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Wildlife Research

Volume 28, 2001© CSIRO 2001

P u b l i s h i n g

© CSIRO 2001 10.1071/WR00003 1035-3712/01/020121

Wildlife Research, 2001, 28, 121–134

Seasonal changes in the diet, food availability and food preference of the greater bilby (Macrotis lagotis) in south-western Queensland

Lesley A. Gibson

School of Biological Sciences A08, University of Sydney, NSW 2006, Australia.Present address: Queensland Parks and Wildlife Service, PO Box 149, Charleville, Qld 4470, Australia.

Abstract. Diet and food availability of the greater bilby (Macrotis lagotis), was examined at two sites and over twosummer and two winter seasons in Astrebla Downs National Park in south-western Queensland. The presence ofboth invertebrate and plant material in almost all faecal pellets (99.6% and 98.5%, respectively) indicated that bil-bies in south-western Queensland are omnivorous. The plant component of the bilby diet consisted predominantlyof Dactyloctenium radulans seeds (frequency of occurrence: 94.4%). The major invertebrate prey items were Isop-tera (termites, 73.1%) and Formicidae (ants, 77.6%). The frequency of occurrence of invertebrates in faecal pelletswas higher during summer than winter, but the frequency of occurrence of plants was relatively constant regardlessof season. Within the invertebrate categories, Isoptera and Acrididae (grasshoppers) displayed the greatest seasonaldifference, with a significantly higher proportion of both categories in the diet during summer than winter at bothstudy sites. The availability of invertebrate prey items also varied with season, and for most taxa, frequency of oc-currence was significantly higher in summer than winter. In contrast, the frequency of occurrence of seeds and bulbs(Cyperus bulbosus) varied only slightly with season. Seasonal changes in food availability were reflected by varia-tion in dietary composition; however, relative proportions of some dietary items differed from their proportionalavailability. Therefore, bilbies were considered to be qualitatively opportunistic; selecting more invertebrates whentheir apparent availability increased, but continually exploiting the available seeds and bulbs. Additionally, althoughbilbies appeared to have some preferences for particular food items, several food items were selected almost ran-domly, and therefore bilbies were also considered to be dietary generalists. Such an opportunistic feeding strategyis advantageous to an arid-zone species as it permits the continuous exploitation of food resources that are unpre-dictable in their availability both spatially and temporally.

L. A. GibsonWR00003

IntroductionThe greater bilby (Macrotis lagotis) is a medium-sized (800–2500 g) burrowing marsupial in the family Peramelidae(Groves and Flannery 1990). Prior to European settlementthe bilby was distributed over 70% of mainland Australia(Southgate 1990a). Since European settlement, the popula-tion size of the bilby has significantly decreased and its dis-tribution has contracted to the driest and least fertile parts ofits former range (Southgate 1990b). The bilby now occurspatchily in the Tanami Desert and in a small area north-eastof Alice Springs in the Northern Territory, the GibsonDesert, the northern Great Sandy Desert, the inland Pilbaraand the southern edge of the Kimberley in Western Australia,and in far south-western Queensland, a total area that consti-tutes approximately 20% of its former range (Southgate1990b). In Queensland, the bilby presently occurs in smallisolated pockets in the Mitchell grass and stony downs in anarea bordered by the towns of Birdsville, Bedourie, Beetootaand Boulia (P. McRae, unpublished).

The diet of the bilby appears to range from insectivory toherbivory depending on habitat, although it is generally con-sidered omnivorous. Smyth and Philpott (1968) documenteda primarily insectivorous diet near Warburton in WesternAustralia, with bilbies feeding mainly on termites, ants andinsect larvae. Watts (1969) concluded that, in central Aus-tralia, the bilby was omnivorous but leaned towards herbivo-ry, its diet consisting of bulbs, fungi, roots, seeds, fruit and,to a lesser extent, insects (mainly Coleopterans). Southgate(1990a) conducted a broad-scale comparison of the bilby’sdiet, from Western Australia, the Northern Territory andQueensland, and concluded that the diet was skewed towardseither insectivory or herbivory depending on locality. South-gate (1990a) recorded the following food items: seeds (mostcommonly Dactyloctenium radulans and Yakirra austral-iense), bulbs (Cyperus bulbosus), fungi (Endergonacae), in-vertebrates (ants, termites, beetles, grasshoppers, spiders)and invertebrate larvae and eggs (Coleoptera, Lepidoptera,Orthoptera). However, there are no detailed studies examin-

122 L. A. Gibson

ing seasonal variations in the diet of the bilby or the seasonalavailability of its food.

The availability of resources in the bilby’s habitat is un-predictable and therefore it should be advantageous for thebilby to have a flexible dietary strategy that responds rapidlyto seasonal changes in resources. In this study, the majorquestion asked was, do seasonal changes in dietary composi-tion of the bilby reflect the seasonal availability of fooditems? In order to answer this question, two preceding ques-tions need to be addressed. Firstly, does dietary compositionof the bilby change with season? Secondly, are there seasonaldifferences in the availability of food? Furthermore, sincepreference for a particular food type is shown when the pro-portion of that food type in the animal’s diet is higher thanthe proportion in the animal’s environment (Begon et al.1990), preferences for particular dietary items of the bilbycan also be addressed. The few studies that have attempted tocompare diet and food availability for an omnivore have ex-amined only the availability of the animal component, ratherthan the overall food availability (e.g. Quin 1985; Scott1995).

Study Area and Methods

Study area

An assessment of bilby diet and food availability was conducted fromAugust 1995 to May 1997 in Astrebla Downs National Park(24°13�05��S, 140°34�48��E) in south-western Queensland. The Dia-mantina River lies 50 km to the east of this 200 000-ha national park,and 180 km to the west is the Simpson Desert. The nearest towns areBedourie (100 km south-west) and Boulia (160 km north-north-west).

Astrebla Downs experiences an arid climate, typified by low and un-predictable rainfall, and high temperatures and evaporation. Long-termtemperature records collected from Birdsville show an average maxi-mum of 38.6°C and a minimum of 24.0°C in January (Bureau of Mete-orology 1997). In July, records show average maximum and minimumtemperatures of 20.6°C and 6.6°C respectively. Long-term rainfall re-corded at Davenport Downs (adjacent to Astrebla Downs) for the period1945–97 shows an average annual rainfall of 231 mm (Bureau of Me-teorology 1997).

Astrebla Downs is situated on deep self-mulching cracking claysoils on flat to gently undulating plains. The dominant vegetation is bar-ley Mitchell grass (Astrebla pectinata). Ephemeral grasses such as Dac-tyloctenium radulans and Iseilema vaginiflorum are seasonallyabundant. Numerous forb species, many of which are ephemeral, arealso found in the area; the more common ones include Atriplex spong-iosa, A. muelleri, A. lindleyi, Boerhavia diffusa, Euphorbia parvicurun-cla, Psoralea cinerea, Salsola kali and Sida trichopoda. Eucalyptuscoolibah occurs commonly in the larger drainage lines. The shrubs Aca-cia victoriae and Senna barclayana are common in the smaller drainagelines. Astrebla Downs has three major drainage lines, but none containpermanent water.

Faecal collection

Diet was determined from analysis of dietary remains in faeces. Freshfaecal pellets were collected from two locations, Ninu (24°09�29��S,140°35�34��E) and Talgoo (24°10�29��S, 140°41�29��E). These loca-tions were selected because they had a high number of occupied bilbyburrows compared with the surrounding area. Pellets were collectedduring two winter (August–September 1995 and June–August 1996)

and two late summer (February–March 1996 and March–May 1997)seasons. For the overall analysis of dietary composition, pellets werecollected during two additional study periods, December 1995 and Sep-tember 1997. Pellets were gathered from two sources: (1) the edge offeed scrapes (small excavations, made by bilbies, that may be up to 25cm deep), and (2) beneath the cage traps of captured animals.

Faecal analysis

One pellet was selected from a group (a group was classed as pelletscollected from the same feed scrape or the same animal) and weighed.Large pellets weighing more than 0.6 g fresh matter were sub-sampled.The pellets were soaked in water overnight and then teased apart in apetri dish. The faecal material was suspended evenly by adding a smallamount of water and then examined underneath a dissecting micro-scope (7–40×).

Plant items in faecal pellets were categorised as: seed, bulb, fruit,‘other plant’ (stem, leaf and root combined) or fungi; and invertebrateprey items were classified to order (where possible). Invertebrates wereidentified by comparison with reference material collected using thetechniques described below. The distinguishing features of the inverte-brate orders listed by Calver and Wooller (1982) were also used in theidentification. Formicidae and Isoptera were identified by the presenceof head capsules and mouthparts. Coleoptera, Orthoptera and Hemi-ptera were distinguished by elytral pieces (Coleoptera), wings, legs andmouthparts. The legs and chelicerae of Araneae were clearly distinct.Larvae were recognised by the presence of body cuticle, often with legsattached, and chewing mouthparts, whereas pupae and eggs were de-tected by their external casing. Larvae were grouped together because,in most cases, it was impossible to identify them to ordinal level on thebasis of the appearance of sections of cuticle. All pupae and egg caseswere also placed into one dietary category because it was extremely dif-ficult to identify these prey items to a lower taxonomic level.

For the description of overall dietary composition, two measureswere used: frequency of occurrence and percentage volume. Frequencyof occurrence was defined as the number of pellets in which each preyitem was found. For each study period and each prey item, the totalnumber of pellets containing that prey item was calculated and dividedby the total number of pellets examined for that study period and ex-pressed as a percentage. The percentage volume of the faecal pellet oc-cupied by each prey item was estimated visually and the averagepercentage volume for each study period was calculated. For the exam-ination of seasonal changes in diet, only frequency of occurrence wasused.

Food availability

The bilby obtains its food on the surface of the ground or digs for sub-terranean food. Therefore, a measure of total food availability requiressampling both above and below the ground. The two techniques em-ployed were pitfall and soil sampling.

Pitfall sampling was used to measure the relative abundance andcomposition of potential invertebrate prey items on the soil surface. Pit-fall traps (n = 240) were set at eight sites (four in Ninu and four in Tal-goo) in areas where there was clear sign of bilby foraging (i.e. feedscrapes). Each site was based on a drainage channel and the minimumdistance between each site was 200 m. A site consisted of three parallellines of ten pitfalls spaced 10 m apart with one line in the drainagechannel, one line on the slope of the drainage channel and one line onthe plain. The distance between the lines varied slightly due to the dif-ferential structure of the drainage system. This pitfall configuration wasdesigned to account for variation in the availability of invertebrates be-tween habitat types.

The benefits and disadvantages of the systematic sampling designused here are listed in Cochran (1977). The major disadvantage of sys-tematic samples is that they may give poor precision when unsuspected

Seasonal diet and food availability of the bilby 123

periodicity is present, although Cochran (1977) found that systematicsamples compared favourably in precision with stratified random sam-ples. The main advantage of the systematic design is that logistically itis much easier to execute than random sampling. In this study, the sys-tematic, rather than random design was chosen mainly because of thedifficulty of locating pitfall jars in an unvarying landscape.

Pitfall traps consisted of glass jars (127 mm deep × 74 mm wide)that were sunk into the ground with their rims level to the soil surface.During each sampling period, the lids were removed and the jars werepartially filled with preservative. A mixture of 5% formalin and 95%ethylene glycol was chosen as the preservative because it was resistantto evaporation, hence the jars needed to be filled only once. After 9 daysof collection the jars were removed and replaced. Following each sam-pling session, the contents of the jars were sieved, sorted and their con-tents identified. Invertebrates were identified to the lowest taxonomicresolution possible using reference texts (Mascord 1980; Davies 1986;Hawkeswood 1987; CSIRO 1991; Zborowski and Storey 1995) andconsultation with the Queensland Museum. From this material a refer-ence collection was established; families of invertebrates were furtherseparated into morphotypes. Pitfall sampling commenced during thefirst week of each study period.

Pitfall composition was expressed as frequency of occurrence, thatis, the number of pitfall traps in which each invertebrate category waspresent. For each study period and each category, the total number ofpitfall traps containing that category was calculated and divided by thetotal number of pitfall traps examined for that study period and ex-pressed as a percentage.

Soil sampling was used to measure the relative abundance and com-position of subterranean seeds, bulbs and invertebrates (i.e. termites,pupae and larvae). Soil samples were collected from two sites at eachlocation (Ninu and Talgoo). During each collection period, five soilsamples measuring 200 mm × 200 mm × 150 mm deep were removedfrom each of the three lines (drain, slope and plain) using a specially de-signed steel sampler and stored in heavy-duty geological survey bags.The samples were collected randomly along each line during each studyperiod. The soil was sieved shortly after collection to avoid desiccationof invertebrates; however, due to the often extreme temperatures, somedead and unrecognisable invertebrates were usually encountered. Also,due to the size and weight of each sample, any manipulation of the sam-ple potentially crushed invertebrates within it. For these reasons, it waslikely that the invertebrates were largely under-estimated.

Initially, a large sieve (1.2 mm) was used to separate the larger par-ticle sizes from the smaller, and the larger material was sorted immedi-ately. This involved the collection of bulbs and invertebrates, and thenumbers of each (per taxon for invertebrates) were recorded for eachsample. A sub-sample measuring 130 mm × 130 mm × 70 mm deep ofthe sieved material was further wet-sieved (0.7 mm). These sampleswere sun-dried and a 2-mL sub-sample was placed in a petri dish andexamined under a dissecting microscope (7–40×) for seeds. Seeds wereseparated into morphotype.

Invertebrates were expressed as the total number of individualswithin each category per study period. Seeds and bulbs were expressedas frequency of occurrence, that is, the number of soil samples contain-ing each category divided by the total number of soil samples.

Dietary selectivity

Levels of overall dietary selection were determined using the propor-tional similarity index (PS) of Feinsinger et al. (1981):

PS = 1 – 0.5 ∑ (pi – qi)

where pi is the proportion of dietary item i in the diet and qi is the pro-portion of category i available. This index ranges from 0 if a species ismaximally selective, to 1 if a species exhibits zero selection (i.e. if fooditems are used in proportion to their availability).

Electivity indices measure the utilisation of individual food resourc-es in relation to their availability in the environment (Lechowicz 1982).To determine the degree of selectivity for each individual food item, theelectivity index (E*) of Vanderploeg and Scavia (1979a) was used; thisis based on the selectivity coefficient Wi (Vanderploeg and Scavia1979b) and the number of available food types:

Wi = (pi/qi) / ∑ (pi/qi)

E* = [Wi – (1/n)] / [Wi + (1/n)]

where pi is the proportion of food i in the diet, qi is the proportion offood i in the environment and n is the number or prey items. This indexranges from –1 (maximum selection against) to +1 (maximum selectionfor), with 0 values indicating non-selective feeding. Vanderploeg andScavia’s E* index represents the feeder’s perception of a food’s value inrelation to both its abundance and the other food types available and, ac-cording to Lechowicz (1982), provides the best electivity index.

Measurements of frequency of occurrence of dietary items and foodavailability categories, expressed as relative proportions, were used inthe calculation of the above indices.

Statistical analysis

Chi-squared analyses were used to test differences in the frequency ofoccurrence of both dietary items and food availability categories be-tween season. Overall Chi-squared tests were performed initially forgrouped invertebrate categories and grouped plant categories using theproportions test (Statistical Sciences Inc. 1993). Categories that con-tained expected frequencies of less than five were deleted from theanalysis. Contingency tables of season (winter and summer) by status(present or absent) for each individual category were constructed andChi-squared tests of independence were performed (SYSTAT 1992).Chi-squared tests were also used to examine differences in the frequen-cy of occurrence between dietary and food availability categories. P <0.05 was used to define statistical significance.

Results

Seasonal conditions

During Winter 1, no rain fell and conditions were essentiallydry for three months prior to this first field trip. During Win-ter 2, 60 mm of rainfall was recorded, although virtually norain fell in the three months prior to this field trip. In Summer1, 38 mm rainfall was recorded over 3 days. Above- and be-low-average monthly falls of rain were recorded in the vicin-ity of the study site in each of the three months prior to thistrip. No rain was recorded during Summer 2, but exception-ally high rainfall occurred in the two months prior to thisfield trip.

Faecal analyses

A total of 268 faecal pellets was analysed. Of these, 206 werecollected during the four major study periods. In all, 16 die-tary categories were recognised: five plant, nine invertebrate,one fungus and one soil (Table 1). Additionally, scorpion re-mains were detected in two pellets and the remains of a lizardwere found in one pellet; these were excluded from the anal-yses due to their small sample size. Fleas, mites and bilbyhair were assumed to be ingested during grooming and weretherefore ignored.

Faecal contents had been finely masticated. For example,only small ants (<1 mm) and seeds appeared whole. Conse-

124 L. A. Gibson

quently, to avoid misrepresentation of faecal contents, inver-tebrate prey items were identified to order. Plant materialwas classified into broader groups. It was not an aim of thisstudy to comprehensively detail the diet of the bilby, as thishas been done elsewhere (see Southgate 1990a), but rather toinvestigate seasonal trends in dietary composition. There-fore, the level of identification selected for this study wasconsidered appropriate.

Overall dietary composition

Invertebrate prey remains were recorded in 99.6% of pelletsand they occupied an average faecal volume of 22.6%(Table 1). Hymenoptera, represented exclusively by ants(Formicidae), and Isoptera (termites) remains occurred mostfrequently in pellets although the volume they each occupiedwas less than 10%. Coleoptera (beetles) occurred next mostoften in pellets, followed by larvae, Acrididae (all Orthopteraidentified were grasshoppers) and Araneae. Pupae and Pen-tatomidae (all Hemiptera identified were pentatomids) oc-curred infrequently. The volume occupied by each of theseinvertebrate groups was less than 5%. The lower frequencyof occurrence and volume occupied by the larger prey itemscompared with termites indicates that termites were a fa-voured dietary item. The ‘unknown’ invertebrate categoryincluded all invertebrate remains that could not be identified;this category formed a small component of the overall diet.

Stems, leaves and roots were grouped together as ‘otherplant’ because it was often difficult to distinguish betweensmall sections of these items. Seeds of Dactyloctenium rad-ulans were analysed separately from the others as it wasclearly the most abundant item in the diet. The bulb Cyperusbulbosus was evident in pellets only by the presence of itsshell.

Plant material occurred in 98.5% of pellets and occupiedan average faecal volume of 51.4% (Table 1). The seed of D.radulans accounted for 42.0% of this volume (whole seedsand fragments combined) and it also occurred most frequent-ly in pellets. Whole D. radulans seeds were recorded in pel-lets, but these occupied an average volume of only 3% (9.0%of the total D. radulans seed volume). The remaining plantcategories each occupied a small average faecal volumewhen compared with D. radulans (Table 1). The faecal vol-ume of all seeds combined was 44.1% and all other plant cat-egories combined was 7.3%. The fruit from the desertcucumber (Cucurbitaceae) was detected in the pellets by thepresence of its seed; this species accounted for 1.3% of thefaecal volume and occurred in 15.7% of pellets.

Fungal spores were obvious due to their clustered natureand dark brown colouring (when disturbed a fine suspensionof brown material was evident). Fungi occurred in 7.5% offaecal pellets and was clearly a minor component of the diet.Soil occurred in all pellets, and occupied 25.3% of pellets by

Table 1. Percentage frequency of occurrence of prey items in bilby faeces during summer and winter, and with seasons pooled (total diet) at Ninu and Talgoo

Percentage volume is also included for the total diet. Results of Chi-squared analyses are also shown (d.f. = 1): n.s., not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001; n = sample size

Prey item Frequency of occurrence (%)Ninu Talgoo Total diet (n = 268)

Winter Summer �2 Winter Summer �2 Freq. occ. Volume(n = 46) (n = 50) (n = 50) (n = 60) (%) (%)

InvertebratesColeoptera 45.7 58.0 1.5n.s. 34.0 36.7 0.1n.s. 38.8 2.2Acrididae 21.7 46.0 6.3* 22.0 60.0 6.3* 32.1 4.8Larvae 50.0 18.0 11.3*** 54.0 23.3 11.0** 33.2 0.9Isoptera 47.8 94.0 25.3*** 56.0 76.7 5.3* 73.1 8.7Formicidae 78.3 86.0 1.0n.s. 82.0 63.3 4.7* 77.6 3.2Araneae 10.9 24.0 2.8n.s. 24.0 25.0 0.0n.s. 23.1 1.0Pentatomidae 0.0 2.0 0.9n.s. 0.0 3.3 1.7n.s. 2.6 1.1Pupae 0.0 12.0 5.9* 4.0 6.7 0.4n.s. 7.5 0.4Unknown 19.6 10.0 1.8n.s. 14.0 15.0 0.0n.s. 13.8 0.3

PlantsD. radulans seed 89.1 92.0 0.2n.s. 98.0 93.3 1.4n.s. 94.4 42.0Other seed 30.0 40.0 0.6n.s. 20.0 30.0 1.9n.s. 32.8 2.1Other plant 41.3 48.0 0.5n.s. 14.0 5.0 1.6n.s. 35.8 2.0Fruit 20.0 30.0 4.6* 30.0 30.0 0.4n.s. 29.1 2.0Bulb 32.6 26.0 0.4n.s. 38.0 26.7 2.7n.s. 26.5 3.3

Fungi 26.1 0.0 14.9*** 3.0 3.3 0.5n.s. 7.5 0.7Soil 100.0 100.0 – 100.0 100.0 – 100.0 25.3

Seasonal diet and food availability of the bilby 125

volume. It was assumed that it was ingested accidentally dur-ing foraging.

Seasonal variation in dietary composition

In the investigation of seasonal differences in dietary compo-sition, the two winter and the two summer periods werepooled to minimise between-year variation in climate. Thetwo study locations, Ninu and Talgoo, were considered sep-arately.

Table 1 illustrates the differences in the frequency of oc-currence of prey items during summer and winter at Ninuand Talgoo; the results of individual Chi-squared analysesare also included. At both sites an overall seasonal differencewas detected for invertebrates (Ninu: χ2 = 232.5, d.f. = 7, P< 0.001; Talgoo: χ2 = 77.7, d.f. = 7, P < 0.001) but not forplants.

Frequencies of occurrence of Isoptera and Acrididae werehigher in summer than winter at both Ninu and Talgoo. Con-versely, the frequency of occurrence of larvae was higher inwinter than summer at both locations. Pupae occurred morefrequently in summer at Ninu and Formicidae occurred morefrequently in winter at Talgoo. Frequencies of occurrence ofthe remaining invertebrate categories all tended to be higherin summer than winter at both locations, apart from ‘un-known’ at Ninu. However, some of these differences wereonly slight and none was significant. All seasonal differencesin frequencies of occurrence of the plant categories were notsignificant. The frequency of occurrence of fungi was higherin winter than summer at Ninu, but not at Talgoo.

Food availability

A list of the orders and families identified in pitfall traps dur-ing this study is shown in Appendix 1. Dermaptera, Manto-dea, Neuroptera, Phasmatodea, Scorpionida, Pseudo-scorpionida and larvae were not included in the analyses be-cause they all comprised <5% of material caught in traps.Diptera and wasps (Hymenoptera) were also excluded fromthe analyses because it was assumed that these predominant-ly airborne insects were active during the daylight hours andso would be unavailable as food to the nocturnal bilbies. Tocomplement the dietary analyses, invertebrates recorded inpitfall traps were grouped to order, unless only a single fam-ily was considered.

The soil-sampling technique used in this study proved tobe ineffective in the monitoring of abundance of subterrane-an invertebrates. In total, only 107 termites (Isoptera), 14 lar-vae and 14 pupae were collected. The patchiness ofsubterranean termite nests and their invisibility on the soilsurface makes termites difficult to sample. The inclusion ofa nest in a soil sample can amount to a relatively largenumber of termites in that sample, but numerous soil sam-ples have to be collected to account for the spatial variationin nest distribution. Pupae and larvae were also difficult tosample because of their patchy distribution. Dactyloctenium

radulans was clearly the most prominent seed in the soilsamples, and was considered separately in the analyses. Thesample sizes of the remaining individual seed categorieswere too small to consider separately and were thereforecombined as ‘other seeds’ (OS). This categorisation comple-ments that of the dietary analyses. Bulbs of Cyperus bulbo-sus also occurred in the soil samples, but their abundancemay have been under-estimated also due to their patchy dis-tribution.

Seasonal variation in food abundance

To complement the dietary analyses, the two winter and sum-mer sampling periods were pooled and the two study loca-tions were analysed separately.

Clear seasonal trends in the frequency of occurrence ofinvertebrates trapped in pitfalls were observed at both Ninuand Talgoo (Table 2), and overall seasonal differences weresignificant at both locations (Ninu: χ2 = 796.0, d.f. = 14, P <0.001; Talgoo: χ2 = 707.9, d.f. = 13, P < 0.001). At Ninu, thefrequency of occurrence of all invertebrate categories, apartfrom Pentatomidae, Lepidoptera and Orthoptera (all ofwhich showed no seasonal differences), were higher in sum-mer than winter. At Talgoo, Pentatomidae, Blattodea, Thysa-nura and Hemiptera showed no differences, but thefrequencies of occurrence of the remaining categories werehigher in summer than winter with the exception of Lepidop-tera, which was higher in winter.

The overall seasonal difference for seeds and bulbs wassignificant at Ninu (χ2 = 10.9, d.f. = 3, P < 0.05) but not Tal-goo. However, there was no difference between winter andsummer in the frequency of occurrence of D. radulans, andof other seeds or bulbs at either location (Table 2).

Relationship between diet and food availability

Comparisons of the frequency of occurrence of each inverte-brate category between pitfall traps and faecal pellets at eachlocation and within each season revealed several differences(Figs 1, 2). Table 3 shows the results of Chi-squared analysescomparing the individual invertebrate categories present inthe faecal pellets with those present in the pitfall traps. Notall invertebrate categories sampled in the pitfall traps wererecorded in the faecal pellets. These categories included:Blattodea, Isopoda, Lepidoptera, Chilopoda, Thysanura,Gryllidae, Hemiptera (apart from Pentatomidae) and thesmaller-sized families of Araneae (Figs 1, 2; Table 3). De-pending on season and location, some of these categories(e.g. Blattodea, Isopoda and Chilopoda) occurred in suchlow frequencies in the pitfall traps that they were not signif-icantly different from the zero values of the diet. The fre-quency of occurrence of Isoptera was consistently higher infaecal pellets than in pitfall traps during both seasons and atboth locations. The frequencies of occurrence of Formicidae,Pentatomidae, Coleoptera and Araneae were all significantlyhigher in the pitfalls than in the diet, but during summer only.

126 L. A. Gibson

Acrididae were mostly consumed in similar frequencies totheir availability. In general, during summer, the frequencyof occurrence of invertebrates collected in pitfall traps tend-ed to be higher than their frequency in faecal pellets whereasduring winter, there were either no significant differences, orthe frequency of invertebrates in faecal pellets was signifi-cantly higher than in pitfall traps (exceptions: Araneae andPentatomidae).

Seasonal changes in the frequency of occurrence of seedsidentified in both the diet and soil samples were small (Figs1, 2; Table 3). The main difference observed was in the rela-tive proportions of ‘other seeds’ consumed within each sea-son in relation to their availability. The frequency ofoccurrence of ‘other seeds’ was significantly higher in thesoil samples than in the diet during both summer and winter.However, D. radulans seeds were consumed in an almostequal frequency to their availability. The slight seasonalchanges in the frequency of occurrence of C. bulbosus bulbsin the diet were reflected in the soil samples, with the fre-quency of occurrence of bulbs at one study site higher duringwinter than summer, and the reverse situation at the othersite.

Dietary selectivity

In the examination of dietary selectivity, a direct comparisonof the relative proportions of prey items in the diet with the

relative proportions of food available in the environmentwould be the preferred option. However, because differenttechniques were required to assess the availability of plantsand invertebrates eaten by the bilby (i.e. pitfall and soil sam-pling), plant and invertebrate preferences are treated sepa-rately.

Firstly, in order to investigate overall dietary selectivity,all pitfall categories were used in the calculations of the pro-portional similarity indices (PS) and electivity indices (E*).At both locations, it appeared that bilbies were slightly moreselective in their feeding during winter (Ninu: PS = 0.56; Tal-goo: PS = 0.63) than summer (Ninu: PS = 0.63; Talgoo: PS= 0.65). Electivity indices (E*) for each category during win-ter and summer at each location are shown in Fig. 3. Thysa-nura, Chilopoda, Lepidoptera, Isopoda, and Blattodea allappeared in the pitfall traps but were not recorded in the dietat either Ninu or Talgoo, hence the –1.00 selectivity of eachof these categories (i.e. not selected at all). At Ninu, duringsummer bilbies selected for Orthoptera, Formicidae, Coleop-tera and more strongly for Isoptera. During winter, the onlycategory they selected for was Isoptera. At Talgoo, Isopterawas the only category selected for, and strongly, in both sea-sons. However, these results can be misleading, mainly be-cause of the broad categorisation of individual invertebratesto order; several species belonging to each order were re-corded in the pitfalls but not in the diet. Therefore, it may ap-

Table 2. Percentage frequency of occurrence of each invertebrate/plant category collected in pitfall traps (invertebrates)/soil samples (plants) during winter and summer at Ninu and Talgoo

Results of Chi-squared analysis are also given: n.s., not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001

Prey item Frequency of occurrence (%)Ninu Talgoo

Winter Summer χ2 Winter Summer χ2

InvertebratesColeoptera 20.8 73.8 134.8*** 21.3 75.0 138.9***

Acrididae 21.3 33.3 8.8** 16.7 31.7 14.7***

Isoptera 1.3 24.6 58.1*** 0.0 5.0 12.3***

Formicidae 85.0 96.3 17.9*** 85.8 96.7 17.6***

AraneaeA 27.9 46.7 8.0*** 21.3 45.4 31.5***

Pentatomidae 11.7 6.7 3.3n.s. 8.8 4.6 3.3n.s.

Orthoptera 42.1 50.8 3.7n.s. 35.8 58.3 24.4***

AraneaeB 53.8 74.2 21.7*** 48.8 66.3 15.0***

Isopoda 4.2 32.1 63.0*** 10.8 28.8 24.3***

Lepidoptera 17.5 12.5 2.4n.s. 19.6 12.1 5.1*

Thysanura 9.6 17.9 7.0*** 9.6 8.8 0.1n.s.

Chilopoda 2.9 15.8 23.6*** 1.7 6.7 7.5**

Hemiptera 30.0 47.5 15.5*** 18.3 23.3 1.8n.s.

Blattodea 1.7 7.5 9.3*** 5.4 7.9 1.2n.s.

PlantsD. radulans seed 96.7 97.9 0.2n.s. 96.7 95.8 0.1n.s.

Other seed 100.0 93.8 3.9n.s. 100.0 95.8 2.5n.s.

Bulb 41.7 27.1 2.5n.s. 50.0 56.3 0.4n.s.

AIncludes Lycosidae and Miturgidae only.BIncludes all spiders

Seasonal diet and food availability of the bilby 127

pear as though orders as a whole were being selected against,but this may be true only for particular species within the or-der. Consequently, the next step was to remove all the pitfallcategories that were not recorded in the diet, including fam-ilies within orders, thereby enabling a closer insight into thelevel of selectivity of actual prey items. Also, because theabundance of termites, as determined by pitfall sampling, isprobably negatively biased (see Discussion), then the E* val-ues calculated for Isoptera are likely to be positively biasedand unrealistic. Because the proportions of each category areinter-dependent in the calculations of PS and E*, the posi-tively biased selection for Isoptera means that the other cate-gories will be biased towards negative selection. To removethis bias Isoptera was also removed from the analysis.

The E* values now indicate less negative selection withinthe other categories (Fig. 4). A more generalist strategy isalso reflected by higher PS values (Ninu: winter – 0.82, sum-mer – 0.89; Talgoo: winter – 0.89, summer – 0.81). Positive

and negative selections were relatively weak and several cat-egories were selected almost randomly (except for Pentato-midae). Acrididae and Coleoptera were the most preferredprey items in relation to their availability. Pentatomidae wasthe most strongly avoided. Formicidae were selected almostrandomly and Araneae were weakly selected against or ran-domly selected. There was no seasonal trend in strength ofeither positive or negative selection, and seasonal differencesin overall selectivity were slight.

PS values for three plant categories (D. radulans, otherseeds, and bulbs) are meaningless; however, E* values(Fig. 5) are still useful for examining the degree of selectionof each category. Of the three plant categories examined,D. radulans seeds were the most strongly selected, althoughvalues approached random selection. ‘Other’ seeds wereavoided in relation to their availability. C. bulbosus bulbswere positively selected, except at one study site duringsummer, when they were strongly avoided. D. radulans and

Fig. 1. Percentage frequency of occurrence of each category col-lected in bilby faecal pellets and pitfall traps (invertebrates)/soil sam-ples (plants) at Ninu during (A) summer and (B) winter. Significantdifferences between diet and food availability are also indicated: *, P< 0.05; **, P < 0.01; ***, P < 0.001. Legend: for – Formicidae; ist –Isoptera; col – Coleoptera; acr – Acrididae; ara1 – Lycosidae &Miturgidae; pen – Pentatomidae; hem – Hemiptera; ort – Orthoptera;ara2 – Araneae; blt – Blattodea; iso – Isopoda; lep – Lepidoptera; chi– Chilopoda; thy – Thysanura; dr – Dactyloctenium radulans seeds;os – other seeds; bu – bulbs.

Fig. 2. Percentage frequency of occurrence of each category col-lected in bilby faecal pellets and pitfall traps (inverts)/soil samples(plants) at Talgoo during (A) summer and (B) winter. Significant dif-ferences between diet and food availability are also indicated: *, P <0.05; **, P < 0.01; ***, P < 0.001. Legend: for – Formicidae; ist –Isoptera; col – Coleoptera; acr – Acrididae; ara1 – Lycosidae &Miturgidae; pen – Pentatomidae; hem – Hemiptera; ort – Orthoptera;ara2 – Araneae; blt – Blattodea; iso – Isopoda; lep – Lepidoptera; chi– Chilopoda; thy – Thysanura; dr – Dactyloctenium radulans seeds;os – other seeds; bu – bulbs.

128 L. A. Gibson

‘other’ seeds were more strongly selected during summerthan winter. Conversely, bulbs were more strongly selected inwinter.

Discussion

Faecal analysis

The benefits and potential disadvantages of using faecal con-tent analyses to determine dietary composition have beenwell documented (e.g. Hall 1980; Churchfield 1982; Fox andArcher 1984; Putman 1984; Quin 1985; Read 1987; Dick-man and Huang 1988; Green 1989; Luo et al. 1994; Dickman1995). The principal advantage of faecal analysis is that nu-merous random samples can be obtained from a populationwithout sacrificing animals. The major problem is that bothsmall and soft-bodied prey items may be either completely oralmost completely digested, and therefore overlooked,whereas large, hard-bodied prey that resist digestion are like-ly to be over-estimated (Dickman and Huang 1988). In thisstudy, soft-bodied invertebrates such as Thysanura, Chilopo-da and small Lepidoptera (moths) were recorded in the fieldbut not in the faeces, perhaps because of complete digestion,but other soft-bodied invertebrates such as insect larvae were

readily identified in faecal pellets. The sampling of stomachcontents substantially reduces this problem, but it requiresthe destruction of the study animal (Fox and Archer 1984).Due to the endangered status of the bilby in Queensland,sampling of stomach contents was not an option in this study.

Each method of quantifying faecal analysis data has prob-lems in terms of over- or under-estimating the importance ofdietary items. Accurately relating the fragments of materialobserved in faeces to the amount ingested is difficult. Differ-ential digestibilities of dietary items may result in absoluteproportions of these items in faeces that do not represent theproportions ingested (Putman 1984). Scoring data as per-centage volume of each prey item per individual is particu-larly susceptible to this problem. Recording data asminimum number of each prey item per individual avoidsthis problem to a certain degree, although prey items thathave been totally digested will still be under-represented. Al-so, the difficulty of estimating how many of each prey itemis represented by a given group of fragments still applies(Putman 1984), and, according to Dickman and Huang(1988), underestimation of the numbers of a particular preyitem will increase as the numbers of that prey item ingested

Table 3. Results of Chi-squared analyses testing for differences in the frequency of occurrence of invertebrate and plant categories between faecal pellets and pitfall traps during summer and winter at Ninu and Talgoo

d.f. = 1 (individual categories). n.s., not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001

Category Ninu TalgooSummer Winter Summer Winter

χ2 P χ2 P χ2 P χ2 P

Common categoriesFormicidae 8.34 ** 1.30 n.s. 59.26 *** 0.48 n.s.Isoptera 85.98 *** 104.97 *** 158.08 *** 148.76 ***Coleoptera 4.99 * 12.74 *** 32.07 *** 3.75 n.s.Acrididae 2.90 n.s. 0.01 n.s. 16.47 *** 0.81 n.s.AraneaeA 8.69 ** 5.96 * 8.25 ** 0.18 n.s.Pentatomidae 1.63 n.s. 5.95 * 1.63 n.s. 4.72 *

OverallC 2179.05 *** 348.45 *** 1103.66 *** 24.74 ***Additional categories

Blattodea 4.00 * 0.78 n.s. 5.07 * 2.84 n.s.Isopoda 21.84 *** 1.99 n.s. 22.40 *** 5.95 *Lepidoptera 6.97 ** 9.44 ** 8.03 ** 11.69 **Chilopoda 9.11 ** 1.38 n.s. 4.23 * 0.85 n.s.Thysanura 10.52 ** 4.79 * 5.65 * 5.20 *Orthoptera 0.91 n.s. 5.89 * 3.63 n.s. 2.19 n.s.AraneaeB 46.10 *** 28.50 *** 33.53 *** 28.50 ***Hemiptera 35.80 *** 18.44 *** 12.31 *** 10.81 **

Overall invertsC 2512.47 *** 823.17 *** 1662.79 *** 254.06 ***Plant categories

D. radulans 1.77 n.s. 2.40 n.s. 1.77 n.s. 0.18 n.s.Other seeds 29.82 *** 38.31 *** 24.01 *** 49.50 ***Bulbs 4.56 * 0.00 n.s. 34.91 *** 15.83 ***

Overall plantsC 63.53 *** 63.63 *** 301.43 *** 155.13 ***

AIncludes Lycosidae and Miturgidae only.BIncludes all spiders.CSubscripts equal degrees of freedom; these differ because categories with expected frequencies <5 were removed.

Seasonal diet and food availability of the bilby 129

increases. Additionally, the importance of small prey itemsmay be overestimated and the contribution of large preyitems potentially underestimated (Dickman 1995). An alter-native method that minimises the errors involved in the quan-tification of fragments is frequency of occurrence (i.e. thenumber of individuals in which each prey type occurred).Dickman and Huang (1988) suggested that this method of re-cording faecal content data is relatively reliable, particularlyif the study animal is a dietary generalist that consumes hard-bodied prey. The main problem with this measure is that thesignificance of rare prey items in the diet may be positivelybiased and common prey items under-estimated (Luo et al.1994).

Overall dietary composition

The presence of both invertebrate and plant material in al-most all faecal pellets (99.6% and 98.5%, respectively) indi-cated that bilbies in south-western Queensland arecontinuously omnivorous in both seasons sampled. Withinthe invertebrate categories, termites (Isoptera) were a fa-voured dietary item, as were ants (Formicidae). Similarly,Southgate (1990a) and Smyth and Philpott (1968) found that

termites (in particular) and ants were major components ofthe bilby’s diet. Ants and termites were also recorded in thediet of the other surviving arid-zone bandicoot, Isoodon au-ratus (Bradshaw et al. 1994; Southgate et al. 1996). This re-sult is not surprising considering the abundance of termitesand ants in Australia’s arid and semi-arid zones (StaffordSmith and Morton 1990; Whitford et al. 1992; Abensperg-Traun 1994; Abensperg-Traun and Steven 1997). Addition-ally, because termites and ants live in colonies, they offer aconcentrated food source (Redford and Dorea 1984;Abensperg-Traun 1994; Abensperg-Traun and Steven 1997).

Overall, there was a close similarity between the dietaryitems identified in the present study with those detected bySouthgate (1990a). Aside from ants and termites, other com-mon invertebrate dietary categories included Coleoptera, lar-vae, Acrididae, Araneae and eggs. Categories additional tothose of Southgate (1990a) were Pentatomidae and pupae,although both occurred infrequently. Common plant catego-ries included seeds (Dactyloctenium radulans included), thebulb of Cyperus bulbosus, fruit and fungi. In this study,

Fig. 3. Electivity indices of each invertebrate category collectedduring summer and winter at (A) Ninu and (B) Talgoo. Fig. 4. Electivity indices of each invertebrate category (only cate-

gories recorded in both faecal pellets and pitfall traps – minus Isop-tera) collected during summer and winter at (A) Ninu and (B) Talgoo.

130 L. A. Gibson

D. radulans seeds were clearly the main plant item con-sumed, occurring in 94% of pellets. Southgate (1990a) alsoidentified D. radulans seeds as a major food source, domi-nating the plant component at sites in south-western Queens-land. It is possible that bilbies obtain a concentrated sourceof seeds by raiding the stores in nests of harvester ants. Theremains of seed-harvesting ant nests were observed in nu-merous bilby feed scrapes. Southgate (1990a) and Smyth andPhilpott (1968) also suggested that ants were consumed co-incidentally while foraging for seeds in harvester ant nests.Whilst fungi have been reported as a common food sourcefor temperate-zone bandicoots (Quin 1985; Moyle 1992;Claridge 1993; Scott 1995), fungi occurred infrequently inthe diet of the bilby. Southgate (1990a) and Watts (1969) alsorecorded small proportions of fungi in the bilby’s diet, al-though Southgate (1990a) suggested that they could be sea-sonally important.

Seasonal changes in dietary composition

The invertebrate component of the bilby’s diet varied acrossseasons. Most of the invertebrate categories occurred in a

higher proportion of faecal pellets during summer than win-ter, although this was significant only for Isoptera and Acrid-idae. Seasonal changes in the diet of other bandicoot specieshave been reported, but most of these studies examined theeffect of dry and wet seasons on dietary composition ratherthan time of the year (i.e. summer, winter) (Heinsohn 1966;Bradshaw et al. 1994; Southgate et al. 1996). Time-of-yearseasonal differences in the dietary composition of I. obesulusand P. nasuta were examined by Quin (1985) and Scott(1995), respectively. Quin (1985) recorded seasonal differ-ences within invertebrate and plant categories, whereas Scott(1995) failed to show any seasonal differences.

Seasonal changes in food availability

Monitoring changes in the abundance of invertebrates by pit-fall trapping has some problems, including the differentialattraction of the preservative to invertebrates, the effects ofground vegetation and the ‘digging-in effect’ (disturbanceassociated with digging pitfall traps) (Southwood 1978). Themain drawback of this technique is that it confounds meas-urement of activity with abundance (Greenslade 1964; Foxand Archer 1984). Pitfall trapping works on the theory thatinvertebrates randomly fall into pitfall traps whilst carryingout normal activities and therefore this method essentiallymeasures invertebrate activity rather than abundance. How-ever, because increased activity of potential prey itemsmeans that they are more likely to be encountered by a pred-ator (Statham 1982), this method of assessing prey availabil-ity on the soil surface was considered appropriate for thisstudy.

Activity of invertebrates varied with season, with a higherproportion of most invertebrates active during summer. Read(1987) also recorded a greater richness and abundance ofarid-zone invertebrates during spring and summer than win-ter, and James (1991) recorded an increase in abundance ofarid-zone invertebrates during a wet season from low levelsduring a dry season. Seasonal fluctuations in invertebrateabundance have also been observed in more temperate re-gions of Australia, with the highest abundance occurringduring spring and summer and the lowest during winter(Statham 1982; Fox and Archer 1984; Bennett and Baxter1989; Green 1989).

Overall, the abundance of seeds varied slightly with sea-son; no significant differences were detected. The most crit-ical factor influencing seed abundance is more likely to bethe occurrence of rain rather than the time-of-year (i.e. tem-perature regulated), although the composition of seeds maychange depending on differential growing seasons (Juradoand Westoby 1992). More rain fell during the winter studyperiods than the summer; however, a larger volume of rainfell during the months leading up to the summer study peri-ods. Given that there is a delay between the appearance ofseeds in the soil and the rain-stimulated germination ofplants, a greater abundance of seeds would be expected dur-

Fig. 5. Electivity indices of Dactyloctenium radulans seeds, otherseeds and Cyperus bulbosus bulbs collected during summer and win-ter at (A) Ninu and (B) Talgoo.

Seasonal diet and food availability of the bilby 131

ing summer than winter. The fact that there was little varia-tion could be an artefact of the sampling technique.According to Southgate (1990a), the abundance of C. bulbo-sus is influenced by a combination of rainfall and season.During the first summer study period the appearance of thevegetative section of C. bulbosus above ground was noted forthe first and only time in the study; however, this increase inabundance was not detected in the soil samples. The patchydistribution of this bulb makes it very difficult to sample.

Relationship between season, diet and food availability

The categories that were absent from the bilby diet in thisstudy were also absent from the diet of the bilbies studied bySouthgate (1990a). Soft-bodied invertebrates such as Lepi-doptera and Thysanura could have been overlooked becausethey were mostly, or completely, digested (Dickman andHuang 1988). Small spiders and Hemipterans may also havebeen missed simply because of their size. Chilopods mayhave been avoided by the bilby because their long bodies andnumerous nerve centres make them difficult to handle andalso because of their poisonous glands and jaws (Read 1987;Fisher and Dickman 1993). Isopoda may also be avoided be-cause their calcium carbonate content may be high enough torender them relatively indigestible. The absence of Blattodeaand Gryllidae from the diet is surprising considering thatthey should be nutritionally rewarding food resources. Inver-tebrate dietary items that were not recorded in the pitfalls (orthat were recorded in very low numbers) were larvae, pupaeand egg cases. The sample sizes of each of these categoriesin the soil samples were also too small to compare with theirproportions in the diet. Therefore, the relationships betweenthe proportions of these food items in the diet with theiravailability were not examined. Plant dietary items that werenot assessed for their availability included fruit, stems,leaves and roots; however, each of these categories constitut-ed a very small proportion of the overall diet (Gibson 1999).

Bilbies did not appear to be quantitatively opportunistic;that is, they did not consume food items in direct proportionto their availability. Nevertheless, seasonal trends in availa-bility of most invertebrate food items were reflected in theirdiet. Quin (1985) recorded a similar result for the southernbrown bandicoot (Isoodon obesulus). Although the propor-tions of only Isoptera and Acrididae were significantly high-er during summer than winter in the diet, several otherdietary categories also appeared in higher proportions duringsummer. Likewise, invertebrate categories recorded in thepitfall traps occurred in higher proportions during summerthan winter, and most of these differences were significant.The disparity lies in the relative proportions of each inverte-brate category within the diet and pitfall traps. Clearly, ter-mites were a favoured food item, but the fact that theirproportion in the diet did not match their availability wasprobably a reflection of inefficient sampling, rather thantheir physical absence. As suggested in the Results, pitfall

sampling is unlikely to accurately measure termite abun-dance, due mainly to their extreme spatial patchiness and ac-tivity patterns. Because most species of termites consumetheir food in situ, they do not need to forage away from theirnests, and those species that do harvest mainly do so via shal-low sub-surface tunnels (Gay 1970; James 1991; Whitford etal. 1992). Therefore, the number of termites caught in pitfalltraps was likely to be an under-estimate of availability. James(1991) and Fisher and Dickman (1993) also observed thattermites were rarely caught in pitfalls. The abundance of ter-mites in the soil samples was not compared with their pro-portion in the diet because this technique was even lesseffective than the pitfall traps.

The availability of invertebrates during summer tended tobe higher than the frequency at which they were consumed.In winter, there were either no significant differences be-tween diet and availability, or the frequency of invertebratesin the diet was significantly higher than their availability.One possible explanation is that because there was such alarge increase in abundance of invertebrates during summer,bilbies may have reached satiation before consuming preyitems to the level of their abundance. In contrast, the lowerabundance of invertebrates during winter may have allowedthe consumption of invertebrates only in similar proportionsto their availability. Alternatively, bilbies could have selec-tively chosen some prey items until they were depleted.

Of the plant groups examined in this study, their frequen-cy in the diet appeared to largely reflect their availability,with one exception – the frequency of occurrence of ‘otherseeds’ was significantly higher in the soil samples than in thediet.

Food preferences

As discussed above, bilbies appeared to totally avoid severalpotential invertebrate prey items. Of those that they did con-sume, only Isoptera was positively selected; which suggeststhat bilbies are specialised termite-consumers (within its in-sectivorous dietary component). However, because pitfallsare unlikely to effectively sample termite activity or abun-dance, this result over-represents the significance of termitesin the diet. Once termites were removed from the equation, amore generalist dietary strategy was indicated.

Conclusion

Because different techniques were used to assess the availa-bility of potential invertebrate and plant prey items, the pro-portions of available plant items, in relation to invertebrateitems, could not be determined. However, some inferencescan be made. Bilbies consumed more invertebrates duringsummer than winter, but the proportion of plants consumedwas relatively constant regardless of season. Corresponding-ly, the availability of invertebrates was higher during summerthan winter, whereas the availability of plants (seeds andbulbs) did not differ. Therefore, bilbies appeared to be qual-

132 L. A. Gibson

itatively opportunistic, selecting more invertebrates whentheir apparent availability increased, but continually exploit-ing the available seeds and bulbs. A species is considered tobe qualitatively opportunistic if it takes most of the types ofprey from the range available (Hall 1980; Statham 1982; Foxand Archer 1984; Quin 1985). In this case, because bilbiesaltered their diet in accordance with seasonal fluctuations infood availability, they were considered to be opportunistic(Begon et al. 1990). Furthermore, because bilbies were notquantitatively opportunistic, then they must fit the qualita-tively opportunistic criterion. This interpretation differsslightly from that of Hall (1980), Statham (1982), Fox andArcher (1984) and Quin (1985), although bilbies were stillsampling from most of the seasonal range of prey availableto them. Bilbies appeared to have some preferences for par-ticular food items, but they also selected several food itemsrandomly, and therefore were also considered to be dietarygeneralists. This opportunistic feeding strategy is advanta-geous to an arid-zone species as it permits the continuous ex-ploitation of food resources that are unpredictable in theiravailability both spatially and temporally.

Acknowledgments

Many thanks to Kyra Kopestonsky, Malcolm Connolly, Man-da Page, Lyn Pullen, Craig Eddie, Geoff Lundie-Jenkins andDon Lundie-Jenkins, and particularly Peter McRae, for as-sistance in the field. A special thanks to Chris Dickman foradvice on the diet analysis and Terry Beutel for statistical ad-vice. Thanks also to Robert Raven and Philip Lawless(Queensland Museum) for generously donating pitfall trappreservative and identifying spiders, and to Geoff Monteith(Queensland Museum) for identifying several species of bee-tles and ants. I am grateful to Chris Dickman, Ian Hume, Pe-ter McRae and two anonymous referees for their valuablecomments on the manuscript. Queensland Parks and WildlifeService provided access to Astrebla Downs National Parkand granted permission to conduct the study (Permit: HO/000076/95/SAA). The study was funded by a grant to IanHume from the Australian Research Council; LAG was sup-ported by an Australian Postgraduate Award.

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Manuscript received 19 January 2000; accepted 7 June 2000

134 L. A. Gibson

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Appendix. Invertebrate taxa collected in pitfall traps

Class Order Family No. of morphotypes

Arachnida Araneae Lycosidae 1Miturgidae 2Corinnidae 4Oxyopidae 1Salticidae 5Lamponae 4Segestriidae 1Zoderiidae 3Gallieniellidae 3Theridiidae 2Ctenizidae 2Unknown 20

Pseudoscorpionida 1Scorpionida 1

Chilopoda Scolopendrida 1Scutigerida 1

Insecta Blattodea Blattidae 3Blaberidae 2

Coleoptera Tenebrionidae 14Carabidae 13Cerambycidae 2Curculionidae 6Histeridae 2Elateridae 1Chrysomelidae 2Bostrichidae 1Scarabaeidae 4Dermestidae 1Coccinellidae 1Melyridae 1Brentidae 1Staphylinidae 1Hydrophilidae 1Hybosoridae 1Trogidae 1Cleridae 1Dytiscidae 1Buprestidae 1

Dermaptera 1Diptera 3Hemiptera Cicadellidae 1

Lygaeidae 9Cydnidae 1Reduviidae 11Fulgoroidea 1Pentatomidae 4Dictyopharidae 1Alydidae 1Miridae 1

Hymenoptera Formicidae 8Mutillidae 2Unknown/wasps 14

Isoptera 3Lepidoptera 3Mantodea Mantidae 1Neuroptera Myrmeleonidae 3Orthoptera Acrididae 19

Gryllidae 9Gryllotalpidae 1Gryllacrididae 1

Phasmatodea Phasmatidae 1Thysanura Lepismatidae 1

Malacostraca Isopoda 5