was ers is used for both. To learn whether the beetle's choice of b u in September, when, paradoxically, flowers were more abun­ location in this medium is directed mainly toward one or the P

e dant in the litter. Moreover, in November there were very few other (or both) of the two resources, we used a combination of h t empty crops, whereas in September an appreciable number methods involving observation and manipulation. It soon be­

y of beetles housed beneath AaN and AaS had not fed before came clear that subcanopy habitats in the Kuiseb riparian b

d their removal. Considerably more mineral matter was ingested community have markedly variable thermal characteristics e t in those two habitats in November that in September and states of litter quality. Thus, while the regional climate is n a (Table 5). relatively aseasonal (Pietruszka and Seely, 1985), diel habitat r g

temperatures exhibit wide ranges, and interspecific litter accu­ e c Food choice: laboratory tests mulations as well as intraspecific substrate organic masses n e Results of the laboratory food choice trials are given in Table c vary appreciably beneath acacia trees. i l

6. A one-way ANOVA with multiple comparisons, followed by Against this backdrop, we determined that sUbcanopy-spe­ r e Dunnett's test comparing fragmented control litter with crop cific relative densities and activity of P. globosa may be rela­ d n contents, indicated significant differences (P < 0,05) in mean tively stable for at least a month, and that, regardless of density u

y proportions at each flower concentration offered. With one level, nearly 50 % of a subcanopy population may exhibit a exception in 18 trials (females eating dry flowers at the low 'habitat fidelity' during that period. Moreover, while densities w e

t concentration) flowers constituted> 85 % of the diet. How­ and activity beneath some trees sometimes decline signifi­ a ever, a Kruskal-Wallis test indicated no significant differences cantly, those beneath other trees may simultaneously in­ G

t among means of flower consup1ption by experimental groups. crease. A picture thus emerges of an extensive local popula­ e n U-test i A Mann-Whitney further revealed no significant differ­ tion of P. globosathat roams intermittently among subcanopy b ence in proportions of fresh or dry flowers at any level of flower habitats, and that does not necessarily follow potential re­ a S concentration. As a whole, these tests show that flowers, source gradients. y b

irrespective offreshness and concentration, are preferred over On the other hand, P. globosa movements within a single d e c u d o r p e R SCALE-RELATED HABITAT USE BY PHYSADESMIA 141

Table 6 Results of food choice trials by Physadesmia g/obosa offered fresh or dry Acacia erioloba flowers at different proportions, and leaves.

Relative proportions Per cent Per cent flowers in crop contents SEt & conditions flowers in of flowers controls n Males n Females n Males & females n

High Fresh 58,3 ± 2,5 (10) 96,8 ± 2,5 (6) 96,2 ± 1,8 (8) 94,6 2,1 (8) Dry 50,4 ± 3,8 (10) 89,3 ± 5,5 (8) 100,0 ± 0,0 (2) 100 (1 )

Medium Fresh 42,2 ± 2,1 (10) 91,4 ± 5,6 (8) 90,6 ± 6,8 (8) 89,0 ± 11,0 (5) Dry 26,0 ± 3,4 (10) 99,0 ± 0,2 (6) 89,6 ± 4,5 (4) 99,8 ± 0,2 (5)

Low Fresh 11,8 ± 1,2 (10) 96,2 ± 2,4 (8) 87,6 ± 6,5 (7) 96,0 2,4 (4) Dry 12,8 ± 1,4 (10) 86,7 ± 6,3 (7) 56,3 ± 22,7 (5) 97,2 ± 2,1 (4)

*Beetles with empty crops were not included in calculations, which showed significant differences between controls and crop contents (ANOVA, P < 0,05) in all but one case. See text for details.

subcanopy area can also be relatively consistent for up to as litter when it serves only as shelter). In effect, the two 3 months. Thus, some pitfall traps under certain study trees resources categories are decoupled, because food is detected regularly caught significantly more beet/es than did other traps and selected at a much finer level of resolution than is sub­ under the same trees. Litter quality was probably not involved, strate not being used as food. .

) because both the arena tests and the habitat manipulation Such a scale-related dichotomy of choice has to our 9

0 tests gave no indication that either quality or specific food type knowledge, not been demonstrated for other terrestrial macro­ 0

2 determines the temporary location of a beetle beneath a tree. detritivores, although patterns of species-specific feeding

d Rather, some combination of factors, independent of the nu­ (Calkins and Kirk, 1973; Rogers, Woodley, Sheldon and e t

a tritional quality of the litter, seems to affect beetle location. Uresk, 1978) and habitat use (Doyen and Tschinkel, 1974; d

( Temperature, and the hygroscopic capacity of litter Holm and Scholtz, 1980; Thomas, 1983; Sheldon and Rogers, r

e (Tschinkel, 1972) as a function of its organic content (Table 1), 1984; Faragalla and Adam, 1985) have been documented for h s are candidates for further study. tenebrion ids. i l

b Food quality, however, is important to P. g/obosa at a much The scale-related behaviour of P. g/obosa may be explained u finer spatial scale. Our subcanopy enclosure tests show that partly in the context of its gut-symbiont relationship. Crawford P

e despite low flower: leaf ratios in the soil-litter substrate, bee­ (1988) has argued that an avoidance of thermal stress, com­ h t

tles conSistently ate more flowers than leaves. These findings bined with selection of thermally optimal habitat conditions, y b were reinforced by results of laboratory choice tests, in which should maximize gut symbiont activity and, therefore, the

d flowers, regardless of their concentrations, were again se­ detritivore's nutritional state. If this view is correct, then as long e t

n lected over more abundant leaves. Since the earlier field as high-value food, such as flowers, is available at levels a

r manipulation tests, in which flowers were placed around one exceeding some critical threshold, non-nutritional factors, such g as micrOClimate, should be largely responsible for where a e fourth of the pitfall traps, had no effect on trap capture, we c

n conclude that choice of a nutritional resource (in this case beetle spends most of its time in a subcanopy habitat. Whether e c flowers over leaves) operates at a spatial scale quite removed this explanation can explain dispersion patterns of other mo­ i l from that involving choice of a non-nutritional resource (such bile detritivores remains to be examined. r e d n u y a ACKNOWLEDGEMENTS w e We thank the following colleagues for critical reviews of Pietruszka, and manuscript typing by Linda Malan, Ruth t a drafts of this manuscript: Richard Bradley, James Brown, Mecklenburg, Diane Thomas and Irene Farmer. We gratefully G

t Manuel Molles, Robert Pietruszka, Ken Schoenly, Eric Tool­ acknowledge the Foundation for Research Development of e

n son and Marlene Zuk. Consultation with Robert Pietruszka the C.S.I.R. for research funds and a visiting fellowship i b during and after completion of the field study was especially (C.S.C.), and the Directorate of Nature Conservation and a S

valuable. We appreciate the field assistance of Deboran Kelso Recreation Resorts, Namibia, for facilities and permission to y

b and Janet Rasmussen, the figure illustration of Carol work in the Namib-Naukluft Park. d e c u d o r p e R 142 SCALE-RELATED HABITAT USE BY PHYSADESMIA

REFERENCES

CALKINS, C. O. and KIRK, V. M., 1973. Food preference of a false PIETRUSZKA, R. D. and SEELY, M. K., 1985. Predictability of two wireworm Eleodes suturalis. Environmental Entomology 2: 105- moisture sources in the Namib Desert. South African Journal of 108. Science 81: 682-685. CHEW, F. S., 1977. Coevolution of pierid butterflies and their crucif­ RAUSCHER, M. D., 1979. Larval habitat suitability and oviposition erous food plants. II. The distribution of eggs on potential foodplants. preference in three related butterflies. Ecology 60: 503--511. Evolution 31: 566-579. ROGERS, L. E., WOODLEY, N., SHELDON, J. K. and URESK, V. A., CRAWFORD, C. S., 1988. Nutrition and habitat selection in desert 1978. Darkling beetle populations of the Hanford Site in southcent­ detritivores. Journal of Arid Environments 14: 111-121. ral Washington. PNL-2465, Pacific Northwest Laboratory, Richland, DOYEN, J. T. and TSCHINKEL, W. F., 1974. Population size, micro­ Washington. geographic distribution and habitat separation in some tenebrionid SEELY, M. K., BUSKIRK, W. H., HAMILTON, W. J. III and DIXON, J. beetles (Coleoptera). Annals of the Entomological Society of E. W., 1980/81. Lower Kuiseb River perennial vegetation survey. America 67: 617-626. South West Africa Scientific Society 34135: 57-86. FARAGALLA, A. A. and ADAM, E. E., 1985. Pitfall trapping of tene­ SHELDON, J. K. and ROGERS, L. E., 1984. Seasonal and habitat brionid and carabid beetles (Coleoptera) in different habitats of the distribution of tenebrionid beetles in shrub-steppe communities of Central Region of Saudi Arabia. Zeitschrift fOr Angewandte En­ the Hanford Site in eastern Washington. EnvironmentalEcology13: tom%gie 99: 466-471. 214-220. GIESS, W., 1962. Some notes on the vegetation of the Namib Desert THERON, G. K., VAN ROOYEN N. and VAN ROOYEN, M. W., 1980. with a list of plants collected in the area visited by the Carp-Trans­ Vegetation of the lower Kuiseb River. Madoqua 11: 327-345. vaal Museum Expedition during May 1959. Cimbebasia2: 1-35. THOMAS, D. B. Jr., 1983. Tenebrionid beetle diversity and habitat HOLM, E. and SCHOLTZ, C. H., 1980. Structure and pattern of the complexity in the eastern Mojave Desert. Coleopterist's Bulletin 37: Namib Desert dune ecosystem at Gobabeb. Madoqua 12: 3--39. 135-147. MORRIS, D. W., 1987. Ecological scale and habitat use. Ecology62: TSCHINKEL, W. R., 1972. The sorption of water vapour by windborne 363-369. plant debris in the Namib Desert. Madoqua 2: 63-68. OVERTON, W. S., 1971. Estimating the number of animals in wildlife VINSON, S. B., 1976. Host selection by parasitoid insects. Annual populations. In: GILES, R. H., ed., Management techniques, pp. Review of Entomology 21: 109-133. 403--453. Wildlife Society, Washington, D.C. . ) 9 0 0 2 d e t a d ( r e h s i l b u P e h t y b d e t n a r g e c n e c i l r e d n u y a w e t a G t e n i b a S y b d e c u d o r p e R Food and Habitat Use by Three Tenebrionid Beetles (Coleoptera) in a Riparian Desert Environment

S. A. Hanrahan' & M. K. Seely2 1Department of Zoology, University of the Witwatersrand, Johannesburg, 2050 South Africa 2Desert Ecological Research Unit of Namibia, P. 0. Box 1592, Swakopmund, 9000 Namibia

Use offood and habitat by three species of diurnal adesmine tenebrionid beetles was assessed by crop analyses and pit trapping. Physadesmia globosa (Haag), Onymacris rugatipennis rugatipennis (Haag) and Stenocara gracilipes Solier were chosen for this study as they are the three most common conspicuous macrodetritivores living in the riparian woodland of the usually dry Kuiseb River in the central Namib Desert. The types of food consumed overlapped extensively among species and months of the year, whereas the overlap of habitat use among species and months was less.

INTRODUCTION 1977; Penrith, 1979; Wharton and Seely, 1982). This paper investigates the diet of the three most common Desert environments generally have abundant detritus with tenebrionid species in the Kuiseb riverine environment by: 1) relatively slow or variable rates of decomposition (Crawford examining crop contents to directly determine what food was and Gosz, 1982). As decomposition by bacteria and fungi is eaten, a method not often used by workers in this field; 2) using limited by the pervading aridity, macrodetritivores are relatively pit-fall traps to establish any possible relationships between important to nutrient and energy cycling (Crawford and Taylor, selected food and habitat in this environment. 1984). Endogenous enzymes (Marcuzzi and Lafisca, 1977) .

) and symbiotic gut fauna (Crawford and Taylor, 1984) enable MATERIALS AND METHODS 9

0 macrodetritivores to use detritus effectively. 0 2

One group of purported macrodetritivores, tenebrionid bee­ The study area was located in the Kuiseb River course near

d 0 0

e tles, are a numerically important component of the biota of the the Namib Research Institute (23 34' S, 15 03' E). t a Namib Desert (Koch, 1962). Within the riparian woodland of C/adoraphisspinosa (Uil.) S. M. Phillips is the dominant grass d (

the Kuiseb River, three species of adesmine tenebrionid bee­ of the river-bed. The perennial woody vegetation of the flood r e tles are among the most common and conspicuous compo­ plain is dominated by Acacia a/bida Del. and A. eri%ba E. h s

i nents of the diurnal epigeaic fauna (Wharton and Seely, 1982): Meyer (Seely, Buskirk, Hamilton and Dixon, 1981). Because l b Stenocara gracilipes Solier, Physadesmia g/obosa (Haag) of the lack of pronounced seasonal change in climate and u

P (Penrith, 1979) and Onymacris rugatipennis rugatipennis water availability, trees and grasses growing in the river course

e (Haag) (Penrith, 1975). produce leaves, flowers and fruit throughout most of the year, h t Distinctive activity patterns with temporal and spatial differ­ contributing substantially to surface litter. This plant material, y b

ences have been documented for the more common species together with other organiC debris, constitutes the detritus that d

e of tenebrionid beetles living in the lower Kuiseb River and accumulates in the environment. t

n adjacent gravel plains (Wharton and Seely, 1982). Despite the To examine food consumed by the three tenebrionid a r apparent importance of food in determining the density and the species, ten individuals of each species were collected by g

e habitats used by these species, little work has been directed hand in the field in September and December 1983 and March c

n toward examining the types of food consumed. Holm and and June 1984. Beetles were immediately placed on ice in the e c

i Scholtz (1980) stated that 'virtually all desert animals are field and crops were removed within three hours. Crops were l

r euriphagous oppol'1;unists' whereas Wharton and Seely (1982) fixed in Pam pie's fluid and stored in 70 % alcohol. e

d described desert tenebrionids as 'opportunistic omnivores' Reference slides were made by dissecting fresh and dry n

u and stated that 'most of the adesmines ... were observed to plant material from species likely to be present in detritus.

y feed on animal remains and faeces while foraging for plant Samples were processed in a Waring blender and mounted in a

w material'. A number of authors have observed the three Hertwig and Hoyer's solution (Bear and Hansen, 1966). e t species under consideration, as well as other tenebrionids, To analyse the food consumed, crops were severed at the a

G foraging near or under particular riverine plant species and base of the oesophagus and at the cardiac valve. Total crop t e have assumed that the beetles were feeding on locally derived contents were washed into 5 ml of water and spread evenly in n i detritus (Holm and Edney, 1973; Penrith, 1975; Hamilton, a Petri dish (53 mm diameter) containing a grid of 2 x 2 mm b a Buskirk and Buskirk, 1976; Hamilton and Penrith, 1977; Roor, units. Presence or absence of animal material, plant material, S y b

d Hanrahan, S. A. and Seely, M. K., 1990. Food and habitat use by three tenebrionid beetles (Coleoptera) in a e c riparian desert environment. In: Seely, M. K., ed., Namib ecology: 25 years of Namib research, pp. 143-147. u

d Transvaal Museum Monograph No.7, Transvaal Museum, Pretoria. o r p e R 144 FOOD & HABITAT USE BY TENEBRIONID8

As Ae CIS o INSECT ~ FLOWERS ~ FLOWER IllD UNKNOWN III!& LEAVES II SAND I!I!I

Fig. 1 Percentage of flower parts, leaves, sand and unidentifiable material available in detritus under Acacia a/bida (Aa), Acacia erioloba (Ae) and C/adoraphis spinosa (Cs).

sand or unidentifiable fragments were recorded for 100 . lLAR ) SEP DEC IUN

9 2 x 2 mm units across 5 rows of the grid. 0

0 To determine relative availability of the four major crop

2 Fig. 2 components in detritus found under three major plant species, d

e Percentage of crop contents from Physadesmia globosa (Pg),

t three samples of surface detritus (0-5 mm) from beneath each

a Onymacris rugafipennis (Or) and Stenocara gracilipes (8g) which plant species were spread in a thin layer in a Petri dish and the d consisted of insect remains, flower parts, unidentifiable material and ( percentage occurrence in four quadrats was estimated (Craw­ r sand. Number of crops examined from each species during each e ford, Hanrahan and Seely, this volume). month is indicated above the bars. h s i

l To examine habitat use by the three tenebrionid species, b twenty pit traps (150 mm diameter, 240 mm deep) were set u P

out: five placed in a region dominated by A. albida, ten placed e

h in areas dominated by A. erioloba (5 north and 5 south of the and unknown material were estimated from samples taken t

y river course); five placed near clumps of C. spinosa where beneath Acacia afbida. Acacia eriofoba and Cfadoraphis b Nicotiana glauca R. Graham also occurred. Trap distribution spinosa (Fig. 1). Small animals and animal fragments, mainly d e

t was in proportion to the occurrence of vegetation types. Traps insect material, represented such a small proportion of the n were emptied at two or three day intervals, the identity and surface detritus that they were not represented in Fig. 1, a r

g numbers of beetles noted, and the beetles released. although closer inspection revealed small insects associated e

c Statistical analyses followed procedures outlined in Sokal with Acacia flowers. n

e and Rohlf (1981) and Siegel (1956). A symmetrical overlap Plant material identified in beetle crops consisted of flowers c i l index (Pianka, 1973), which ranges from 0 (no overlap) to 1 of A. afbida, A. erioloba and C. spinosa, and leaves of Acacia r

e (complete overlap), was used to evaluate overlap of food and species. Anthers and stamens of the flowers were most com­ d habitat use within and between species. monly eaten and leaf material was rarely consumed. The n u

animal material in beetle crops consisted almost exclusively of y a RESULTS insect fragments, mainly unrecognizable except as parts of w

e insect cuticle. Ant and dipteran and lepidopteran larvae t a Detritus samples taken from beneath individual plants were remains were occaSionally identifiable to order. The few whole G found to be derived predominantly from plants in that immedi­ insects consumed were probably inadvertently eaten because t e ate vicinity and did not consist of a random admixture of of their association with flower matter. Sand was also found in n i

b detritus from all plant species growing in the riverine habitat. the crops, but it is not known if it was consumed inadvertently a

S All plant material in the detritus had wind-blown dust and sand or is involved in digestion. All three tenebrionid species had

y adhering to its surface. Proportions of flowers, leaves, sand mixed diets containing all food items listed. In three of the four b d e c u d o r p e R FOOD & HABITAT USE BY TENEBRIONIDS 145

• P.a1obo•• mO.raa.UpeDo.l. ~ S.,r.cUlpe.

IUL A.UG SIP OCT HOV DBC IAK PBB MAR A.PR MAY IUK 1983 1884-

Fig. 3 Monthly percentages of annual totals of Physadesmia globosa, Onymacris rugatipennis and Stenocara gracilipes captured in 20 pit-traps during 1983 and 1984. o C.spinosa ~ A.albida months in which we examined diet, there was an overwhelm­ ~ A..erlo1oba N' ing preference for flower material (Fig. 2). Using a symmetrical overlap index (Pianka, 1973) we found III A.erioloba S a high degree of overlap between the species of food types PI Or Sa P, Or Sa . consumed in all months (Table 1). We also found high overlap )

9 values for a single species among seasons (Table 2). The high 0

0 overlap values for crop contents among species and months 2

d confirm that the beetles feed upon similar types of food most e t of the time, agreeing with the casual observations of Robinson a d and Seely (1980). Moreover, all four items found in the beetle (

r crops also occurred in detritus under A. a/bida, A. eri%ba and e h C. spinosa (Fig. 1), although the proportions differed from s i l those found in the beetle crops. b u Individuals of the three tenebrionid species were captured P

e in every pit trap during every month of the year (Fig. 3). Total h t numbers captured were 18 228 P. g/obosa, 2 346 O. rugati­

y pennis, and 555 S. gracilipes. For all three species the distri­ b

d bution throughout the year was non-uniform (P. g/obosa ::: e t 930,29; O. rugatipennis::: 713,91; S. gracilipes = 120,73; chi­ n a square, all species df::: 11, P < 0,001) although the Kendall r g

coefficient of rank correlation indicated that values for the e c species varied independently (P. g/obosa vs 0. rugatipennis ::: n e 0,424, P 0,055; P. globosa vs S. gracilipes 0,182,

c = i l

P = 0,412; 0. rugatipennis vs S. gracilipes::: 0,242, P == r e O,271). A degree of seasonality was suggested for O. d n rugatipennis, which was trapped most frequently in the winter u months of June and ~Iuly. The percentage trapped in those two y a months (36 %) is more than twice that which would be ex­ SEP DEC MAR IUN' w e pected from uniform monthly captures. In contrast, numbers t a of S. graci/ipes declined throughout most of the year; this trend Fig. 4 G

t reversed the following year. No pattern of monthly capture was Percentage of individuals of Physadesmia globosa, Onymacris e n apparent for P. globosa. rugatipennis and Stenocara gracilipes trapped in habitats dominated i b For comparison with the food analyses, the percentage by Cladoraphis spinosa, Acacia albida and A. erloloba (north and a south of the river-bed) during four months of the year. Total number S occurrence of the beetles within each habitat type has been of individuals in a sample is indicated above each bar. y b depicted for September, December, March and June (Fig. 4) d e c u d o r p e R 146 FOOD & HABITAT USE BY TENEBRIONIDS

Table 1 Table 2 Mean monthly overlap values (± SE) between species for food and Mean monthly overlap values (± SE) within a species for food and habitat during four months at different seasons habitat during four months at different seasons.

Food Habitat Food Habitat

Physadesmia globosa x Physadesmia globosa 0,91 ±0,02 0.87±O,03 Onymaeris rugatipennis 0,92± 0,04 0,65±O,04 Onymacris rugatipennis O,85±0,05 0.86±0,04 Physadesmia globosa x Stenocara gracilipes 0,90 ± 0.04 0,48± 0,07 Stenocara graei/ipes 0,72 ±0,10 0.68 ± 0.04

Onymaeris rugatipennis x Stenoeara graeilipes 0,90± 0,06 0,37 ± 0,04

deserts and grasslands. Based on field observations, dissec­ tion of digestive tracts and literature citations, Rust (1986) and symmetrical overlap indices calculated. When habitat use found four detritivores and one herbivore among the ten­ between species was compared for the four months, mean ebrionids of a sand dune insect community in the Great Basin. overlap values (Table 1) indicated greater variation between Staminate conifer cones represented an average of 51 % of species in habitat use than in food type consumed. When the diet of six beetle species in Arizona, as determined by habitats occupied by a single species were compared on a feeding observations (Doyen and Tschinkel, 1974). The re­ monthly basis, the mean overlap values (Table 2) indicated mainder of the diet consisted of living or dry vegetation. Fifty­ that P. globosa and O. rugatipennis were trapped fairly con­ two plant species were ingested as living, dead or litter material sistently in the same sites; S. gracilipes showed slightly greater by 13 beetle species in a community in western Washington, variability in its use of habitat. although flower material was not particularly mentioned Inspection of Figs 1 and 4 shows thatthere may be a positive (Rogers, Woodley, Sheldon and Uresk, 1978). In the latter relationship between relative amount of time spent near C. study, arthropod remains ranged from < 1 % to 19 % of the

. spinosa, as indicated by pit-fail trap captures, and amount of contents of the digestive tract of the various species. In the )

9 insect material consumed. The Spearman's rank correlation latter two studies, there was a high degree of overlap in diets 0

0 coefficient was, however, not significant, falling between among species examined, similar to that found in the Kuiseb 2 0,10> P > 0,05. detritivores. d e t Our results from the Kuiseb agree with previous descriptions a

d DISCUSSION of food and habitat use derived from a number of shorter term (

r studies in the same area (Hamilton and Penrith, 1977; Penrith, e

h Analyses of crop contents of the three Kuiseb species show 1979; Wharton and Seely, 1982). Although previous authors s i l that these tenebrionid beetles are definitely detritivores, as has did not always differentiate between habitat and food use. they b

u been suggested previously by a number of authors. They are generally provided anecdotal observations of selected forag­ P not, however, indiscriminate omnivores as they consistently ing areas (e.g., Penrith, 1975; Hamilton et al., 1976; Hamilton e h selected specific food types from detritus irrespective of spe­ and Penrith, 1977; Roer, 1977; Wharton and Seely, 1982). Our t

y cies of plant under which the detritus occurred. Leaves are the analyses expand on these observations and provide direct b major food type available in detritus (Fig. 1), whereas flowers evidence as to the types of food eaten. d e t were more commonly eaten. Insects, particularly dry cuticular The above results do not provide any evidence for exclusive n

a fragments, represent less than 1 % of detritus whereas they habitat use by any of the three species of tenebrionid beetles r g formed a conspicuous part of the crop contents (Fig. 2). Those investigated. They also leave unanswered the further question e c materials consumed appeared to be the more nutrient-rich as to whether beetles occur randomly within their environment n e components of the detritus. according to microhabitat or according to differences in avail­ c i l Although the Kuiseb tenebrionids were found to be entirely ability or relative attractiveness of particular food types. Craw­ r e detritivores, characteristics oftheir diet do not differ widely from ford et al. (this volume) provide a detailed evaluation of the d

n those of the more herbivorous tenebrionids of North American question with respect to Physadesmia globosa. u y a w e t a ACKNOWLEDGEMENTS G

t We wish to thank G. L. Prinsloo of the National Collection of the Foundation for Research Development of the C.S.I.R., the e

n Insects, Pretoria, for identification of insect fragments from University of the Witwatersrand and the Transvaal Museum. i b beetle crops. C. S. Crawford, A. C. Marsh and R. D. Pietruszka The Directorate of Nature Conservation and Recreation a S

commented on a draft of this manuscript and R. D. Pietruszka Resorts, Namibia, provided facilities and permission for work y

b provided statistical advice. For financial assistance we thank in the Namib-Naukluft Park. d e c u d o r p e R FOOD & HABITAT USE BY TENEBRIONIDS 147

REFERENCES

BEAR, G. D. and HANSEN, R. M., 1966. Food habits, growth and of some psammophilous tenebrionid beetles from South West reproduction of white-tailed jackrabbits in southern Colorado. Col­ Africa. Madoqua 10: 191-193. orado Agricultural Experiment Station Technical Bulletin 90: 1-59. PENRITH, M.-L., 1975. The species of Onymacris Allard (Coleoptera: CRAWFORD, C. S. and GOSZ, J. R., 1982. Desert ecosystems: their Tenebrionidae). Cimbebasia (A)4: 47-97. resourceS in space and time. Environmental Conservation 9: 181- PENRITH, M.-L., 1979. Revision of the western southern African 195. Adesmiini (Coleoptera: Tenebrionidae). Cimbebasia (A)5: 1-94. CRAWFORD, C. S., HANRAHAN, S. A. and SEELY, M. K., 1990. PIANKA, E. R., 1973. The structure of lizard communities. Annual Scale-related habitat use by Physadesmia globosa (Coleoptera: Review of Ecology and Systematics 4: 53-74. Tenebrionidae) in a riparian desert environment. In: SEELY, M. K., ROBINSON, M. D. and SEELY, M. K., 1980. Physical and biotic ed., Namib ecology: 25 years of Namib research, pp. 135-142. environments of the southern Namib dune ecosystem. Journal of Transvaal Museum Monograph No.7, Transvaal Museum, Pretoria. Arid Environments 3: 183-203. CRAWFORD, C. S. and TAYLOR, E. C., 1984. Decomposition in arid ROER, H., 1977. Areas and adaptation of the Namib Desert beetle environments: role of the detritivore gut. South African Journal of Onymacris r. rugatipennis (Haag 1875) (Col.: Tenebrionidae, Ad­ Science80: 170-176. esmiini) to the Kuiseb river bed in South West Africa. Zoologische DOYEN, J. T. and TSCHINKEL, W. R., 1974. Population size, Jahrbiicher, Abteilung far Systematik 104: 560-576. microgeographic distribution and habitat separation in some ten­ ROGERS, L. E., WOODLEY, N., SHELDON, J. K. and URESK, V. A., ebrionid beetles (Coleoptera). Annals of the Entomological Society 1978. Darkling beetle populations (Tenebrionidae) of the Hanford of America 67: 617-626. Site in south central Washington. PLN-2465, UC-11, Rockwell HAMILTON III, W. J., BUSKIRK, R. E. and BUSKIRK, W. H., 1976. Hanford operations. Social organisation of the Namib Desert tenebrionid beetle RUST, R. W., 1986. Seasonal distribution, trophic structure and origin Onymacris rugatipennis. Canadian Entomologist 108: 305-316. of sand obligate insect communities in the Great Basin. Pan-Pacific HAMILTON, W. J. III and PENRITH, M.-L., 1977. Description of an Entomologist62: 44-52. individual possible hybrid tenebrionid beetle and the habitat prefer­ SEELY, M. K., BUSKIRK, W. H., HAMILTON III, W. J. and DIXON, J. ence of the parental species. Canadian Entomologist 109: 701-710. E. W., 1981. Lower Kuiseb perennial vegetation survey. Journal of HOLM, E. and EDNEY, E. B., 1973. Daily activity of Namib Desert the South West Africa Scientific Society 34135: 57-86. arthropods in relation to climate. Ecology54: 45-56. SIEGEL, S., 1956. Nonparametric statistics for the behavioural sci­ HOLM, E. and SCHOLTZ, C. H., 1980. Structure and pattern of the ences. McGraw-Hili Book Company, New York. Namib Desert dune ecosystem at Gobabeb. Madoqua 12: 5-39. SOKAL, R. R. and ROHLF, F. J., 1981. Biometry, 2nd edn. W. H. KOCH, C., 1962. The Tenebrionidae of southern Africa XXXI. Com­ Freeman and Company, San Francisco. prehensive notes on the tenebrionid fauna of the Namib Desert. WHARTON, R. A. and SEELY, M. K., 1982. Species composition and Annals of the Transvaal Museum 24: 61-106. biological notes on Tenebrionidae of the lower Kuiseb River and MARCUZZI, G. and LAFISCA, M. T., 1977. The digestive enzymes adjacent gravel plains. Madoqua 13: 5-25. . ) 9 0 0 2 d e t a d ( r e h s i l b u P e h t y b d e t n a r g e c n e c i l r e d n u y a w e t a G t e n i b a S y b d e c u d o r p e R The Microenvironment Associated with Welwitschia mirabilis in the Namib Desert

B. A. Marsh Desert Ecological Research Unit of Namibia, P. 0. Box 1592, Swakopmund, 9000 Namibia & Zoology Department, University of Cape Town, Rondebosch, Cape Town, 8000 South Africa

Welwitschia mirabilis (Hooker fiLl creates an island of relatively moderate thermal and moisture regimes below its large leaves. In the environmentally stressful habitat in which the plant occurs, soil temperatures may be 40°C lower below the plant than in nearby unshaded areas, and the plant thereby provides critical shelter to many desert organisms.

INTRODUCTION coast. At this site the insolated soil surface temperatures regularly exceed 70°C and W. mirabilis represents the most Plants modify the environment in which they grow by creating abundant perennial vegetation. A moderate thermal climate shade and shelter and by accumulating wind-blown debris, silt below the plants could represent a critical refuge that enables and sand in addition to the litter they themselves produce. animals to survive in this environment. Although the shade and litter associated with desert plants is usually far less than that of plants in temperate and tropical METHODS forests and grasslands, even a little shade and organic matter, particularly if concentrated around shrubs, may be of signifi­ Study site .

) cance locally in a hot desert environment (Noy-Meir, 1979/80). All work was carried out at 'Welwitschia Wash' (15° 36' E; 9

0 Welwitschia mirabilis (Hooker fil.) is a long-lived gymno­ 23° 37' S), 20 km east of the Namib Research Institute. The 0 2

sperm which occurs in discrete localities (Kers, 1967; Rodin, wash is barren and W. mirabilis plants form the main constit­ d

e 1953) in the Namib Desert. This plant has long straplike leaves uent of the vegetation, the only other perennial vegetation of t a (0,5 to 1,0 m broad and 1,0 to 2,0 m long) which create an importance being Adenolobus pechuelli, Orthanthera albida d (

effective umbrella of shade below the plant. Welwitschia and Sutera maxii, which provide little in the way of canopy r e mirabilisoften occurs in environmentally stressful areas where cover. During the day, owing to direct radiation, re-radiation of h s

i other plant cover is sparse. Despite the possible importance heat from the steep rocky sides of the wash as well as lack of l b of the climate-moderating effect of W. mirabilisto animals, this air movement in the wash, insolated soil surface temperatures u

P aspect has not received any direct attention. Schulze, Eller, may be extremely high, temperatures of 78°C having been

e Thomas, Willert and Brinckmann (1980) have examined the recorded during the study. h t thermal relationships of W. mirabilis and state that these plants y b

modify air and soil temperatures quite considerably. Their Soil Moisture d

e study, involving W. mirabilis in a grassland and a cool coastal Samples for analysis of soil moisture were collected at t

n region of the Namib, recorded that air and soil temperatures 15hOO on three occasions (Table I). Soil samples were taken a r below the leaves were equal to, or slightly lower than, ambient at a depth of 50 mm: g

e air temperature despite high temperatures on the insolated soil c

n surface. Unfortunately, they do not give quantitative data con­ a) On the southern side but close to the stem of W. mirabilis e c

i cerning the insolated soil surface temperatures. Furthermore, plants. l

r the leaves themselves were only 4-6 °C above air tempera­ b) In an exposed area, one to two metres south of the plant's e

d ture. This was thought to be due to the high reflectivity of the leaf tips. n

u leaf (Schulze et al., 1980). The central layer of the cuticle

y contains crystals of calcium oxalate that may play an important Soil moisture was determined gravimetrically by drying a a

w role in reflecting radiant energy (Bornman, 1972) as may the pre-weighed soil sample in an oven at 70°C for a minimum of e t fibre bundles that run parallel to the leaf axis (Eller, Willert, four days. a

G Brinckmann and Baasch, 1983). t e The present study examines the extent to which W. mirabilis Temperature measurements n i plants modify their local thermal and moisture environment in Temperatures associated with W. mirabilis plants were mea­ b a the central Namib Desert at a site about 70 km inland from the sured using copper constantan thermocouples and a qigital S y b

d Marsh, B. A., 1990. The microenvironment associated with Welwitschia mirabilis in the Namib Desert. In: Seely, e c M. K., ed., Namib ecology: 25 years of Namib research, pp. 149-153. Transvaal Museum Monograph No.7, u

d Transvaal Museum, Pretoria. o r p e R 150 WELWITSCHIA MICROENVIRONMENT

thermometer (Bailey Instruments, Model BAT-12). Thermo­ Table 1 couples were 30 gauge and had tips embedded in epoxy. Mean (X) and standard deviation (S.o). of the percentage soil mOis­ ture in soil samples from below leaves of W. mirabilis and exposed Readings were taken at approximately hourly intervals for 24 sites nearby; n represents the number of samples taken. to 48 hours on three occasions in 1982. On each occasion, soil surface temperatures were also measured at an exposed site close to the W. mirabilis plants. Air temperature in the shade Percentage percentage was measured at a height of 1 m with a mercury thermometer. soil moisture soil moisture below plant at elglosed site Date n X±S.D. X±S.D. 1) From 17h30 on 25 January 1982 to 17h30 on 26 January 1982 the following temperatures were measured: soil surface temperature below W. mirabilis leaves where the 10 February 1983 9 0,41 ± 0,08 0,17±0,03 ground was covered with a 20-30 mm layer of litter; 7 April 1983 10 0,37± 0,08 0,26 ± 0,06 40 mm below the soil surface in an exposed area; 40 mm 13 August 1983 20 0,43 ± 0,12 0.25 ± 0,07 below the soil surface under W. mirabilis leaves, where the soil was covered with a 20-30 mm layer of litter. 2) From 12hOO on 19 April 1982 to 14hOO on 20 April 1982 the following temperatures were measured: soil surface 13 August 1983 and P < 0,01 for 7 April 1983. t-test for two temperature below W. mirabilis leaves, ground not cov­ means). The percentage soil mOisture was not higher on days ered by litter; surface temperature below W. mirabilis during which fog was experienced in the mornings (10 Febru­ leaves, ground covered by 20-30 mm layer of litter; under ary and 7 April) than on a fogless day (13 August). a 40-50 mm layer of litter in the stem depression. 3) From 19hOO on 21 February 1982 to 18hOO on 23 Febru­ Temperature measurements ary 1982 the following temperatures were measured on On 26 January 1982 surface temperatures at the exposed the east and west sides of the plant: soil surface temper­ site reached 78°C while surface temperatures below the ature below leaf tips of plant; soil surface temperature plant's leaf reached a maximum of just over 40 °C (Fig. 1). about 30 mm from the stem of the plant and under a Temperatures at the exposed site were greater than 40°C 20-30 mm layer of litter. for approximately 10 of the 24 hours monitored (Fig. 2). Temperatures at a depth of 40 mm below the soil surface at the exposed site reached a maximum of just over 60°C while

. RESULTS at 40 mm depth below the plant, temperatures peaked at )

9 36,6 °C and the range in temperature over 24 hours was 6,5 °C 0

0 Soil moisture compared with a range of 36°C at a depth of 40 mm at the 2 The percentage soil moisture at a depth of 50 mm (Table 1) exposed site. d e t was significantly higher below W. mirabilis plants than at On 20 April 1982, surface temperatures reached a maximum a

d nearby exposed areas (P < 0,001 for 10 February 1983 and of 54°C at the exposed site and 31,4 °C under W. mirabilis ( r e h s i l b u (1) 20 April 1982 (2) 26 January 1982 P e h t y b Litter d e t n a r g e c n e c i l r e d n u y a w e t a G t e Fig. 1 n i b Transverse section through Welwitschia mirabilis, showing maximum temperatures recorded on 1) 20 April 1982 and 2) 26 January a

S 1982 in the following positions: a: 1 m above the soil; b: insolated soil surface; c: 40 mm below insolated soil surface; d: below

y leaves on shaded, litter-free SOil; e: soil surface beneath a 20-30 mm deep layer of litter; f: 40 mm below soil surface that was b covered with a 20-30 mm deep layer of litter; and g: in stem depression under a 40-50 mm deep layer of litter. d e c u d o r p e R WELWITSCHIA MICROENVIRONMENT 151

80

70

60 E 1£ 50 .3 f! c. E'" 40 I-'"

30

20

1800 2000 2200 2400 0200 0400 0600 0800 1000 1200 1400 1 800 1800 Time (h)

Fig. 2 Temperatures recorded during 25 and 26 January 1982: a: on exposed soil; b: at 40 mm depth below exposed soil; c: below W. mirabilis leaves - on the soil surface; and d: at 40 mm depth. Soil was covered by 20-30 mm litter for (c) and (d).

leaves (but not covered with litter), while temperatures under shade. W. mirabilisleaves, where the ground was covered with litter, Temperatures on the east side peaked earlier, the maxima

. reached 28 cC. The maximum temperature under litter in the were lower than those on the west side, and exhibited a smaller )

9 stem of the plant was 34°C (Fig. 1). Whereas the range of range. Even the frayed leaf tips moderated ground tempera­ 0

0 temperatures was 18,5 °C for bare soil surfaces below the tures considerably. At 14hOO the difference in temperature 2

d leaves and under the stem litter, the range was 10°C below between the soil surface below leaf tips and exposed soil e t litter under the leaves (Fig. 3). surfaces was approximately 10°C (both east and west sides); a d On 20 February 1982, exposed surface temperatures however, at 16hOO the temperature below leaf tips on the west (

r reached a maximum of 67 QC (Fig. 4). At anyone time temper­ side was virtually the same as the exposed soil surface, while e h atures on the east and west side of the plant varied consider­ the temperature on the east side below leaf tips was 15°C s i l ably, both at the leaf tips and close to the stem in constant lower. b u P

e 60 h t

y a b 50 d e t n a r 40 g G e !!- c b n :>e C e 30 c ~ i d 0> l

C. r E e I- d '" 20 n u y a 10 w e t a G t e 1200 1400 1600 1800 2000 2200 2400 0200 0400 0600 oaoo 1000 1200 1400 n i Time (h) b a S

Fig. 3 y b Temperatures recorded during 19 and 20 April 1982: a: on exposed soil; b: under 40-50 mm litter in W. mirabilis stem

d depression; c: below W. mirabi/is leaves - on bare ground; and d: soil surface under 20-30 mm litter. e c u d o r p e R 152 WELWITSCHIA MICROENVIRONMENT

70

60

b

50 E a !!: ::> ~., 40 c. d E l-'" C

30 e

20

1800 2200 0200 0600 1000 1400 1800 2200 0200 0600 1000 1400 1800 Time (h) . ) 9 0

0 Fig. 4 2

d Temperatures recorded during 21, 22 and 23 February 1982: a: on exposed soil; b: soil e

t surface under western leaf tips; c: soil surface under eastern leaf tips; d: soil surface west a of stem; and e: east of stem. (d) and (e) were under 20-30 mm litter. d ( r e h s i l b u P

DISCUSSION 1977; Sartos, DePree and Whitford, 1978) and for refuge and e

h feeding of other animals (Cloudsley-Thompson, 1962; t

y Welwitschia mirabilis creates a microclimate below its leaves Larmuth, 1979; Seely, De Vos and Louw, 1977). The b in which temperature and moisture regimes are considerably microhabitats below W. mirabilis attract numerous animals, d e

t more moderate than those prevailing in the harsh environment including birds, snakes, small mammals, lizards, chameleons, n in which it lives. Thus when the temperature of exposed soil scorpions, spiders and a variety of insects (Bornman, 1971). a r

g was over 70°C, soil temperatures below W. mirabilis plants Marsh (1987) has also established that a relatively rich micro­ e

c barely reached 40°C. Even soil temperatures below frayed arthropod community flourishes in the soil mound below this n

e leaf tips were substantially lower (up to 15°C) than at exposed species. Many of the large animals do not live permanently c i l sites under certain conditions. Furthermore, temperatures below W. mirabilis nor do they utilize the plant as a food r

e varied considerably depending on the point of measurement resource. Their occasional presence below the plant is prob­ d

n relative to the plant. Since leaves tend to become frayed at the ably largely due to the thermal refuge that the plant provides. u

tips, temperatures were generally higher with increasing dis­ Furthermore, it is likely that the humidity beneath the plants is y a tance from the stem. Thermal regimes at easterly and westerly greater than elsewhere because of transpiratory water loss w

e positions under the plant displayed considerable differences and this may also enhance the attractiveness of the microen­ t a and the amount of litter collected within the stem depression vironment for animals. The presence of a permanent diverse G

t and at the base of the stem moderated temperatures. There micro-arthropod fauna below the plants and their scarcity e

n is therefore a mosaic of temperatures within the microhabitats elsewhere in this habitat (Marsh, 1987), is largely because of i b associated with W. mirabilis at anyone time. a relatively rich food resource base in the form of litter but their a

S Microhabitats associated with desert plants often provide ability to persist there can also be attributed to the more y favourable conditions for arthropod activity (Charley and West, b favourable microclimate created by the plant. d e c u d o r p e R WELWITSCHIA MICROENVIRONMENT 153

ACKNOWLEDGEMENTS I thank M. K. Seely. A. C. Marsh and G. N. Louw for advice Recreation Resorts. Namibia. gave permission to do research and for their comments on the manuscript. I am also grateful in the Namib-Naukluft Park and provided living and working to the Research Assistants of the Desert Ecological Research facilities. The work was funded by the Transvaal Museum and Unit of Namibia. especially Lorian Osberg and Karen Nott. for the Foundation for Research Development of the C.S.I.A., field assistance. The Directorate Nature Conservation and Pretoria.

REFERENCES

BORNMAN. C. H., 1971. Welwitschia rediscovered. Lantern 20: 2-9. mirabilis in the Namib Desert. South African Journal of Zoology BORNMAN, C. H .• 1972. Welwitschia mfrabilis: paradox of the Namib 22(2): 89-96. Desert. Endeavour 31 (113): 95-99. NOY-MEIR. I.. 1979/80. Structure and function of a desert ecosystem. CHARLEY, J. L. and WEST, N. E.. 1977. Micro-patterns of nitrogen Israel Journal of Botany 28: 1-19. mineralization activity in soils of some shrub-dominated semi-desert RODIN, R. J., 1953. Distribution of Welwitschia mirabilis. American ecosystems of Utah. Soil Biology and Biochemistry 9: 357-366. Journal of Botany 40(4): 280-285. CLOUDSLEY-THOMPSON, J. L., 1962. Bioclimatic observations in SARTOS, P. F., DEPREE, E. and WHITFORD, W. G., 1978. Spatial the Red Sea hills and coastal plain, a major habitat of the desert distribution of litter and micro-arthropods in a Chihuahuan Desert locust. Proceedings of the Royal Entomological SOCiety of London ecosystem. Journal of Arid Environments 1: 41-48. (A)37: 27-34. SCHULZE, E. D., ELLER. B, M., THOMAS, D. A., WILLERT, von D. ELLER, B. M .• WILLERT, von D. J .• BRINCKMANN, E. and BAASCH, J. and BRINCKMANN, E., 1980. Leaf temperatures and energy R., 1983. Ecophysiological studies of Welwitschia mirabilis in the balance of Welwitschia mirabilis in its natural habitat. Oecologia 44: Namib Desert. South African Journal of Botany 2: 209-223. 258-262. KERS, L. E., 1967. The distribution of Welwitschia mirabilis Hook F. SEELY, M. K., DEVOS, M. P.and LOUW, G. N., 1977. Fog imbibition, Svensk Botanisk Tidskrift 61 (1): 97-125. satellite fauna and unusual leaf structure in a Namib Desert dune LARMUTH. J., 1979. Aspects of plant habit as a thermal refuge for plant Trianthema hereroensis. South African Journal of Science 73: desert insects. Journal of Arid Environments 2(4): 323-327. 169-172. MARSH. B. A., 1987. Micro-arthropods associated with Welwitschia . ) 9 0 0 2 d e t Present address of author: a

d B. A. Marsh (

r Department of Zoology e h University of Namibia s i l Private Bag 13301 b u Windhoek P 9000 Namibia e h t y b d e t n a r g e c n e c i l r e d n u y a w e t a G t e n i b a S y b d e c u d o r p e R