Ecology of Desert Environments, pp. 301 - 332, 2001 Editor: Ish war Prakash I Publishing Co., Scientific Publishers (), P.O. Box 91, Jodhpur http://www.scientificpub.com 00 IIivet, P. and Cox, bls., Amsterdam,

Wrangel Island.

719 pp. , Paris, France : Realized Assemblages in the Namib, Kalahari and Kara-Kum Deserts . the Biology of .••ca.demic Publs.,

I the Hengduan era (Col. Chrys.) tnelidae. Biondi, Harold Heatwole 199-309. Department of Zoology, North Carolina State University, ns, China. Acta Raleigh, N C 27695-7617, USA nountains (Col.

the Hengduan Abstract of China (Col. Baited transects were used to study the assemblages of in the Kalahari and Kara-Kum deserts and across dunes, a plain, and a dry nae. of riverbed in the Namib Desert. Species-richness was related to amount of vegetation, with the shrub desert of the Kalahari having over twice as Iuan Mountain many species as the other, more severe deserts. The point species- rth, D. G. (ed.). richness increased with overall species-richness up to an asymptote of five species. The mean number of species occurring at baits stabilized at slightly more than one species. The assemblages were inequitable, with 11. Chrys.) from one to three species accounting for most of the visits to baits. In the more highly vegetated deserts foraging ants examined virtually the entire 1e in Galeruca desert floor on a daily basis. rth, D. G. (ed.), In the Namib, most species of ants were scavengers or omnivores; no strict granivores were found. In the other deserts only the scavenger guild was studied. In all deserts, ants dominated the scavenger guild, with other taxa coming to sardine baits only infrequently. Foraging took place more often at night and in the morning than in the middle of the day; at most sites it was more intense on the ground than in shrubs. The Kara-Kum differed in that most activity was associated with shrubs. The three most common species in the Namib were partially segregated on the basis of different times of peak activity. No overt combat was observed, and in the less speciose assemblages there was no evidence of interference. With increasing species-richness, however, there was exclusion of some species from baits, and in the Kalahari a dominance hierarchy was observed. 302 Heatwole Ant

Introduction ~ assemblages. time can be com This paper represents a continuation of a study of changes in assemblages of ants across environmental gradients and habitat boundaries. The approach is to present Even withir the results from the simplest assemblages first, and then proceed toward more may nest in wac complex ones, eventually providing a general review of a series of habitats ranging Colonies of sot from tundra and desert to tropical rainforest, with examples of each from two or more vicinity, therebj continents. Previous contributions described changes in ant assemblages from tundra, reasons, a given across the treeline to birch, aspen and pine forests in northern Lapland, i.e., near the the realized (J. , distributional limits of ants in a cold region (Heatwole, 1989). That was followed by particular point examination of ant assemblages at the opposite extremes of inhospitability, heat and I· important meas salinity, in two starkly arid habitats, the Atacama Desert in Peru and the Chott El species that ac Djerid in the Tunisian Sahara (Heatwole, 1996). A study of dunes in the Arabian ecologicalimpor Desert (Heatwole, 1991) extended comparison to a somewhat milder dune desert. The In summarj present study expands those comparisons to three additional deserts, each with its /I (1) regional list own combination of climatic and vegetative characteristics. richness, with ti The .structure of desert ant assemblages is highly dynamic and its analysis at any Assemblage one season or year provides an incomplete picture (Whitford, 1978). Dietary niche roles played by breadths may show considerable variation within a species through time, and among the apportionm species at a given time; diet may be opportunistic and shift without apparent pattern omnivores, prec (Marsh, 1985a). Species vary in the rigidity of their activity patterns, some and/or honeyde: maintaining the same cycle at all seasons, others shifting seasonally, especially from ants would be summer nocturnality to winter diurnality (Marsh, 1988; Heatwole & Muir, 1989). qualitative list I Some species close their nests and are not active on the surface for long periods. of one kind of f Such inactivity may be seasonal but does not necessarily show regular patterns and otherwise grani may be opportunistically related to food supply and/or rainfall, with large (Heatwole, 199: year-to-year differences (Marsh, 1985a; Heatwole & Muir, 1991). These species have a the Namib dun pulsed effect on the ecosystem. During active periods their foraging may be intense 1985a, 1990).A and their trophic effect great. When cloistered underground and relying on stored multiple guilds food they exert little immediate influence and can be considered as functionally seeds, but occas "absent" from the assemblage. For these reasons it is necessary to distinguish guild, but plaj between the "total assemblage" (comprising both active and inactive components) and participation in the "realized assemblage" (that part of it playing an immediate role in community to baits of diffe dynamics). than one guild. Not only does composition of the realized assemblage change temporally, but it The present also does so spatially, even to the extent of differing between north-facing and . assemblage stn south-facing slopes in the same habitat (Donnelly & Giliomee 1985). Often ants show assemblages ac mosaic distributions, with the species composition of a given patch being controlled by structure. a particularly dominant species (Majer, 1972). Different patches may have different mixes of species, although having some species in common. The assemblage of a given patch or habitat can be designated an ex assemblage, with its number of species being the ex species-richness. As one adds new patches or different habitats in the same The investi general area, the number of species increases and the larger assemblage can be Kalahari, and t considered as the ~ assemblage and its number of species as the ~ species-richness. Heatwole (199E One can ascertain the realized ex and ~ assemblages for a particular season or time, from below the and over a prolonged period accumulation of data can lead to estimates of total ex and abutting eastw: cold Benguela c Ant Assemblages in the Namib, Kalahari and Kara-Kurn. deserts 303,

P assemblages. Finally, the total list of species from all patches in all habitats over time can be considered the regional assemblage. [ages of ants s to present Even within a patch, species may not be uniformly distributed. Particular ones oward more may nest in wood or under rocks and only forage within a certain radius of such sites. .ats ranging Colonies of some species may inhibit establishment of founder queens in their two or more vicinity, thereby denying access to rival species. For these and possibly many other rom tundra, reasons, a given point in the habitat may not be visited by all ofthe species present in .e., near the the realized 0: assemblage. The number occurring and potentially interacting at a followed by particular point is the "point species-richness". Biologically, this is probably the most ;y, heat and important measure of biodiversity. In terms of community dynamics, the number of e Chott EI species that actually make contact and interact at a specific point is of greater he Arabian ecologicalimportance than the overall species-count. desert. The In summary, the various measures of species-richness, in descending order, are: ch with its (1) regional list, (2) ~ species-richness, (3) 0: species-richness, and (4) point species- richness, with the last three being divided into a total and realized richnesses ysis at any Assemblages are composed of different guilds, as defined by the various ecological !tary niche roles played by component species. The trophic structure of an assemblage indicates and among the apportionment of species among the various guilds. Major guilds among ants are mt pattern omnivores, predator-scavengers, granivores, and feeders on sweet substances (nectar rns, some and/or honeydew), although other minor trophic specialists also exist. Most species of cially from ants would be considered omnivores if trophic designation were based solely on a 1989). qualitative list of their known foods. However, many species feed on a preponderance 19 periods. of one kind of food and take only small amounts of other items. For example, many tterns and otherwise granivorous species include small amounts of matter in their diet rith large (Heatwole, 1991). Similarly, Camponotus detritus, mainly a consumer of honeydew in :ies have a the Namib dunes (Curtis & Seely, 1987), is also an opportunistic scavenger (Curtis, )e intense 1985a, 1990). An assemblage structure should take into account such participation in on stored multiple guilds. For example, an abundant omnivorous ant, primarily feeding on nctionally seeds, but occasionally taking carrion, would be a major participant in the granivore istinguish guild, but play a relatively minor role in the scavenger guild. The extent of rents) and participation in particular ones may vary in time or space. The relative attractiveness Immunity to baits of different types permits assessment of the extent of membership in more than one guild. Iy, but it The present study employs the above definitions. It depicts differences in realized cing and assemblage structure among different deserts; in one desert, the Namib, it compares mts show assemblages across habitat boundaries at a particular time, and examines guild .rollsd by structure. differcnt )f a given Study Areas ies being .he same The investigation was carried out in three different deserts, the Namib, the e can be Kalahari, and the Kara-Kum. Their general characteristics have been summarized by richness. Heatwole (1995). The Namib Desert occupies a coastal strip in southwestern Africa lor time, from below the Orange River northward through Namibia and into , and ,al 0: and abutting eastward onto the Kalahari-Karoo system. The Namib owes its origin to the cold Benguela current running northward along the Namibian coast. The major land 304 Heatwole An form consists of extensive areas of mobile sand, but in addition there are gravelly to tenebrionid be sandy flats, rocky outcrops, hills, and a narrow, sandy littoral strip. represented. The Kalahari is not clearly defined and has been interpreted in various ways, Wind is ve including whether it is a desert at all. In its strictest sense it is considered as the dry food (Fitzflimoi region of Botswana, Namibia, and , lying between the Okavango Delta and Etosha and Zambezi rivers to the north and the Orange River to the south The study (Thomas and Shaw, 1995). Sometimes it is also considered as including the Karoo, an temperatures 1 arid region extending westward into Namibia (McGinnies et al., 1970). Southwestern hottest month, dunefields support a shrub savannah; towards the northeast, ground cover generally (Goudie, 1972; increases and shrubs progressively give way to trees. The Kalahari t The Kara-Kum Desert, located in central Turkmenistan, is a continental desert forming part of the greater Turkestan Desert complex of Central Asia. The Kara-Kum The study consists of extensive areas of sand dunes interspersed with loessial and alluvial Letlhakeng, BI plains, depressions of finer-textured soils (clays), and river terraces and flood plains flat sand. The (McGinnies et al., 1970). Ground vegeta were no dwell The Namib Si:e grazing in the and annual rs Research in the Namib Desert was carried out at the Namib Desert Research The study wa Station, near Gobabeb, Namibia (23°34'S : 15°03'E). The area is characterized by two prolonged droi types of desert, (1) large, active sand dunes, some with vegetation and others devoid of it, and (2) a flat, sparsely vegetated interdune valley consisting of a plain of sand The Kara-KuIJ and gravel with rocky outcrops. The transition between the two is abrupt; in the vicinity of the research station it is demarcated by the Kuiseb River, with dunes on The inves one side and plain on the other. The Kuiseb River flows only episodically and was dry Biological Sta during the present study. Its course is marked by trees and shrubs in strong contrast Biological Pro to the more barren habitats on either side. with varying vegetation, sn The climate, geology, soils, vegetation, and fauna of the study site have been tops, and larg described often in the course ofthe many biological investigations carried out there (e. small, consolii g. Fitzflimons, 1964; Goudie, 1972; Scholz, 1972; Seely & Stuart, 1976; OIlier, 1977; the vegetation Seely et al., 1979-1981; Holm & Scholtz, 1980) and bibliographies of papers treating the area are available in various issues of the Namib Bulletin. The Kara 1979). Mean Seasonal change in air temperature is slight, with mean temperature of the coldest month coldest month (July 17.TC)being only 6.5°C lower than that of the hottest month with practical (March 24.2°C). Temperatures in excess of 40°C occur; subzero values are not out in 1995in frequent. Ground temperatures are much more extreme, with sand surfaces showing still high. a maximum daily range of about 50°C and an annual one of 70.5°C. Precipitation is slight but variable. At the study area the annual mean is 16.8 mm (range 0-26.3 mm), with the highest daily amount recorded being 16.5 mm. However, fog occurs on an average of 102 days per year and precipitates an average of 31 mm of The tech moisture annually, more than contributed by rainfall. Various of the and (Heatwole, H reptiles rely upon condensation offog as a source of water (.•Hamilton. & Seely, 1976). cord was unrs Away from dry river courses, vegetation is sparse and the biomass of both plants immediately; and in dry periods is among the lowest reported for any non-polar terrestrial also placed a ecosystem (Seely & Louw, 1980). A major resource driving the biotic community is ground. Thw plant detritus imported by wind (Seely, 1978). A diverse fauna of detritivorous vegetation Wl Ant Assemblages in the Namib, Kalahari and Kara-Kum deserts 305 lre gravelly to tenebrionid beetles builds upon this base; reptiles and arachnids also are well represented. various ways, Wind is very important in the Namib as it brings two critical resources: fog and red as the dry food(FitzSimons, 1964). avango Delta to the south The study was carried out in 1979 in February, a time of year when air and soil the Karoo, an temperatures are near maximum and rainfall usually is most frequent. March is the Southwestern hottest month, followed by February, and most rainfall occurs from January to April er generally (Goudie, 1972; Seely & Stuart, 1976; Robinson & Seely, 1980). The Kalahari Site nental desert [leKara-Kum The study site in the Kalahari was about 10 km south of the village of and alluvial Letlhakeng, Botswana (24°05'S : 25°03'E). There the soil was ofloose, but stabilized, I flood plains flat sand. The vegetation was a mixture of Acacia and other shrubs up to 2 m tall. Ground vegetation was sparse and there were many large patches of bare sand. There were no dwellings in the vicinity, but inhabitants of Letlhakeng had some cattle grazing in the area. Mean summer (January) temperature at this site is about 30°C and annual rainfall is about 400 mm, mostly occurring in summer (Werger, 1986). ert Research The study was conducted in November 1986, just after early rain had broken a Irized by two prolonged drought. rthers devoid nlain of sand The Kara-Kurn Site ~rupt; in the ith dunes on The investigation in the Kara-Kum Desert was conducted at the Repetek and was dry Biological Station (38°14'N : 68°11'E), formerly the location of an International ong contrast Biological Programme. The vicinity of the station contains a variety of dune deserts with varying amounts of vegetation ranging from live dunes with sparse to no vegetation, small consolidated dunes with shrubs on the sides but relatively bare e have been tops, and large dunes with shrubs and large trees at their bases (South Valley). The out there (e. small, consolidated dunes were the focus of the present study. A detailed account of pllier, 1977; the vegetation is provided by Rodin (1979). iers treating The Kara-Kum is the hottest of the central Asian continental deserts (Rodin, 1979). Mean temperature is 32°C for the hottest month (July) and 1.TC for the rture of the coldest month (January). Most rainfall occurs in winter and spring (mean 113 mm) ttest month with practically none in summer and autumn (mean 2 mm). The study was carried es are not out in 1995 in September, i.e., in autumn when conditions were dry and temperatures ces showing still high. lis 16.8 mm Materials and Methods n. However, of31 mm of The technique employed was similar to that described in previous papers iropods and (Heatwole, 1989, 1991, 1996). Ribbons were tied on a cord at 10-m intervals. As the ely, 1976). cord was unreeled in a straight line across the desert, a bait was placed on the ground both plants immediately at the location of each ribbon. If shrubs or trees were nearby a bait was r terrestrial also placed at the base of the nearest one and on its trunk one meter above the mmunity is ground. Thus, for each baiting station there were three baits when arborescent btritivorous vegetation was present and one when there were no trees or shrubs. The baits were 306 Heatwole An examined at different times of day, each time replacing or replenishing depleted baits. accounted for The species of ants present and their activities were noted on each occasion and prominence of voucher specimens preserved in 70% alcohol for later identification. On a few continents. occasions some baits could not be found (especially at night) or because they had been consumed. Abundance of individuals of each species was scored as: Nil, Few (1-10), or Many (>10). Activity was scored for each species as: FIA (foraging in area but not Table feeding), ATT (repeatedly attempting to reach a bait but prevented from doing so by ants of another species already in attendance), F (feeding), C (carrying food away) or MOB (mobilized; ants completely surrounding bait and defending it). Each bait was Species considered a point for estimating point species-richness. PONERINAE There were slight differences in methodology among sites as necessitated by local Pachycondyla Ul conditions or availability of time. In the Namib Desert, the cord was unreeled MYRMICINAE transversely across the dry riverbed ofthe Kuiseb River (20 stations, three baits each) Crematogaster c and extending into the plain on one side (17 stations, one bait each) and into sand Crematogaster s dunes on the other (15 stations: 10 on bare dunes with one bait each in the open, and Meranoplus spit 5 with one bait each placed at the edge of a shrub), thus including three habitats, Monomoriuim b instead of only one homogeneous type as in the other two deserts. Sardines, a nearly Monomorium de universal ant attractant (Heatwole, 1989), were employed as baits at all study sites. Monomorium in In the Namib, the study was broadened to also include seeds (wheat germ) and Monomorium Sl.< treacle; all three baits were placed simultaneously at each baiting site. The baits were Ocymynnex ban examined over a 24-hour period at morning (0730-0930 hrs), afternoon (1500-1630 Oxymyrmex pice hrs), evening (2020-2045 hrs), and night (2300-2330 hrs). Ocymyrmex ioeu At the Kalahari site, baits were laid at 20 stations during late morning and Pheidole sp.2 examined at mid-day (1200-1300 hrs) and twice in the afternoon (1400-1500 and Pheidole ienuitu 1600-1700 hrs). The intent was to extend observations into the night and following Tetrcmorium co morning. However, in the politically charged atmosphere of that time (1986) in an Tetramorium gl area subject to clandestine activities from across the border, a suspicious human population was unconvinced that my transect line with ribbon markers really Anoplolepis mel represented scientific research. They were more effective in persuading me than I was Anomolepis stei. in persuading them and convinced me their threats against my life, unless I Anoplolepis trin immediately vacated the area, were genuine. I acquiesced and terminated my Camponotus de. observations; consequently, comparisons between the Kalahari and other areas can be Camponotus rul made only for the diurnal period. Camponotus me At the Kara-Kum site, 29 stations were employed; there were two readings (on Camponotus me consecutive days) of the transect in both morning and at mid-day, and one each in the Camponotus oli afternoon and at night. Camponotus rOI The species names used here are those provided by the taxonomists identifying Camponotus ru, voucher specimens (see Acknowledgements), as modified by nomenclatural changes Camponotus sp and taxonomic reassessments occurring since the original identifications (Bolton, capen 1995). Plagiolepis brui DOLICHODEB Results and Discussion Tapinoma sp.

Taxonomic Composition ,',day time only 1. found by gene Twenty-nine species of 12 genera in four subfamilies were found during the study 2. may include 1 (Table 1). The subfamilies Myrmicinae (14 species) and Formicinae (13 species) ~'l.daytime only Ant Assemblages in the Namib, Kalahari and Kara-Kum. deserts 307 lepleted baits. accounted for all but two species. Marsh (l986b, 1990) previously noted the occasion and prominence of these two subfamilies in African deserts, compared to deserts of other In. On a few continents. they had been Few (1-10), or area but not Table 1. Ants collected during the present study from the study areas m doing so by Namib food away) or Species Kalahari" Kuiseb Plain Veg. Bare Kara- ~ach bait was R Dune Dune kum PONERINAE tated by local Pachycondyla undescribed sp. 1 + vas unreeled MYRMICINAE ee baits each) Crematogaster castanea + + md into sand Crematogaster subdentata + the open, and Meranoplus spininodis + tree habitats, Monomoriuim. barbatulum. + ines, a nearly Monomoriuni delagoense + + + 11 study sites. Monomorium indicurn. husnezoioi + It germ) and Monomorium. subopacuni + he baits were Ocyrnyrmex barbiger + + + + n (1500-1630 Oxymyrmex picardi + Ocymyrmex weitzecheri + + morning and Pheidole sp2 + 00-1500 and Pheidole tenuinodis + + + md following Tetramorium. caespitum + (1986) in an Tetramorium. gladstonei + cious human FORMICINAE rrkers really Anoplolepis melanaria + ie than I was Anomolepis steingroeveri + + ife, unless I Anoplolepis trimenii + minated my Camponotus detritus + areas can be Camponotus fulvopilosus + Camponotus maculatus + readings (on Camponotus mayri + e each in the Camponotus olivieri + Campo notus robecchii troglodytes + IS identifying Camponotus rufoglaucus + ral changes Camponotus sp. 3 + ons (Bolton, Lepisiota capensis + + + Plagiolepis brunni + DOLICHODERINAE Tapinoma sp. +

" day time only 1. found by general collecting but not in transect g the study 2. may include more than one unidentified species (13 species) 3. daytime only Ant. 308 Heatwole Kuiseb riverbed Three genera (Crematogaster, Monomorium, and Camponotus) were represented desert. There ~ in the realized assemblages of all three deserts; the Kalahari and Namib sites shared underground w~ an additional four genera, and the Kalahari and Kara-Kum sites shared one genus moister habitat, not detected at the Namib site. Two genera occurred only at the Kalahari site, and vegetation prodi one only in the Namib. Camponotus was the genus best represented in the study permitting occuj areas with eight species recorded; Monomorium was represented by four species, there are suffici: Ocyrnyrmex and Anomolepis by three each, Crematogaster and Pheidole each by two, as Ocymyrmex. ( and all others by a single species each. It is likely that given genera were represented diverse habitat ~ in different deserts by ecological equivalents. For example, Curtis (1985b, 1990) noted that Camponotus detritus is restricted to the dunes of the Namib Desert where it replaces the closely related Camponotus [uluopilosus that occupies the rest of arid Table 2. Ow southern Africa (and was found at the Kalahari site). Only four species were shared between the Namib and Kalahari sites (Crematogaster castanea, Ocymyrmex weitzecheri, Pheidole tenuinodis, and Lepisiota Species capensis). The Kara-Kum did not share any species with the African deserts. Ocymyrmex barbi, At the Namib site, 11 species of ants in 8 genera were found in the transected Monomoriuni dele area. Six genera were represented by one species each, one (Ocymyrmex) by two Lepisiota capensu species and one (Camponotus) by three. At the Kalahari site, 18 species from 11 Anoplolepis steing genera were found. Seven genera were represented by a single species each, three Pheidole tenuinod genera tOcymyrmex, Anomolepis and Pheidole) by two species each, and one Camponotus maci (Camponotus) by five species. In the Kara-Kum desert four species from three genera Tapinoma sp. were present; the genus with two species was Monomorium. Camponotus robe, Ocymyrmex weitz, Species-Richness Crematograster C( Realized ex species-richness at the Namib site varied among the different habitats, Camponotus det/"l with the dry riverbed having the greatest number of species (8). The plain and the TOT AL SPECIE~ vegetated part of the dune desert each had five species. Only one species, Ocymyrmex Only one sp barbiger, was found in the dunes away from the vicinity of vegetation (Table 2). extent and that Although these four habitats shared a number of species, all except bare dunes also structural diver had one or more unique members in its assemblage. Only one species, Ocymyrmex richness in the barbiger, occurred in all four habitats investigated. Monomorium delagoense was hypotheses seen present in three, being absent only from non-vegetated parts of the dune desert. Two species from the riverbed also were found on the plain; another one also occurred in Eighteen sp vegetated dunes. The remaining six species were found in only one habitat, often only recorded at baii on one or a few occasions. Of these, three were restricted to the riverbed, two to species-richness vegetated dunes and one to the plain. The greatest faunal overlap (4 shared species) unidentified SpE was between the dry riverbed and the plain. Vegetated dunes shared three species would have revI with the dry riverbed and two with the plain. but not in the t (Prins, personal The. Namib site was the only area in which more than one habitat in the same region was studied and for which realized ~ species-richness could be calculated The realizer directly from the data of the present study. The realized ~ species-richness was 11 Point Species-ru species collectively for all four habitats in the transect. Thus, the realized ex species- richnesses of bare dunes, vegetated dunes, flat desert, and the dry riverbed The numbs represented respectively 9%, 46%, .46%, and 73% of the ~ species-richness of the Kara-Kum and entire site. Marsh (1985a) indicated that in Namibian gravel desert single surveys species found s yielded about 70% of the total species known to occur in the assemblage. The dry Ant Assemblages in the Namib, Kalahari and Kara-Kum deserts 309

ere represented Kuisebriverbed had a greater realized species-richness than either kind of adjacent unib sites shared desert. There are several possible contributing causes. (1) The riverbed has ared one genus underground water (Seely & Louw, 1980) and therefore more nearly resembles alahari site, and moister habitats. (2) Microclimatic diversity is greater there. The arborescent ted in the study vegetation produces patches of shade ameliorating surface temperatures, thereby by four species, permitting occupancy by species intolerant of harsher conditions. At the same time iole each by two, there are sufficient open, exposed patches to accommodate thermophilic species such ere represented as Ocymyrmex, (3) It has a more complex vegetation structure and therefore a more 85b, 1990) noted diversehabitat geometry. Desert where it the rest of arid Table 2. Occurrence of ants in different habitats along the Kuiseb River transect, Namib Desert. + indicates presence of a species Kalahari sites Dune Desert is, and Lepisiota leserts. Species Bare Shrubby Dry riverbed Plain Ocymyrmex barbiger + + + + 11 the transected Monomorium delagoense + + + yrmex) by two Lepisiota capensis + + ~speciesfrom 11 Anoplolepis steingroeveri + + :cies each, three Pheidole tenuinodis + + each, and one Camponotus maculatus + om three genera Tapinoma sp. + Camponotus robecchii troglodytes + Ocyniyrmex weitzecheri + Crematograster castanea + fferent habitats, Campo notus detritus + e plain and the TOTAL SPECIES 1 5 8 5 .ies, Ocymyrmex Only one species in the riverbed (Lepisiota) foraged on tree trunks to any great ation (Table 2). extent and that species was also found in flat desert as well (Fig. 2). Consequently, bare dunes also structural diversity of the habitat does not seem to be critical in elevating species- ies, Ocymyrmex I richness in the river bed, and either (or a combination of both) of the microclimatic 'delagoense was hypotheses seem more likely. une desert. Two also occurred in Eighteen species were collected from the Kalahari site, 17 (94%) of which were bitat, often only recorded at baits in the transect. This may be a slight underestimate of realized ex ·iverbed, two to species-richness as the category "Pheidole sp." possibly includes more than one shared species) unidentified species. Nocturnal observations, had they been possible, almost certainly sd three species would have revealed additional species. The only species found by general collecting, but not in the transect, was an undescribed Pachycondyla sp., near P. kruegeri Forel (Prins, personal communication). tat in the same d be calculated The realized species-richness ofthe Kara-Kum site was four. ·ichness was 11 tlized ex species- Point Species-richness " dry riverbed The number of species co-occurring at baits varied among deserts. In the Irichness of the Kara-Kum and in the dunes and on the plain of the Namib, seldom were two or more I single surveys species found simultaneously at the same baits, and then only at low frequencies iblage. The dry 310 Heatwole Ant

(4-20%) (Table 3). By contrast the more heavily vegetated habitats (the dry bed of the Kuiseb River and the Kalahari site) had as many as 3-7 species co-occurring at a small proportion of the baits. Furthermore, dual occurrences were observed there more frequently and often involved a greater proportion of baits than in the sparser deserts. '

Table 3. Percentage of sardine baits attended by various numbers of ant species in the Namib, Kalahari, and Kara-Kum deserts. 0 = baits on grounmd in the open,'B = baits on ground at bases of shrubs or trees; S = baits up in shrubs or trees, Dash indicates no data taken

NAMIB KARA-KUM RIVERBED DUNES':' PLAIN KALAHARI No,ofSpp, at Bait 0 B S 0 B S 0 B 0 0 B S MORNING 0 93 57 71 0 0 29 100 40 70 1 7 43 30 80 82 71 0 60 10 2 0 0 0 20 12 0 0 0 20 3 0 0 0 0 6 0 0 0 0 NOON/AFTERNOON 0 99 75 84 5 6 18 80 100 40 8 0 83 1 1 25 16 60 61 59 20 0 50 40 73 17 2 0 0 0 25 33 24 0 0 10 27 17 0 Figure 1. Relatil 3 0 0 0 10 0 0 0 0 0 10 7 0 Kalahari, and K 4 0 0 0 0 0 0 0 0 0 3 3 0 slight differences 5 0 0 0 0 0 0 0 0 0 3 0 0 with the point sl 6 0 0 0 0 0 0 0 0 0 7 0 0 species. It appe: 7 0 0 0 0 0 0 0 0 0 2 0 0 individual foodit EVENINGINIGHT 0 97 55 72 10 9 25 100 70 10 There are pr 1 3 41 28 84 91 71 0 20 90 an asymptote WE 2 0 4 0 3 0 4 0 10 0 time (Fig. 1). 01 3 0 0 0 3 0 0 0 0 0 habitats of the p the assemblage I ':'Bare dunes (baits onlyin open)and vegetated dunes (baits onlyat base ofshrub) respectively species may taki contribute to an In the Kalahari, multiple occupancy of baits seemed to be more frequent in the to observe baits c open than at the base of shrubs, both of which had more frequent multiple occupancy than baits in shrubs (Table 3). At the other sites multiple occupancy usually was Equitability higher at the base of shrubs than in the open or in shrubs. The number of species interacting at any point would be expected to increase with In the presei an increase in the number of species available for interaction. Thus, one would expect apportionment 0 that at the sites with higher species-richness, point species-richness would also resources (baits) increase. This was verified by the present study. There was an increase in the In the Nami maximum number of species recorded together at baits as realized ex species-richness Lepisiota capensi increased (Fig. 1). Ignoring slight diel and inter-habitat differences, the mean number Two other specir of species observed per bait only increased up to an asymptote of about five species, large numbers of Ant Assemblages in the Namib, Kalahari and Kara-Kum. deserts 311

8 the dry bed of the ---- MAXIMUM POIJ'IIT SPP.OF..s-RlcnN~S co-occurring at a ----*- MEAN SPECIES PER ATl1;:NUI;:O DAIT .e observed there 7 .J --<>-- M1W< SPIlClE5 PBI. BArr an in the sparser

6

ant species in the ~ 5 he open; B =: baits . Dash indicates ~ !:3 4 ~ <'-l \[ KALAHARI ~ 3 o B S 2 2

ol~ I I I I , i i o 2 4 6 8 10 12 14 16 18 8 0 83 40 73 17 REALIZED ALPHA SPECIES-RICHNESS 27 17 0 Figure 1. Relation of point species-richness to realized ex species-richness in the Namib, 10 7 0 Kalahari, and Kara-Kum deserts. Calculation of mean ex species-richness ignored the 3 3 0 slightdifferences among microhabitats and times of day. 3 0 0 with the point species-richness thereafter maintained only slightly greater than one 7 0 0 species. It appears, that regardless of number of species in the habitat in general, 2 0 0 individual food items seldom are harvested by more than a few species. There are probably two reasons fer the apparent point species-richness reaching an asymptote well below the number of species active and available at a particular time (Fig. 1), One is that ecologic segregation, such as found in the more speciose habitats of the present study, separates species in space, such that not all species in the assemblage are equally available at any time or place. Furthermore, dominant hrub)respectively species may take over food sources and drive subordinate species away, and thus contribute to an underestimate of the actual point species-richness. One would need ~frequent in the to observe baits continuously to correct for this artifact. rltiple occupancy ncy usually was Equitability l to increase with In the present study, equitability is not used in the conventional sense of the one would expect apportionment of individuals among species, but rather as the apportionment of ness would also resources (baits) among species. increase in the In the Namib, the scavenger guild was inequitable. In the Kuiseb riverbed species-richness Lepisiota capensis found 75% or more of the baits in all three microhabitats (Fig. 2). he mean number Two other species, Ocymyrmex barbiger and Monomorium delagoense, also found iout five species, large numbers of baits in the open and at the base of shrubs but negligible numbers 312 Heatwole Ant}

100 • BArn [N TIlE OPEN

IS BAm AT TIm BASES OF SHRUBS OR TREES 80 o DAm IN SI(RliIIS UR TREES

en !:: -< 60 IXl u... 0

ffi 40 ~ ~

20

0 2 •.. ., .... '" e S .':l '" '" bl) 2 0 ;;'" ~" .;:) " .~" = ~ ~ >< "" ., ., ," a 'v; "§ 0: "0 0" !-" 0< 's. :-, -e o .s .s. 0 E! ...l EO ~ .~ " e'", .".. :-, 0 >. '.> <= -" "0" S 5 0 0 "'" S '" a u '" 5 c '-' ~ -< 0 Figure 2. Equitability of ant species at sardine baits in different microhabitats in the dry riverbed of the Kuiseb River, Namib Desert. in shrubs. All other species were poorly represented in all microhabitats. On the plain the same three species still represented the top three species in the scavenger guild (Fig. 3), but the ranks of Lepisiota and Monomorium were transposed. There were too few stations in the vegetated dunes for meaningful analysis of equitability; however, Figure 3. Equitabi both Monomorium and Ocymyrmex were represented there, although even more baits (interdune valley) were found by Pheidole tenuinodis, a species that was infrequent or absent in the this assemblage, bi other two habitats. the lowest-rankin The ant assemblage in the Kalahari was highly inequitable. Only a few species shrubs (Fig. 4). Te were found at a large number of baits, with most species being represented at onlya the second-ranked few (Fig. 4). Crematogaster castanea was clearly the most common species as in all species. All specie three microhabitats it occurred at a higher proportion of the baits than any other baits in all micr species. It was found at 65% of the baits in the open, at 90% of those at the base of participants in gui shrubs and at 15% of those in shrubs. Species in this genus commonly nest in trees In the Kara-K and often forage arboreally. However, in the study area C. castanea nested in dead baits associated w wood on the ground, a habit that gave it easy access to all three microhabitats. The compared to 15%( ranking of species after C. castanea varied with microhabitat. In the open, in the open. Anoplolepis trimenii occupied the second-highest proportion of baits (40%), but was Ant Assemblages in the Namib, Kalahari and Karo-Kuni deserts 313

100

OR TREES

80

~

~ 40 ~ cUJ,

20 --r-.;:: i! u ;"j '0 ~ 0 . 0 :c o .. c.. ;;1 6 «S «S -<.) s-, ~.." .Q <.i :.> -;;) ~ o 8 -e >< q,) .0.'".. S (/) '";:I ohabitats in the dry 8 .. '0., •.. >. 0 :5 ....0l .. >S. =0 0 Q, iitats. On the plain 8 <.i 0 0 osa r:: he scavenger guild 0 u led. There were too ~ ritability; however, Figure 3. Equitability of ant species at sardine baits in different microhabitats on a plain gh even more baits (interdunevalley) in the Namib Desert. Note that an additional species was present in It or absent in the thisassemblage, but did not visit sardine baits at this site. the lowest-ranking species at the base of shrubs and was not represented at all in Only a few species shrubs(Fig. 4). Tetramorium gladstonei was the third-ranked species in the open and presented at only a thesecond-ranked one at the base of shrubs and therefore a common ground-foraging in species as in all species.All species, other than these three, were represented at 20% or fewer of the its than any other baits in all microhabitats and thus were either less abundant or were mainly hose at the base of participants in guilds other than the scavenging one. nonly nest in trees In the Kara-Kum, Crematogaster subdentata was the highest ranked in terms of nea nested in dead baits associated with shrubs, finding over 40%. of the baits associated with shrubs, microhabitats. The comparedto 15%. or fewer (Fig. 5) for any other species. No species found many baits tat. In the open, inthe open. tits (40%), but was An 314 Heatw()le

5

100 4 • BAITS IN ll{E OPilN ~ lI."m. ,4T 'llIli BASHOF SHRUBS 80 0 BAITS IN SHRllll5 ~ U'J ~ 3 Eo- CL, ~ 0 a:l 60 ~ i5 z 2 ~ ffi 40 g: ~ If 20

0 ...• ce o'l '0 c, .~ •.. '" ,~ 'C S ::l ,~ ·c 0. '"' '8 " •.. U "C >-. ;:> "C U 5 ';::" r::: «l ~ l) ::l a .., .., '" ;:> o 0.- E c I'>. s 0 ,8 'l) 0 '0 E3 '2'".- >... '" 0 c:: e ~ '0 ;:> II> 'Q}" Q. '" ;::>., a '" ee 0 0 'i)i, «l 0.. " '~ P "C c, ,"" •... E E 00 •• o 0 '0 >. •... 0 ....l e ... 0 '" 0 0 >. C =ct 0 'u a ';;j u •• 0 C .t:: ii: c 6 s U 0 Cl. ..• 0 0 "" 8 Po 0 U'" 8 ~ c >e. ...., Cl. •I..I). .t: 0 u ~ >< ~ e 8 o f-< 0 0= OJ ..• ~ u u

Figure 5. Equi Kara-Kum DeSE climbing up int Figure 4, Equitability of ant species at sardine baits in different microhabitats in the in the open all Kalahari Desert. restricted to tl Monomorium ij Foraging Microhabitat At the veg The various deserts differed in the extent to which foraging occurred in shrubs to a gre association' with vegetation as opposed to the open, Ants were more strongly as in the Kara associated with vegetation in the Kara-Kum than any other desert. There, more baits often than els were visited by ants at the bases of shrubs than anywhere else, followed closely by somewhat man baits in shrubs; both these microhabitats had visitations by ants a whole order of individual spec magnitude higher than baits in the open (Table 3). The only microhabitat in which all four species occurred was at the bases of shrubs, with baits on the open ground and In contrast those in shrubs visited by two species each (Fig, 5). Tetramorium caespitum was the open than found only at the bases of shrubs, avoiding open ground, but at the same time not more often at t Ant Assemblages in the Namib, Kalahari and Kara-Kum deserts 315

50 • BATI'S IN 'i1W omN

IS! BAITS AT TIlE BASES OF SHRUBS

o BAITS UP IN SHRUBS 40 TIlE OPEN

'JlIIi BASil OF SHRUBS

SHRUDS ~ CO 30 u, o ~ z B 20 g0:::

10

I i ~ •'':: • c:i.. 0 ., s ~ S ;> ;; '" S S ~ ~ o ,'.::: -0 ~ ;:I '":I " '" o :0 Q. (5 -.=::l -;;; "0' 0: '0"" .. d ~ «S 0 ~ •.. .=- ''0" t) I'>. '" ..ocl U 0: "0 E "" 8 0 «S 0.. ';;;' '-< ,~ e U 0 S ... ::I C i'i": ~ ., 0 'I: •• I: 6 0 o '"oJl 0 .8 0 6 Ei =0 '.".. "" 0 .... a c ~ 0 0... 0 [-< u ~ Figure 5, Equitability of ant species at sardine baits in different microhabitats in the Kara-Kum Desert, climbing up into shrubs, Monomorium barbatulurri was a ground-forager, being found ihabitats in the in the open and at the base of shrubs, By contrast Crematogaster subdentata was restricted to the vicinity of shrubs, either at their bases or up in the foliage, Only Monomoriurri indicum. was eurytopic; it foraged in all three microhabitats, At the vegetated Namib sites, ants also tended to forage in association with g occurred in shrubs to a great extent, but the differences among microhabitats was not so marked more strongly as in the Kara-Kum. In the Namib, baits at the bases of shrubs were visited more ere, more baits often than elsewhere, but only slightly so, and those in the open were visited wed closely by somewhat more often than those up in shrubs (Table 3), The foraging microhabitats of whole order of individual species are discussed below (see section of Ecologic Segregation), .at in which all en ground and In contrast to the other deserts studied, in the Kalahari more species foraged in zaespitum. was the open than near vegetation; those foraging in association with vegetation did so same time not more often at the base of shrubs than up in the foliage, All species, bar one, from the 316 Heatwole Ant.

transect were recorded from baits in the open, whereas only ten of those species were station. By contr recorded at baits near the bases of shrubs and only two at baits in shrubs (Fig. 4). only 7% ofthe 01 Seven species were found exclusively at baits on the ground in the open, one species In contrast t (Camponotus sp.) was found only at the base of shrubs, and no species was exclusive granivores than to baits in shrubs. Only two species visited baits in all three microhabitats, the (1985a) found a ubiquitous C. castanea and the relatively uncommon Camponotus rufoglaucus Gravel desert n (recorded at 10% or fewer ofthe baits in any microhabitat). reported here. I Trophic Specialization not dominant ir related in part In the Namib habitats a variety of baits, designed to attract all trophic categories, gravel deserts ir was employed and consequently the species-richness of the entire realized ant the gradient of assemblage was treated, as well as its various trophic components. There were no pointed out tha strict granivores as no species fed only or primarily at wheat germ. The only species granivorous ant attracted to wheat germ was Monomorium delagoense. It was the most omnivorous realized ant ass species present and the only one attracted to all three kinds of baits (Table 4). Three absent from the other species were omnivores in that they were attracted to both sardines and treacle. Whitford (1978: Five species were scavengers and only attracted to sardines. The remaining two large guilds ofg species were found foraging near baits but not actually partaking of them. One of by omnivores. them, Camponotus detritus, is known to feed mainly on honeydew secreted by aphids and scale insects but also takes small amounts of pollen, nectar, dead animal material Activity and possibly lizard and bird faeces (Curtis 1982, 1985a, 1990). Although four species were attracted to treacle, it was not the preferred bait of any of them. Clearly, all Activity can species were involved to some extent in the scavenger guild and results are at particular till comparable to those from other areas in which only that guild was addressed. type of activity j

Proportion I Table 4. Occurrence of ants at different baits along the Kuiseb River transect, Namib hottest desert s Desert. "No Bait" means ants were foraging in the vicinity of a bait but not actually noted being onl feeding. + indicates presence frequent during hotter (noon/afti Species Sardine Seeds Treacle No Bait During the Monomorium delagoense + + + depending on VI Ocymyrmex barbiger + + first day, all t Lepisiota capensis + + observation we Anoplolepis steingroeueri + + directly to sun 1 Pheidole tenuinodis + heat of the grou Tapinoma sp. + Camponotus robecchii + day, the mid-d. Ocymyrmex weitzecheri + ground was un Crematograster castanea + exposed baits h Camponotus detritus + baits that had r Camponotus maculatus + In the cooh TOTALSPECIES 9 1 4 2 whole were not Frequency of attendance at baits was greatest for sardines. Eighty-two per cent of combined multi the examinations of individual sardine baits revealed ants of at least one species in than in the mr attendance. The comparable value for treacle was 55% and that for wheat germ only riverbed (TablE 3%. Ants often fed on sardine baits and ignored other types of baits at the same observations th Ant Assemblages in the Namib, Kalahari and Kara-Kum. deserts 317

e species were station.By contrast, ants were present at treacle but not the corresponding sardine in hrubs (Fig. 4). only7%of the observations of ants at treacle. in, one species In contrast to the above results, Chew & De Vita (1980) found greater numbers of was exclusive granivores than omnivores in an ant assemblage in an American desert, and Marsh ohabitats, the (1985a)found a high proportion of granivorous ants in a Namibian gravel desert. s rufoglaucus Gravel desert may have greater numbers of granivores than the dunes and plain reported here. Indeed, Marsh (1990) posed the question: "If granivorous species are not dominant in the dune sea, why?" However, the differences might be weather- related in part and may not be as great as first appears. Marsh's (1985a) study in hic categories, graveldeserts included two periods of rainfall and his study area was farther along on ; realized ant the gradient of productivity where numbers of species were higher. Marsh (1990) I'here were no pointed out that after episodic rain seeds are superabundant and are harvested by le only species granivorous ants. Thus, during the surge of seed production after precipitation the st omnivorous realized ant assemblage in dune areas may contain granivores that are inactive and 'able 4). Three absent from the realized assemblage at other times, such as that ofthe present study. es and treacle. Whitford (1978) pointed out that desert areas with abundant annual plants have 'emaining two large guilds of granivorous ants whereas those with few annual plants are dominated them. One of byomnivores. eted by aphids iimal material Activity rh four species ill. Clearly, all Activity can be expressed in different ways, i.e., (1) the proportion of baits visited d results are at particular times of day, (2) the numbers of species abroad at those times, or (3) the essed. type of activity in which the ants are engaged. These measures are discussed in turn. Proportion of Baits Visited: Visitation at baits by ants in the Kara-Kum, the , Namib hottest desert studied, was low at all times of the diel cycle, the highest occupancy .not actually noted being only 45% (Table 3). The proportion of baits visited was about twice as frequent during the cooler parts of the day (morning, evening/night) than when it was hotter (noon/afternoon). No Bait During the day, baits associated with shrubs had greatly different occupancy depending on whether they were exposed to direct sunshine or not. At noon of the first day, all the baits at the base of shrubs that were shaded at the time of observation were attended by Crematogaster subdentata; none of those exposed directly to sun had attendant ants of any species. Up in shrubs away from the intense heat of the ground, ants were present at both shaded and unshaded baits. The second day, the mid-day reading produced identical results except one shaded bait on the ground was unattended. In the afternoon reading of the transect, four previously exposed baits had become shaded but had not yet attracted any ants, whereas three + baits that had previously been shaded but were now in sun still had ants. + In the cooler Namib, diel differences in visitation by the ant assemblage as a 2 whole were not great and were not always reduced at mid-day. For example, the two per cent of combined multiple and single visitations were more frequent in the middle of the day one species in than in the morning on the plain, and more frequent than in the evening in the aeat germ only riverbed (Table 3). No diel comparisons could be made in the Kalahari because is at the same observations there were made only during the day. 318 Heatwole A

Species Active: In the Namib, the greatest number of species was active at night at this site, in and the second greatest number in the morning, i.e., the coolest times of day. some species ( Afternoon and evening had low numbers of active species (Table 5). The three species that were found active only by day and the three species found active only by night were all rare species observed on only one or a few occasions. Consequently, their Table 6. Kin( activity periods might span a greater part of the diel cycle than Table 4 would baits. Values suggest. Five species were active both at night and during at least one diurnal period. visited, e.g., 1 Two (Anoplolepis and Crematogaster) were active at night and in the morning. each of two Lepisiota and Monomorium were active at all four times of the diel period and Ocymyrmex barbiger was active at all but the morning one. The activity cycles of the last three ?pecies are treated in more detail below (see Ecologic Segregation). TIME OF DAY NAMIBTRAN: Morning Table 5. Occurrence of ants at baits along the Namib Desert transect at different Noon!Afternooi times of day EveninglNight Species Morning Afternoon Evening Night KARA-KUMTl Lepisiota capensi , + + + + Morning Monomorium delagoense + + + + Noon!Aftemooi Ocymyrmex barbiger + + + EveninglNight Anoplolepis steingroeueri + + KALAHARI Crematograster castanea + + Daytime Ocyrnyrmex weitzecheri + Interference 0, Camponotus detritus + Tapinoma sp. + No eviden Pheidole tenuinodis + four species 1 Camponotus robecchii + probably capa Camponotus maculatus + than one SpE TOTAL SPECIES 6 4 3 8 Monomorium. In the Kara-Kum, three of the four species were observed foraging at all times of Crematogastei day; Tetramorium caespitum, the rarest species, was not seen to be active at mid-day In the N~ or afternoon. . baits without Kind of Activity: The kinds of activity and its intensity changed during the die! occasions), LE cycle. In the Namib and Kara-Kum deserts, there was a preponderance of bait, three Sj mobilizations around baits and of feeding and carrying food in the mornings and at together. Ho, night; at only a few baits were ants merely foraging in the area (Table 6). In the abundance of middle of the day, however, mobilizations dropped markedly and there was a genera! species to rea! shift from the higher categories of activity to the lower ones. By night the trend On two oc toward greater numbers of feeding ants and more frequent mobilizations was restored merely forag (Table 6). observations In the Kalahari only daytime values are available. They differ from daytime feeding Mono results in the other deserts. The intensity of activity was bimodal, with many baits feeding, Ocym showing high activity levels (mobilizations) and many others low ones (few ants either Interferer feeding or foraging in the area), but few at intermediate levels (many feeding). Almost guild of terre: all mobilizations were by one species, Crematogaster castanea, whereas the other was observec species usually were present as only a few individuals., either foraging in the area, prevented otl: feeding, or attempting to reach baits around which C. castanea was mobilized. Thus, but upon cont Ant Assemblages in the Namib, Kalahari and Kara-Kum deserts 319 active at night at this site, interspecific interactions may have influenced the intensity of activity of times of day. somespecies (see next section). ~three species only by night quently, their Table 6. Kinds of activities in which ants engaged at different times of day at sardine 'able 4 would baits. Values are in percentage of total ant I baits (number of species x number of baits liurnal period. visited, e.g., 1species at one bait = 1anti bait; two species at one bait, or one species at the morning. each of two baits = 2 ant I baits, etc.). For abbreviations of types of activity, see text el period and y cycles of the --. FEEDING ANDIOR CARRYING ;ion). TIME OF DAY FIAIATT FEW MANY MOB NAMIB TRANSECT Morning 7 34 23 36 'It different Noonl Afternoon 25 45 22 8 EveninglNight 9 30 36 25 Night KARA-KUM TRANSECT + Morning 0 35 30 35 + Noon/Afternoon 3 57 24 16 + EveninglNight 0 13 46 42 + KALAHARI + Daytime 18 41 3 38 Interference Competition + No evidence of interference competition was observed at the Kara-Kum site. All + four species recruited nest mates and mobilized around baits, and therefore are + probably capab I of vigorous defense of food resources. However, only once was more than one species observed at a bait. On a night reading of the transect, one 8 Monomorium. barbatulum. was feeding on a bait also containing a number of feeding t all times of Crematogaster subdentata, e at mid-day In the Namib study area, multiple species were observed feeding at the same baits without fighting. The species-pairs involved were: Monomorium-Lepisiota (7 ring the diel occasions), Lepisiota-Ocymyrmex barbiger (4), Monomorium-Anoplolepis (1). At one nderance of bait, three species (Ocymynnex barbiger, Lepisiota, Monomoriurri) fed peaceably lings and at together. However, on several instances species were excluded from baits by an le 6). In the abundance of another species; although combat was not observed, attempts of one as a general species to reach a bait was thwarted by another species already mobilized there. It the trend On two occasions, Ocymyrmex barbiger was in possession of a bait and Lepisiota vas restored merely foraged in the area, but at another bait the reverse occurred. Other observations (once each) of mixed species at baits involved Ocymyrmex barbiger om daytime feeding Monomorium not; Camponotus robecchii feeding Lepisiota not; Monomorium. many baits feeding, Ocymyrmex weitzecheri not. r ants either Interference seemed to play a somewhat greater role in the structuring of the lng), Almost guild of terrestrially-foraging scavengers in the Kalahari. Although no direct combat s the other was observed, some species controlled baits and mobilized around them and In the area, prevented other species from feeding. Other species sometimes approached the bait lized. Thus, but upon contacting the controlling ants, would back off and approach from a different 320 Heatwole

direction, only to be repelled again. Other species merely foraged in the area of the "attempting bait without overtly attempting to get through the cordon of ants. In some cases, a subordinate small number of ants, of several species, shared a bait peaceably, but as numbers of and M. sub. one species built up others were displaced and the bait was taken over by one species the other s exclusively. Occasionally, a new species would arrive and displace all previous where it wa species. Using these observations, species could be ranked as dominant or species ever subordinate to other species. Less cor The clear dominant of the scavenger guild of the diurnal ant assemblage was short durat Crematogaster castanea (Fig. 6). It was associated at different baits and times with 12 However, oi other species, of which it was dominant to 11 (the relation to the 12th species was not of the pri clear). Furthermore, it had a high incidence of being the sole species at a bait, Crematogas perhaps because of exclusion of potential rivals during the interval between data. Ocym observations. This species laid odour trails, rapidly recruiting nest mates to mobilize dominant t( around food resources. It is likely that this attribute contributes to its ability to species. In t control baits. That is not to say that this species never shared a bait with other and no cleai species. When total numbers of individuals at a bait were low C. castanea was Majer's (19 observed feeding at various times at the same baits as Anoplolepis trimenii, Lepisiota individuals capensis, Meranoplus spininodis, Monomorium. subopacum. and Tetramorium dominate. ( gladstonei. However, as additional members of C. castanea were recruited, the other dominant tl species were displaced and could no longer feed, but rather were relegated to hierarchy, themselves Althoug species wer tenuinodis, recruited llE noted that diffusely, bi sufficient m Althoug Phd

Ecologic See Combat '\. -.;,~ Tttramorlatrl i.e., intersp 7 .blbtolleJ place of fors in the Kara Interspecifir relevant. In Equitability of ecologics studied thr Figure 6. Dominance hierarchy among some ants of the assemblage in the Kalahari delagoense, Desert. Arrows point from the dominant species toward the subordinate one. and eurytop Ant Assemblages in the Namib, Kalahari and Kara-Kum deserts 321 irea of the "attempting" or merely foraging in the vicinity of the bait. On some occasions the Ie cases, a subordinate species left the vicinity of the bait altogether. Two species, T. gladstunei .umbers of and M. subopacum; seemed to coexist longer with C. castanea at baits than most of me species the other species. M. subopacum frequently crawled under the bait and fed there l previous where it was protected from direct contact with other species. However, both these ninant or species eventually were prevented from feeding by C. castanea. Less common species undoubtedly meet each other infrequently, and during the blage was short duration of the present study not all potential interactions were observed. es with 12 However, of the combinations that were observed a summary of relative dominances ss was not of the principal components of the assemblage was constructed (Fig. 6). at a bait, Crematogaster castanea was dominant to all species for which there were adequate between data. Ocymyrmex weitzecheri also was relatively high in the hierarchy and was D mobilize dominant to at least three other species, and known to be intimidated by only two ability to species. In the middle of the hierarchy was a group of species that often shared baits zith other and no clear relationships could be determined among them; these probably accord to anea was Majer's (1972) category of "non-dominant" in regard to each other. Numbers of Lepisiota individuals of these species never built up at baits sufficiently for anyone species to ramorturri dominate. Ocymyrmex picardi was subordinate to three species but in turn was the other dominant to at least one other. Plagiolepus brunni was the lowest species in the egated to hierarchy, and was subordinate to at least five species, some of which were themselves subordinate to one or more other species. Although nearly all mobilizations were by Crematogaster castanea, three other species were observed to mobilize once or a few times each. They were Pheidole tenuinodis, Tetramorium gladstonei, and Ocymynnex ioeitzecheri. The first two recruited nest mates via odour trails; the last was not observed to do so. Marsh (1990) noted that Ocymyrmex species exhibit highly flexible behaviour, usually foraging diffusely, but when confronted with a particularly rewarding food source can recruit hefdak '1'. sufficient numbers to exploit it.

Ph,14lol. Although interaction of species seemed to play a role in the structuring· of the te:a ••• l:oodJI guild of terrestrially-foraging scavengers in the Kalahari, this did not seem to be the .,J,. case in shrubs. There were few species that scavenged arboreally and those that did, j L.pUdot. t:.apcnlil did so infrequently. Baits in that location were seldom found and competition among ants for food in that microhabitat seems unlikely. Ecologic Segregation Combat among sympatric ant species may be reduced by ecological segregation, i.e., interspecific partitioning of resources through differences in time of foraging, place of foraging, or type of preferred food. In the dunes and plain of the Namib, and in the Kara-Kum, the number of species was low and only one species was common. Interspecific interactions were rare and an assessment of ecologic segregation was not relevant. In the Kalahari, there were three relatively common species (see section on Equitability) but only daytime observations were made and the temporal component of ecologic segregation could not be treated. However, the Kuiseb River transect was studied throughout the diel cycle and had three abundant species, Monomorium Kalahari delagoense, Ocymyrmex barbiger, and Lepisiota capensis. They were the most common and eurytopic species of the Namib study area and consequently lend themselves well 322 Heatwole Ant.

to an analysis of resource utilization. Preferred bait (sardines) was used to analyze Riverbed whether there were subtle differences in temporal and spatial niches that might TREE segregate them ecologically. Riverbed In the dry bed of the Kuiseb River and on the plain these species collectively, and NIGHT often individually, occupied a greater proportion of the sardine baits than did all OPEN other species combined (Table 7). Only in the dunes where absolute numbers of baits Dune (0) occupied were low for all species were other species combined more commonly at baits Dune (S) than these three. Ocymyrmex and Lepisiota occupied a greater proportion of baits in Riverbed the dry riverbed than they did on the plain. The same was true for Monomorium on Plain occasions, but at other times it showed greater occupancy on the plain. BASE Riverbed Table 7. Occurrences of ants at sardine baits along a transect through the Kuiseb TREE River, Namib Desert. N = number of baiting stations. (0) indicates section of transect Riverbed on open dune; (S) indicates section of transect on dune with shrubs. There were no observations in the evening in the rocky desert There was I Monomorium an Ocymyrmex Lepisiota Monomorium Other NoAnts and trees, but st Station N No. % No. o/r No. % No % No % with relatively h MORNING Lepisiota wa OPEN proportion of tlu Dune (0) 10 0 0 0 0 0 0 0 0 10 100 only time its occ Dune (S) 5 0 0 0 0 0 0 3 60 2 40 it, was in the aft Riverbed 20 0 0 14 70 8 40 2 10 0 0 and Lepisiota oc Plain 11 0 0 3 27 9 82 0 0 0 0 high proportion I BASE the trunks (Fig. Riverbed 17 0 0 15 88 4 24 1 6 0 0 rather high inte TREE occupied baits h also a high pre Riverbed 16 0 0 12 75 0 0 1 6 3 19 mobilizations WE AFTERNOON was there a high OPEN not feeding at th Dune (0) 10 2 20 0 0 0 0 0 0 8 80 Ocymyrme:x Dune (S) 5 0 0 0 0 0 0 0 0 5 100 afternoon, with I Riverbed 20 15 75 9 45 3 15 0 0 1 5 this species to 36 Plain 11 3 27 3 27 1 9 1 9 4 immobilized by BASE with daily vapor Riverbed 17 6 35 16 94 1 6 0 0 0 0 In the present st TREE large numbers ol 0 0 2 13 Riverbed 16 4 25 14 88 0 0 Monomoriun EVENING occurrence at s: OPEN afternoon and ( Dune (0) 10 0 0 0 0 0 0 0 0 10 100 mobilizations an Dune (S) 5 1 20 0 0 1 20 0 0 4 80 individuals. In 1 Riverbed 13 0 0 11 85 0 0 0 0 2 15 merely foraging BASE numbers) with C Ant Assemblages in the Namib, Kalahari and Kara-Kum deserts 323

ed to analyze Riverbed 10 o o 9 90 o o o o 1 10 .s that might TREE Riverbed 9 o o 6 67 o o o o 3 33 .lectively, and NIGHT than did all OPEN nbers of baits Dune (0) 10 0 0 0 0 0 0 0 0 10 100 nonly at baits Dune (S) 3 0 0 0 0 0 0 2 67 1 33 on of baits in Riverbed 19 1 5 13 68 5 26 2 11 5 26 nornorium on Plain 10 0 0 2 20 7 70 0 0 1 10 BASE Riverbed 14 0 0 12 86 0 0 0 0 2 14 the Kuiseb TREE In of transect Riverbed 14 0 0 12 86 0 0 2 14 3 21 ere were no There was partial spatial segregation of foraging sites in the Kuiseb riverbed. Monomorium and Ocymyrmex occurred mainly in the open and at the bases of shrubs No Ants and trees, but seldom up in them (Fig. 2). Lepisiota occupied all three microhabitats No % with relatively high frequency. Lepisiota was the most abundant and ubiquitous forager. It occupied a greater proportion of the sardine baits than did either of the other two species (Fig. 2). The 10 100 only time its occupancy was below 70% and the only time another species superseded 2 40 it, was in the afternoon in the open; then Ocymyrmex occupied about 75% of the baits 0 0 and Lepisiota occurred at less than 50%. Lepisiota occurred at all times of day at a 0 0 high proportion of the sardine baits in the open, at the base of shrubs or trees, and on the trunks (Fig. 2). Furthermore, at those baits where it occurred it maintained a 0 0 rather high intensity of activity (Fig. 7). At all times of day a high proportion of the occupied baits had few to many ants feeding. In the morning and evening there was 3 19 also a high proportion of the occupied baits with mobilizations of this species; mobilizations were lower in frequency in the afternoon and evening. At no time of day- was there a high frequency of baits at which this species was foraging in the area but not feeding at the bait. 8 80 5 100 Ocymyrmex was a mid-day specialist, being found in abundance only in the afternoon, with negligible occurrences at other times (Fig. 7). Marsh (1985b) indicated 1 5 this species to be a diurnal, thermophilic scavenger that foraged for insects 4 36 immobilized by high temperatures and desiccation; its foraging success increased with daily vapour pressure deficit; maximal foraging activity occurred at about 52'C. 0 0 In the present study it fed in low numbers at most occupied baits but mobilizations or large numbers of feeding individuals were much less frequent. 2 13 Monomorium had almost the opposite patten}. in that it showed greatest occurrence at sardine baits at night and in the morning but lower ones in the afternoon and (at this site) absent in the evening (Fig. 7). It never occurred in 10 100 mobilizations and was usually represented at occupied baits by only a few feeding 4 80 individuals. In the afternoon there was a high frequency of baits at which it was 2 15 merely foraging in the area and not feeding (Fig. 7). It overlapped (in reduced numbers) with Ocymyrmex only in the afternoon period. 324 Heatwole Ant A

surface examined By contrast, sh 80 liO EVENING comprehensively the Kara-Kum, th 60 60 but those associat

"

40 40 Table 8. Conu Namib, Kalahar shrubs; S = 1met 20 20 ~ baits that u >Sl~ ~-<-i 0 SitelMicrohabitat o > '.> __ •••••• >li\ilH:l flA ~ KARA-KUM 80 o NIGHT 8 0 AFTERNCx::t4 B S 60 60 NAMIB r-r- Bare Dunes 40 40 o , I VegetatedDune 20 B 20 , I Plain o ~ I o o Kuiseb Riverbei ACTIVITY o B Figure 7. Frequency of various activities by ants at baits at different times of day in the S Namib Desert. FIA = foraging in area, but not feeding. KALAHARI Thus, although there was some overlap, the species were segregated to some o extent in ways that probably obviated, or at least diminished, interference with each B other. Whitford & Ettershank (1975) also found differences in diel pattern in foraging S* among ants in an American desert. ':'may be an undere Comprehensiveness of Foraging Non-ant Taxa Att A great proportion of the earth's land surface is examined on a daily basis by foraging ants. The extent of such coverage varies from one habitat to another and can Incidence of 1 be estimated by the proportion of baits that are visited by ants at some time or other majority were bel during a complete diel cycle. flies (and on one I to sardines locate In the Kalahari and in the Namib plain and dry bed ofthe Kuiseb River all baits also came to whe: on the ground (open and base of shrubs) were found by at least one species of ant trees were found sometime during the total period of observations (Table 8), and values were even high during individual observation periods (Table 3). Thus, on a daily basis, the entire crickets feeding ( ground surface of these sites are examined by foraging ants of one species or another, observations of n and few food resources would be overlooked for long. The proportion of the ground ants. No aggressi Ant Assemblages in the Namib, Kalahari and Kara-Kum deserts 325

surface examined by ants in the dunes of the Namib was low, even in vegetated parts. By contrast, shrubs in the Kalahari and the Kuiseb River were not so comprehensively examined by ants and some were left undiscovered altogether. In the Kara-Kum, the pattern was different, with few baits ever being found in the open, but those associated with shrubs being examined more comprehensively (Table 8).

Table 8. Comprehensiveness of foraging by ants in various microhabitats in the Namib, Kalahari and Kara Kum deserts. 0 = on ground in the open; B = at bases of shrubs; S = 1metre up in shrubs. Comprehensiveness is expressed as the percentage of baits that was found by at least one species of ant during the study period.

SitelMicrohabitat Comprehensiveness offoraging lM''lilllHll ~ KARA-KUM o 10 B 69 S 45 NAMIB Bare Dunes o 20 Vegetated Dunes B 60 Plain o 100 ~-~ Kuiseb Riverbed TY o 100 B 100 nes of day in the S 94 KALAHARI egated to some o 100 rence with each B 100 tern in foraging S" 35

* may be an underestimate as observations were made only in the daytime Non-ant Taxa Attracted to Baits daily basis by .nother and can Incidence of taxa other than ants at baits was low in the Namib (Table 9). The Ietime or other majority were beetles, especially tenebrionids. The only other taxa were crickets and flies (and on one occasion a silverfish present but not feeding). Beetles were attracted to sardines located in the open in all habitats but the bare dunes; in the riverbed they I River all baits also came to wheat germ in the open and at the base of trees and shrubs. Few baits in ! species of ant were even high trees were found by non-ant taxa, and treacle was seldom visited. An exception was isis, the entire crickets feeding on treacle in trees at night in the riverbed (Table 9). At 63% of the. iies or another, observations of non-ant taxa at baits, they were feeding with one or more species of . of the ground ants. No aggressive interactions between beetles and ants were observed . 326 Heatwole Ani Table 9. Percentage of baits fed upon by taxa other than ants in the Namib Desert. All values refer to beetles unless indicated otherwise

HabitatlBait Morning Afternoon Evening Night In general, : BARE DUNE of vegetation an Sardine 0 0 0 0 method, the Ka Wheat Germ 0 0 0 0 than double th deserts and spa Treacle 0 0 0 0 species) and hal VEGETATED DUNE all. Dry stream! Sardine 40 0 20 0 the Kuiseb Rive Wheat Germ 0 0 0 0 as the small, sh Treacle 0 o - 0 0 shrubs, many of DRY RIVERBED Sardines Table 10. Real Open 5 10 8 20 Base 6 5 0 12** Tree 0 5* 0 0

Wheat Germ Desert Vegel Open 0 10 0 20 Kalahari Shrul Base 12 0 0 6 Namib Dry I Tree 0 5 0 0 Arabian Dune Treacle Kara-Kum Dune Open 0 0 0 0 Namib Dune Base 6 0 0 0 Namib Span Tree 0 0 0 19**" Atacama Drys FLAT PLAIN Sahara Salt t Sardines 0 18 9 Namib Dune, Wheat Germ 0 0 9 Atacama Rock~ Treacle 0 0 9 ':'flies "'* includes crickets *** mainly crickets These result baited transects. In the Kalahari study area, only once was a species other than an ant observed at sparsely vegetat: a bait: an orthopteran up in a shrub. Twice, baits in the open were missing and had to season and yea: be replenished; perhaps they had been taken by a non-ant species. (Heatwole & Mu (but not always Scavenging species other than ants also occurred at baits at the Kara-Kum site. vegetated habita During morning readings of the transect, flies occurred at an average of 10.5% of the Rojas & Fragoso baits in the open and at 5% of those at the base of shrubs. Corresponding values for found total CJ.. SpE the mid-day transects were 11% (open) and 16% (base). At the afternoon reading sites and from IJ there were flies on 10% of the baits in the open and on 4% of those at the base. In species-richness addition, one spider was observed (but not feeding) at a bait at the base of a shrub doubtless would I during the afternoon and tracks indicated a beetle had attended one bait during one of the mornings. No animals other than ants attended baits either in shrubs or at any Marsh (1986 location at night. On no occasion did ants and other kinds of scavengers occur February and A} simultaneously at baits. trophic categorie: species-richness ~ of ~ species-richr Ant Assemblages in the Namib, Kalahari and Kara-Kum deserts 327 '[amibDesert. All General Discussion

Night In general, species-richness of the scavenger guild of ants correlates with amount ofvegetation and severity of conditions. Among studies employing the baited transect o method, the Kalahari shrub desert had the highest realized species-richness, more o than double that of the next richest site (Table 10). Vegetated dunes in various o deserts and sparsely vegetated Namib plain all had similar, intermediate values (4-8 species)and habitats devoid of vegetation had either only a single species or none at all.Dry streambeds varied depending on their size and vegetation. The large bed of o theKuiseb River with its abundant trees and shrubs had four times as many species o asthe small, shallow gullies of the Atacama which had only low, sparsely distributed o shrubs,many of which were dead (Heatwole, 1996).

Table 10. Realized a species-richness of various desertic habitats investigated by the 20 baited-transect method 12** o a species- Desert Vegetation/Habitat richness Authority 20 Kalahari Shrub desert 17 Present study 6 Namib Dry bed of Kuiseb River (trees & shrubs) 8 Present study o Arabian Dunes with shrubs 8 Heatwole (1991) Kara-Kum Dunes with shrubs 4 Present study o Namib Dunes with sparse shrubs 5 Present study o Namib Sparsely vegetated plain 5 Present study 19*** Atacama Dry stream bed with sparse small shrubs 2 Heatwole (1996) Sahara Salt bed 1 Heatwole (1996) 9 Namib Dunes without vegetation 1 Present study 9 Atacama RockY..£lain o Heatwole (1996) 9 These results accord with those obtained by a variety of methods other than baitedtransects. For example, the a realized species-richness of an ant assemblage on . ant observed at sparsely vegetated dunes of the preSaharan steppe ranged from 4 to 9 depending on ssing and had to season and year (Heatwole & Muir, 1989) for a total a species-richness of 14 (Heatwole & Muir, 1991). The steppe had 7 species active in each of three summers (but not always the same 7). Thus, its species-richness resembled that of similarly Kara-Kum site. vegetated habitats from the present study (Table 10). In the Chihuahuan Desert, ~of 10.5% of the Rojas& Fragoso (2000), using a combination of pitfall trapping and hand-collecting, nding values for foundtotal a species-richness to vary from 11 to 13 (mean 12) among three grassy ternoon reading sites and from 11 to 21 species (mean 16) among eight shrubby ones, out of a total ~ at the base. In species-richness of 32 species. Realized a species-richnesses at particular times base of a shrub doubtlesswould have been somewhat lower. bait during one shrubs or at any Marsh (1986a) found that the realized ~ species-richness for the months of .avengers occur February and April (the same season as the present study in the Namib) for all trophic categories of ants in Namibian gravel desert was 27 species. The realized a species-richness at individual sites gradually increased from 6 to 22 species (22-82% of ~ species-richness) with further distance inland along a gradient of increasing 328 Heatwole representation productivity, structural complexity of the vegetation and diversity of plants, seeds segregation by and insects. Although some species were present at many sites, the mix of species dominance hit differed at each one. His values included six samplings over two months and species. Immr consequently may have included some temporal replacements. In gravel desert at structure of th Ganab, Marsh (1985) reported a total species-richness of 13 (17-month period). including at OJ Marsh's values cover a range of conditions and accordingly span the values presented hottest desert in table 10. Although it is difficult to assess where along the spectrum of vegetation with shrubs, w these various localities belong, it would appear that the gravel deserts of the Namib The four s might have greater species-richnesses than one would predict from vegetation cover different impo alone. abundant ant The proportion of the realized ex species-richness at anyone time to the total ex there. By conti species-richness requires study over extended periods encompassing all seasons, and dunes, where i comparison with the ~ species-richness requires a complete faunal list for all habitats ranked ninth i: in the region. These requirements were not fulfilled for all study areas. However, In the Namib i some information can be gleaned for some ofthem. and third ranJ Twenty-seven species have been listed from the vicinity of Repetek in the habitats. Pheic Kara-Kum (Krivohatskii, 1985); accordingly, the 4 species from the present transects of baits it visit would represent 15% of ~. However, this is probably an underestimate as the (Table 2) or b: complete list contains some synonyms, human commensals, and some species that vegetated dum may no longer be present (Mishagina, personal communication). The species on the ranked sevent total list came from a variety of habitats in addition to the one transected in the species there. ] present study. General collecting in the small dunes did not reveal any species in the fewest vis addition to the four species obtained in the transect. However, additional species (also roles in differe present on Krivohatskii's list) were collected around the buildings of the Biological insignificant il Station and nearby at South Valley, a moister, more shaded area of tall trees at the different locati base oflarge dunes. species should and functionin, The regional list of native ants from the Namib Desert contains 37 species (Marsh, 1986b, 1990). Of these, .33 occur in gravel desert (not included in the present There has study) but only 13 occur in the dune sea despite the fact that within the dune seas are abiotic factors interdune valleys (the plain of the present study) that resemble gravel deserts. Thus, prevailing ther the realized ex value of five species for the Namib dunes and five for the plain each the harsher de represents 39% of the total species recorded from that habitat. In the Chihuahuan conditions rest Desert, Rojas & Fragoso (2000) found that total ex species-richness averaged 38% of Heatwole, 199: total ~ species-richness at grassy sites and 50% at shrubby ones. Namib display Overlap of diet Morton (1993) reviewed the literature on species-richness of desert ants and varied greatly. concluded that Australian deserts were more speciose by an order of magnitude (ca. importance in 1 2000 species), especially in predators, scavengers, and feeders on sweet materials, aggression wa than American ones (161 species) and probably most other deserts of the world, and deserts. In the that this generally high biodiversity was reflected in higher local (total ex) species- importance of richnesses. By contrast, granivorous ants had similar total ex species-richness in all The least veg deserts. (Kara-Kum) di In the present study, equitability varied among the different deserts along a heavily vegeta sequence of increasing vegetation. All assemblages were inequitable with a few access to baits species being abundant and a greater number being less common. However, in the and/or a domir less densely vegetated, and less speciose sites, only one species occupied the majority applicable to a of baits, whereas in the more densely vegetated sites three species shared high Ant Assemblages in the Namib, Kalahari and Kara-Kum deserts 329

representation. In the latter case, those three species either showed ecological of plants, seeds segregation by microhabitat or time of activity (Kuiseb River), or displayed a clear ie mix of species dominance hierarchy (Kalahari) that influenced distribution of resources among .wo months and species. Immediate environmental conditions probably influenced the temporal gravel desert at structure of the assemblages. At most sites, foraging was more intense on the ground, 7-month period). including at open sites, with fewer baits visited in shrubs or trees. However, in the values presented hottest desert (Kara-Kum) the reverse occurred and most foraging was associated urn of vegetation withshrubs, with little occurring on open ground. irts of the Namib The four species shared between the Namib and Kalahari deserts had greatly vegetation cover different importances in the two locations. Crematogaster castanea was the most abundant ant at the Kalahari site and it dominated the rest of the ant assemblage me to the total a. there. By contrast, at the Namib site, it was found only in one habitat, the vegetated r all seasons, and dunes,where it was relatively rare. Lepisiota capensis showed the opposite pattern. It st for all habitats ranked ninth in the Kalahari and was not known to dominate any other species there. areas. However, In the Namib it was the most common visitor to sardine baits in the Kuiseb riverbed and third rank of four species on the plain; it was not found in the other Namib Repetek in the habitats. Pheidole tenuinodis ranked 14th out of 17 species in terms of the proportion present transects ofbaits it visited in the Kalahari (Fig. 4). In the Namib, it did not occur on the plain (Table2) or bare dunes, and only infrequently in the riverbed (Fig. 2); however, on 'estimate as the orne species that vegetated dunes it visited more baits than any other species. Ocymyrmex weitzeckeri ranked seventh of 17 species in the Kalahari and dominated at least four other ae species on the speciesthere. In the Namib it occurred only in the dry riverbed where it accounted for .ransected in the the fewest visitations at baits. Clearly, particular species may play very different al any species in roles in different habitats, ranging from dominant and abundant in one, to relatively onal species (also insignificant in others. Detailed, long-term, comparative studies of such species at of the Biological different locations, under a variety of physical conditions, and in different matrices of f tall trees at the species should provide valuable insights into the factors determining the structure andfunctioning of assemblages. rtains 37 species .ed in the present There has been considerable controversy as to the relative roles of biotic versus the dune seas are abiotic factors as determinants of community structure and function. One of the fel deserts. Thus, prevailing themes is that biotic factors such as competition are of little significance in or the plain each the harsher deserts, but become operative in the milder ones, particularly if those conditions result in greater productivity and more stable conditions (see review by the Chihuahuan Heatwole, 1995). Marsh (1985a) found that the ants of a barren gravel plain in the averaged 38% of Namib displayed considerable variation in diet and without a consistent pattern. Overlap of diet among species at a given time, and within a species through time, also desert ants and varied greatly. He concluded that competition for food was not likely to be of major of magnitude (ca. importance in this assemblage. Davidson (1977) similarly concluded that interspecific sweet materials, aggression was unimportant in structuring some ant assemblages in American of the world, and deserts. In the present study, the results were consistent with the hypothesis that (total a.) species- importance of biotic interactions are inversely related to environmental harshness. es-richness in all The least vegetated habitats (Namib dunes and plain) and the hottest one (Kara-Kum) displayed no evidence of interference. By contrast, at the milder, more ; deserts along a heavily vegetated sites, some species were seen to unsuccessfully attempt to gain .able with a few access to baits surrounded by ants of another species (Kalahari and Kuiseb River), However, in the and/or a dominance hierarchy was evident (Kalahari). The hypothesis may not be pied the majority applicable to all arid situations, however. For example, Heatwole (1996) observed cies shared high 330 Heatwole.

combat between the only two species occupying a hyperarid site in the Atacama Desert. Donnelly, D. & Formicidael Holldobbler & Wilson (1990) reviewed the dominance-impoverishment rule that Society of S( "the fewer the ant species in a community [assemblage], the more likely the FitzSimons, V. F community [assemblage] is to be dominated behaviorally by one or a few species with large aggressive colonies that maintain absolute territories". They suggested that Goudie, A. (197 rather than dominate species impoverishing assemblages, the reverse was the case, . Central Nar i.e., conditions (such as harshness) that cause low species-richness also promote Hamilton, W. J. development of aggressive dominance. They also noted that the rule did not hold in unguiculari:

some terrestrial situations. In the present study, the smaller realized assemblages Heatwole, H. I had only one common species, whereas the more speciose ones had several common d'Entomolog ones, thus supporting the rule. However, in none of the assemblages did any species Heatwole, H. (H maintain absolute territories, and contrary to the "rule" it was the more speciose Journal ofA assemblages that showed the greatest degree of interference. Heatwole, H. (19 Heatwole, H. (19 Acknowledgments and the Cho' I am grateful to Dr. Mary Seely, Director of the Namib Desert Research Station, Heatwole, H. & • and to the Directorate of the Repetek Biological Station for access to the facilities of Tunisia. J under their charge, to Dr. A. J. Prins of the South African Museum, CapeTown, and Heatwole, H. & 1 Ms. Jeanne V. Mishagina, Repetek Desert Biological Station, for identification of steppe ofTUJ voucher specimens, to Dr. Clarke Scholtz for many courtesies, and for providing Holldobler, B. & laboratory facilities at the Department of Entomology, University of Pretoria, and to University. 'i my wife Audry for assistance in the field in Namibia. The project was supported by Holm, E. & Schol the Internal Research Funds ofthe University of New England (Australia), the North Gobabeb. Me Carolina Agricultural Research Service of North Carolina State University (USA), Krivohatskii, V. and the author's private funds. National ACE Majer, J. D. (197 References 62: 151-160. Marsh, A (1982). Bolton, B. (1995). A New General Catalogue of the Ants of the World. Cambridge: Harvard Universitv Press. 504 pp. . MuseumBul Marsh, A. C. (H Chew, R. M. & De Vita, J. (1980). Foraging characteristics of a desert ant assemblage: community. i . functional morphology and species separation. Journal of Arid Environments 3: 75-83. Marsh, A. C. (19 Curtis, B. (1982). Camponotus detritus. Namib Bulletin, supplement to the Transvaal Museum Bulletin No.4: 9 thermophilic Marsh, A. C. (H Curtis, B. (1985a). The dietary spectrum of the Namib Desert Dune ant Camponotus detritus. .Insectes Sociaux, 32: 78-85. Journal of Al Curtis; B. A. (1985bl. Observations on the natural history and behaviour of the dune ant, Marsh, A. C. (191 Camponotus detritus Emery, in the central Namib Desert. Madoqua 14: 279-289. Desert. Made Curtis, B. A. (1990). Behaviour and ecophysiology of the Namib Dune ant, Camponotus detritus Marsh, A. C. ( (: Formicidae), Transvaal Museum Monograph No.7: 129-133. Environment Curtis, B. A. & Seely, M. K. (1987). Effect of an environmental gradient upon the distribution Marsh, A. C. W and abundance of the dune ant, Camponotus detritus, in the central Namib Desert. Monograph b Journal of Arid Environments 13: 259-266. Morton, S. R. (15 Davidson, D. W. (1977). Species diversity and community organization in desert seed-eating Ricklefs, R. ants. Ecology 58: 711-724. 159-169. Chic Ant Assemblages in the Namib, Kalahari and Kara-Kum. deserts 331

III the Atacama Donnelly, D. & Giliomee, J. H. (1985). Community structure of epigaeic ants (Hymenoptera: Formicidae) in fynbos vegetation in the Jonkershoek Valley. Journal of the Entomological shment rule that Society of Southern Africa 48: 247-257. more likely the FitzSimons, V. F. M. (1964). Life in an Ancient Desert. New Scientist No. 377: 340-343. few species with Goudie, A. (1972). Climate, weathering, crust formation, dunes, and fluvial features of the V suggested that , Central Namib Desert, near Gobabeb, South West Africa, Madoqua (Series 2), 1: 15-31. .'se was the case, .ss also promote Hamilton, W, J. III & Seely, M. K (1976), Fog basking by the Namib Desert beetle, Onymacris e did not hold in unguicularis. Nature, 262: 284-285, zed assemblages Heatwole, H. (1989), Changes in ant assemblages across an arctic treeline, Revue several common d'Entomologie du Quebec, 34: 10-22. s did any species Heatwole, H, (1991), The ant assemblage of a sand-dune desert in the United Arab Emirates. 1e more speciose Journal of Arid Environments, 21: 71-79. Heatwole,H. (1995). Energetics of Desert Invertebrates. Berlin: Springer Verlag. 266 pp. Heatwole, H. (1996). Ant assemblages at their dry limits: the northern Atacama Desert, Peru, and the Chott El Djerid, Tunisia, Journal of Arid Environments, 33: 449-456, & lesearch Station, Heatwole, H. Muir, R. (1989). Seasonal and daily activity of ants in the pre-Saharan steppe of Tunisia. Journal of Arid Environments, 16: 49-67. r to the facilities Cape Town, and Heatwole, H. & Muir, R. (1991). Foraging, abundance and biomass of ants in the pre-Saharan identification of steppe of Tunisia. Journal of Arid Environments, 21: 337-350. ad for providing Holldobler, B. & Wilson, E. O. (1990). The Ants. Cambridge: The Belknap Press of Harvard .Pretoria, and to University. 732 pp. ras supported by Holm,E. & Scholtz, C, H. (1980). Structure and pattern ofthe Namib Desert dune ecosystem at .ralia), the North Gobabeb. Madoqua 12: 3-39. niversity (USA), Krivohatskii, V. A. (1985). The Insects of Repetek, List of Species [In Russian]. Ashgabad: National Academy of Turkmenistan SSR. 71 pp. Majer, J. D. (1972). The ant mosaic in cocoa farms. Bulletin of Entomological Research 62: 151-160. Marsh, A (1982). The ants of the Namib Desert, Namib Bulletin, supplement to the Transvaal mbridge: Harvard Museum Bulletin No, 4: 7-8. Marsh, A. C, (1985a), Forager abundance and dietary relationships in a Namib Desert ant ; ant assemblage: community. South African Journal of Zoology, 20: 197-203. ents 3: 75-83. Marsh, A. C. (1985b). Microclimatic factors influencing foraging patterns and success of the "ransuaal Museum thermophilic desert ant, Ocymyrmex barbiger. Insectes Sociaux 32: 286-296. Marsh, A. C. (1986a). Ant species richness along a climatic gradient in the Namib Desert. mponotus detritus. Journal of Arid Environments 11: 235-241.

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Deserts a upon water a rainfall and p: patchy. An iJ patchiness of Patches differ and the abioti vegetation pat the fauna, clii desert eCOSYSl effects of faur