Namib Ecology 25 Years of Namib Research

~~~ o Desert Research Foundation of Namibia

Gobabeb Libraru

Edited by

M. K. Seely

Oesert Ecological Research Unit of Namibia, Gobabeb, Namibia

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December 1990 Desember

Transvaal Museum Monograph No.7

Transvaal Museum Monografie Nr. 7

Transvaal Museum, Pretoria Food and Habitat Use by Three Tenebrionid (Coleoptera) in a Riparian Desert Environment

S. A. Hanrahan 1 & M. K. Seell 1Oepartment of Zoology, University of the Witwatersrand, Johannesburg, 2050 South Africa 20esert Ecological Research Unit of Namibia, P. 0. Box 1592, Swakopmund, 9000 Namibia

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

INTRODUCTION 1977; Penrith, 1979; Wharton and Seely, 1982). This paper investigates the diet of the three most common Desert environments generally have abundant detritus with tenebrionid species in the Kuiseb riverine environment by: 1) relatively slow or variable rates of decomposition (Crawford examining crop contents to directly determine what food was and Gosz, 1982). As decomposition by bacteria and fungi is eaten, a method not often used by workers in this field; 2) using limited by the pervading aridity, macrodetritivores are relatively pit-fall traps to establish any possible relationships between important to nutrient and energy cycling (Crawford and Taylor, selected food and habitat in this environment. 1984). Endogenous enzymes (Marcuzzi and Lafisca, 1977) and symbiotic gut fauna (Crawford and Taylor, 1984) enable MATERIALS AND METHODS rnacrodetritivores to use detritus effectively. One group of purported macrodetritivores, tenebrionid bee- The study area was located in the Kuiseb River course near tles, are a numerically important component of the biota of the the Namib Research Institute (230 34' S, 150 03' E). Namib Desert (Koch, 1962). Within the riparian woodland of Cladoraphis spinosa (L.fil.) S. M. Phillips is the dominant grass the Kuiseb River, three species of adesmine tenebrionid bee- of the river-bed. The perennial woody vegetation of the flood tles are among the most common and conspicuous compo- plain is dominated by Acacia albida Del. and A. erioloba E. nents of the diurnal epigeaic fauna (Wharton and Seely, 1982): Meyer (Seely, Buskirk, Hamilton and Dixon, 1981). Because Solier, Physadesmia globosa (Haag) of the lack of pronounced seasonal change in climate and (Penrith, 1979) and Onymacris rugatipennis rugatipennis water availability, trees and grasses growing in the river course (Haag) (Penrith, 1975). produce leaves, flowers and fruit throughout most of the year, Distinctive activity patterns with temporal and spatial differ- contributing substantially to surface litter. This plant material, ences have been documented for the more common species together with other organic debris, constitutes the detritus that of tenebrionid beetles living in the lower Kuiseb River and accumulates in the environment. adjacent gravel plains (Wharton and Seely, 1982). Despite the To examine food consumed by the three tenebrionid apparent importance of food in determining the density and the species, ten individuals of each species were collected by habitats used by these species, little work has been directed hand in the field in September and December 1983 and March toward examining the types of food consumed. Holm and and June 1984. Beetles were immediately placed on ice in the Scholtz (1980) stated that 'virtually all desert are field and crops were removed within three hours. Crops were euriphagous opportunists' whereas Wharton and Seely (1982) fixed in Pam pie's fluid and stored in 70 % alcohol. described desert tenebrionids as 'opportunistic omnivores' Reference slides were made by dissecting fresh and dry and stated that 'most of the adesmines ... were observed to plant material from species likely to be present in detritus. feed on remains and faeces while foraging for plant Samples were processed in a Waring blender and mounted in material'. A number of authors have observed the three Hertwig and Hoyer's solution (Bear and Hansen, 1966). species under consideration, as well as other tenebrionids, To analyse the food consumed, crops were severed at the foraging near or under particular riverine plant species and base of the oesophagus and at the cardiac valve. Total crop have assumed that the beetles were feeding on locally derived contents were washed into 5 ml of water and spread evenly in detritus (Holm and Edney, 1973; Penrith, 1975; Hamilton, a Petri dish (53 mm diameter) containing a grid of 2 x 2 mm Buskirk and Buskirk, 1976; Hamilton and Penrith, 1977; Roer, units. Presence or absence of animal material, plant material,

Hanrahan, S A. and Seely, M. K., 1990. Food and habitat use by three tenebrionid beetles (Coleoptera) in a riparian desert environment. In: Seely, M. K., ed., Namib ecology: 25 years of Namib research, pp. 143-147. Transvaal Museum Monograph No.7, Transvaal Museum, Pretoria. 144 FOOD & HABITAT USE BY TENEBRIONIDS

o ~ fLOWERS l?1J FLOWER IITII UNKNOWN

• LEAVES III SAND

P, Or S, P, OrS, P, OrS, P, Or S, 10 II 10 10 10 II II 1010 10 108 10 n UNKNOWN

80 • SAND

Fig. 1 Percentage of flower parts, leaves, sand and unidentifiable material available in detritus under Acacia a/bida (Aa), Acacia etiotobe (Ae) 20 and C/adoraphis spinosa (Cs).

sand or unidentifiable fragments were recorded for 100 SEP DEe lLAR JUN 2 x 2 mm units across 5 rows of the grid. To determine relative availability of the four major crop Fig. 2 components in detritus found under three major plant species, three samples of surface detritus (0-5 mm) from beneath each Percentage of crop contents from Physadesmia g/obosa (Pg), Onymacris rugatipennis (Or) and Stenocara gracilipes (Sg) which plant species were spread in a thin layer in a Petri dish and the consisted of insect remains, flower parts, unidentifiable material and percentage occurrence in four quadrats was estimated (Craw- sand. Number of crops examined from each species during each ford, Hanrahan and Seely, this volume). month is indicated above the bars. To examine habitat use by the three tenebrionid species, twenty pit traps (150 mm diameter, 240 mm deep) were set out: five placed in a region dominated by A. a/bida, ten placed in areas dominated by A. erlolobe (5 north and 5 south of the and unknown material were estimated from samples taken river course); five placed near clumps of C. spinosa where beneath Acacia a/bida, Acacia eriotobe and C/adoraphis Nicotiana g/auca R. Graham also occurred. Trap distribution spinosa (Fig. 1). Small animals and animal fragments, mainly was in proportion to the occurrence of vegetation types. Traps insect material, represented such a small proportion of the were emptied at two or three day intervals, the identity and surface detritus that they were not represented in Fig. 1, numbers of beetles noted, and the beetles released. although closer inspection revealed small associated Statistical analyses followed procedures outlined in Sokal with Acacia flowers. and Rohlf (1981) and Siegel (1956). A symmetrical overlap Plant material identified in crops consisted of flowers index (Pianka, 1973), which ranges from 0 (no overlap) to 1 of A. a/bida, A. eriotobs and C. spinosa, and leaves of Acacia (complete overlap), was used to evaluate overlap of food and species. Anthers and stamens of the flowers were most com- habitat use within and between species. monly eaten and leaf material was rarely consumed. The animal material in beetle crops consisted almost exclusively of RESULTS insect fragments, mainly unrecognizable except as parts of insect cuticle. Ant and dipteran and lepidopteran larvae Detritus samples taken from beneath individual plants were remains were occasionally identifiable to order. The few whole found to be derived predominantly from plants in that immedi- insects consumed were probably inadvertently eaten because ate vicinity and did not consist of a random admixture of of their association with flower matter. Sand was also found in detritus from all plant species growing in the riverine habitat. the crops, but it is not known if it was consumed inadvertently All plant material in the detritus had wind-blown dust and sand or is involved in digestion. All three tenebrionid species had adhering to its surface. Proportions of flowers, leaves, sand mixed diets containing all food items listed. In three oi the four

~-. ------~~ - .. -".J':':"~ • ...••••• FOOD & HABITAT USE BY TENEBRIONIDS 145

20 •• P·llobosa m O.rUlatipenn18 ~ S·lracilipel !WI e ~ :z:••• ~10 III: 110 ""

IUL AUG SEP OCT NOY DEC IAN FED WAR APR WAY IUN 1983 1984-

Fig. 3 Monthly percentages of annual totals of Physadesmia globosa, Onymacris rugatipennis and Stenocara gracilipes captured in 20 pit-traps during 1983 and 1984. o C.spinosa ~ A.albida months in which we examined diet, there was an overwhelm- §11 A.erioloba N ing preference for flower material (Fig. 2). Using a symmetrical overlap index (Pianka, 1973) we found m A.erioloba S a high degree of overlap between the species of food types consumed in all months (Table 1). We also found high overlap Pa Or Sa Pg Or Sa Pa Or Sa Pa Or Sa c:a aD N N values for a single species among seasons (Table 2). The high 00 CD NOlO ~ e- 'ID ""N 'ID'" 'ID overlap values for crop contents among species and months ::: ~ 10 :::2.0 N N C'l N C'l C'l 100 confirm that the beetles feed upon similar types of food most of the time, agreeing with the casual observations of Robinson and Seely (1980). Moreover, all four items found in the beetle crops also occurred in detritus under A. albida, A. erioloba and 60 C. spinosa (Fig. 1), although the proportions differed from those found in the beetle crops.

Individuals of the three tenebrionid species were captured IlQ in every pit trap during every month of the year (Fig. 3). Total ~60 numbers captured were 18 228 P. globosa, 2 346 0. rugati- E-< Z pennis, and 555 S. gracilipes. For all three species the distri- r:iI t.> bution throughout the year was non-uniform (P. globosa = n= 930,29; 0. rugatipennis = 713,91; S. gracilipes = 120,73; chi- fl:! 4-0 square, all species df = 11, P < 0,001) although the Kendall coefficient of rank correlation indicated that values for the species varied independently (P. globosa vs 0. rugatipennis = 0,424, P = 0,055; P. globosa vs S. gracilipes = 0,182, 20 .y. P = 0,412; O. rugatipennis vs S. gracilipes = 0,242, P = 0,271). A degree of seasonality was suggested for 0. rugatipennis, which was trapped most frequently in the winter months of June and July. The percentage trapped in those two DEe .ruN months (36 %) is more than twice that which would be ex- SEP MAR pected from uniform monthly captures. In contrast, numbers of S. gracilipes declined throughout most of the year; this trend Fig. 4 reversed the following year. No pattern of monthly capture was Percentage of individuals of Physadesmia globosa, Onymacris apparent for P. globosa. rugatipennis and Stenocara gracilipes trapped in habitats dominated by Cladoraphis spinose, Acacia albida and A. erioloba (north and For comparison with the food analyses, the percentage south of the river-bed) during four months of the year. Total number occurrence of the beetles within each habitat type has been of individuals in a sample is indicated above each bar. depicted for September, December, March and June (Fig. 4) 146 FOOD & HABITAT USE BY TENEBRIONIDS

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

Food Habitat Food Habitat

Physadesmia globosa x Physadesmia globosa 0,91 ± 0,02 0,87 ± 0,03 Onymacris rugatipennis 0,92 ± 0,04 0,65 ± 0,04 Onymacris rugatipennis 0,85 ± 0,05 0,86 ± 0,04 Physadesmia globosa x Stenocara gracilipes 0,90 ± 0.04 0,48 ± 0,07 Stenocara gracilipes 0,72±0,10 0,68 ± 0,04

Onymacris rugatipennis x Stenocara gracilipes 0,90 ± 0,06 0,37 ± 0,04

deserts and grasslands. Based on field observations, dissec- tion of digestive tracts and literature citations, Rust (1986) and symmetrical overlap indices calculated. When habitat use found four detritivores and one herbivore among the ten- between species was compared for the four months, mean ebrionids of a sand dune insect community in the Great Basin. overlap values (Table 1) indicated greater variation between Staminate conifer cones represented an average of 51 % of species in habitat use than in food type consumed. When the diet of six beetle species in Arizona, as determined by habitats occupied by a single species were compared on a feeding observations (Doyen and Tschinkel, 1974). The re- monthly basis, the mean overlap values (Table 2) indicated mainder of the diet consisted of living or dry vegetation. Fifty- that P. globosa and 0. rugatipennis were trapped fairly con- two plant species were ingested as living, dead or litter material sistently in the same sites; S. gracilipesshowed slightly greater by 13 beetle species in a community in western Washington, variability in its use of habitat. although flower material was not particularly mentioned Inspection of Figs 1 and 4 shows that there may be a positive (Rogers, Woodley, Sheldon and Uresk, 1978). In the latter relationship between relative amount of time spent near C. study, remains ranged from < 1 % to 19 % of the spinosa, as indicated by pit-fall trap captures, and amount of contents of the digestive tract of the various species. In the insect material consumed. The Spearman's rank correlation latter two studies, there was a high degree of overlap in diets coefficient was, however, not significant, falling between among species examined, similar to that found in the Kuiseb 0,10> P> 0,05. detritivores. Our results from the Kuiseb agree with previous descriptions DISCUSSION of food and habitat use derived from a number of shorter term studies in the same area (Hamilton and Penrith, 1977; Penrith, Analyses of crop contents of the three Kuiseb species show 1979; Wharton and Seely, 1982). Although previous authors that these tenebrionid beetles are definitely detritivores, as has did not always differentiate between habitat and food use, they been suggested previously by a number of authors. They are generally provided anecdotal observations of selected forag- not, however, indiscriminate omnivores as they consistently ing areas (e.g., Penrith, 1975; Hamilton et al., 1976; Hamilton selected specific food types from detritus irrespective of spe- and Penrith, 1977; Roer, 1977; Wharton and Seely, 1982). Our cies of plant under which the detritus occurred. Leaves are the analyses expand on these observations and provide direct major food type available in detritus (Fig. 1), whereas flowers evidence as to the types of food eaten. were more commonly eaten. Insects, particularly dry cuticular The above results do not provide any evidence for exclusive fragments, represent less than 1 % of detritus whereas they habitat use by any of the three species of tenebrionid beetles formed a conspicuous part of the crop contents (Fig. 2). Those investigated. They also leave unanswered the further question materials consumed appeared to be the more nutrient-rich as to whether beetles occur randomly within their environment components of the detritus. according to microhabitat or according to differences in avail- Although the Kuiseb tenebrionids were found to be entirely ability or relative attractiveness of particular food types. Craw- detritivores, characteristics of thei r diet do not differ widely from ford et al. (this volume) provide a detailed evaluation of the those of the more herbivorous tenebrionids of North American question with respect to Physadesmia globosa.

ACKNOWLEDGEMENTS We wish to thank G. L. Prinsloo of the National Collection of the Foundation for Research Development of the C.S.I.R., the Insects, Pretoria, for identification of insect fragments from University of the Witwatersrand and the Transvaal Museum. beetle crops. C. S. Crawford, A. C. Marsh and R. D. Pietruszka The Directorate of Nature Conservation and Recreation commented on a draft of this manuscript and R. D. Pietruszka Resorts, Namibia, provided facilities and permission for work "" provided statistical advice. For financial assistance we thank in the Namib-Naukluft Park. FOOD & HABITAT USE BY TENEBRIONIDS 147

REFERENCES

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