Patterns in the local assembly of Egyptian rodent faunas: Areography and species combinations

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ORIGINAL INVESTIGATION

Patterns in the local assembly of Egyptian rodent faunas: Areography and

species combinations

Mohammad Abu Bakera,�, Bruce D. Pattersonb

aDepartment of Biological Sciences, University of Illinois at Chicago, 845 W. Taylor St, Chicago, IL 60607, USAbDepartment of Zoology, Field Museum of Natural History, 1400 S. Lake Shore Drive, Chicago, IL 60605, USA

Received 12 May 2009; accepted 30 August 2009

Abstract

We implicate ecological processes in assembly patterns of Egyptian desert rodents using spatial variation ofdistribution and composition of local assemblages. We also compare our assemblages to prior analyses of NorthAmerican and Australian small mammal communities in terms of species richness and representation of trophic groups.

We studied patterns of occurrence of 29 rodent species among 335 collecting events in 308 sites using museumspecimen records resulting from a country-wide survey. The studied taxa vary greatly in their natural histories andecology. Fully 69% of studied species (20) were localized in 30 or fewer sites (9% of all sites). Site incidence was closelycorrelated with the geographic range size of species, while local abundance of a species showed little relationship to itsgeographic range. The species richness of local assemblages ranged from a lone species at 73 sites to 11 species at3 sites. A total of 214 different combinations were recorded of which 164 (77%) were unique to a single site. G. gerbillus

was both the most abundant and most ubiquitous species. Coexistence with other species was positively correlated withincidence and geographic range size. Body mass distribution was remarkably uniform for the fauna as a whole, andinfluenced the geographic distributions and abundance of individual species.

Our sites have low species richness and substantial variability in species composition, which also characterize desertrodent communities elsewhere. Habitat requirements, exclusive distributions of sibling species, low primaryproductivity, and Egypt’s location all influenced species assembly. The Egyptian and Australian deserts supportedhigher proportions of low richness assemblages compared to North America. As in North America, Egyptian siteswere dominated by granivorous species.& 2009 Deutsche Gesellschaft fur Saugetierkunde. Published by Elsevier GmbH. All rights reserved.

Keywords: Egypt; Rodents; Areography; Body mass; Trophic structure

Introduction

The assembly of species communities is a fundamentalissue in community ecology. Community assemblyremains contentious and incompletely understood de-spite decades of intense scrutiny (e.g., Strong et al. 1984;

atter & 2009 Deutsche Gesellschaft fur Saugetierku09.08.008

r. Tel.: +1 312 912 2655;

[email protected] (M. Abu Baker).

Cody and Diamond 1975). Relatively simple islandsystems provided important early insights, which havegradually become incorporated into continental systems(MacArthur and Wilson 1967; Terborgh 1974).Although generalizations are difficult, biogeographyand history typically interact to determine the potentialsource pools for assemblages (Patterson 1999; Ricklefsand Schluter 1993). Other factors, often mediated bysome general processes, involve admitting and excludingselected species at more local levels (Graves and Rahbek

nde. Published by Elsevier GmbH. All rights reserved.

Mamm. biol. 75 (2010) 510–522

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dx.doi.org/10.1016/j.mambio.2009.08.008

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M. Abu Baker, B.D. Patterson / Mamm. biol. 75 (2010) 510–522 511

2005). Given the hierarchy of spatial and temporalscales affecting the species in local assemblages, studiesof community assembly must be broad in scope andintensively detailed to illuminate the complementaryeffects of causal factors (Brown 1995; Rosenzweig 1995).

Desert rodents offer an excellent system for studyingfactors that govern species assembly. Communities inarid zones tend to be relatively simple, thereby offeringthe analytical advantages of islands in teasing apartcausal factors. On the other hand, desert rodents areoften quite diverse, abundant, and conspicuous, makingthem excellent study subjects. The desert rodent systemhas been explored for a host of relevant ecologicalprocesses and interactions, including resource partition-ing (Bowers and Brown 1982), foraging theory (Brownet al. 1994), predator-prey relationships (Sweitzer et al.1997; Daly et al. 1992), community assembly (Brownand Kurzius 1987; Fox and Brown 1993; Fox 1987),coexistence mechanisms (Brown 1989a; Brown 1989b;Kotler and Brown 1988), macroecology (Brown andKurzius 1987; Brown 1995; Shenbrot et al. 1999),convergence (Ben-Moshe et al. 2001; Mares 1993), andcharacter displacement (Dayan and Simberloff 1994).

Prior analyses of desert rodent community assemblyin America, Asia, and Australia have identifiedcommonalities and differences that implicate generalprocesses and underscore regional or historical con-tingencies (Brown and Kurzius 1987; Fox 1987; Foxand Brown 1993; Shenbrot and Rogovin 1995; Rogovinet al. 1994; Rogovin 1995; Kelt et al. 1999; Morton et al.1994).

Species and population attributes such as abundance,distribution and body mass patterns are fundamental toareography and macroecology. With most speciesreported from few sites, uni-modal, right skewedfrequency distributions have been the general patternsof range size (Brown and Kurzius 1987; Morton et al.1994). Hanski (1982) developed the ‘‘core-satellite’’model of community organization for instances whereregional species distributions show a bimodal frequencydistribution in terms of the number of sites where theyoccur. The relationship between abundance and dis-tribution among ecologically similar species is generallypositive, suggesting the abundant species widely dis-tributed and rare one with restricted distribution (Brown1984; Brown 1995; Gaston et al. 1997). Brown (1984)explained this relationship with a niche-based modelbased on trade-offs between specialist (narrow habitatand restricted range) and generalist (broad habitat andrange) species.

Body size frequency distributions are often highlymodal and strongly right-skewed at the continentalscale, while being nearly uniform at local scales (Brown1995; Brown and Nicoletto 1991; Kelt and Brown 1998).Uniformity suggests that local assemblages are non-random subsets of continental faunas (Brown and

Nicoletto 1991). Kelt and Brown (1998) hypothesizedthat ecological assortment (through local daily interac-tions between species) and evolutionary adjustment(through regional evolution of body size and characterdisplacement) cause local ecological displacement thatminimize competition. Most studies show a negativerelationship between body size and population abun-dance, with highest abundance reached by relativelysmall (but not the smallest) species; from here,abundance declines with decreasing and increasing bodymass, with a steep left-boundary line (Gregory andBlackburn 1995; Brown 1995; Blackburn et al. 1993).Across continents, small mammal communities typi-

cally demonstrate low species richness (2-4 species persite), substantial variability in species composition (highbeta diversity), large number of species combinations,and increased degrees of coexistence with incidence(Kelt et al. 1996). However, deserts differ widely introphic structure and mechanisms responsible for them(Kelt et al. 1996, 1999): interspecific competition inNorth American deserts and habitat selection in Asiandeserts. The essential nature of desert rodent commu-nities has been evaluated on a large scale in a‘‘Gleasonian’’ manner as somewhat unique assemblagesof species that can persist together by individual speciessucceeding or failing idiosyncratically (Brown andKurzius 1987; Morton et al. 1994; Shenbrot et al.1994). Here, species are thought to respond to thespatial distribution of variables that determine theirindividual niches (Brown and Kurzius 1987; Mortonet al. 1994; Shenbrot et al. 1994; Kelt et al. 1999). As aresult, local communities are fluidly structured withrespect to species composition (Kelt et al. 1999).

Despite their huge extent, African deserts have beenlargely overlooked in formulating patterns and under-standing processes of assemblage structure for desertrodents. At more than 9,000,000 km2, the Sahara islarger than all of Australia and rivals the conterminousUnited States in size. As a major distributional barrier,the Sahara limits the distributions of both Palearcticand Afro-Tropical faunas; its biotas include elementsof both, as well as many endemic species (Lomolinoet al. 2006).

We compiled distributional data for rodents in theArab Republic of Egypt, using specimen records inmuseum collections from a country-wide survey. Thissystem provides a unique and previously unexploredopportunity to address questions regarding the relativeroles of biotic attributes in community structure in anarid environment. We apply a macroecology approachusing basic graphical and analytical statistics to assessspecies distribution among sites, co-existence of speciesand differences in the biotic attributes of coexistingspecies (body size, trophic levels and abundance). Weuse the areography and species combination patterns totest whether Egyptian rodent communities exhibit the

M. Abu Baker, B.D. Patterson / Mamm. biol. 75 (2010) 510–522512

same low alpha diversity and high spatial heterogeneitythat characterize desert rodents elsewhere. Our studyalso aims to address similarities and differences in localspecies richness and trophic structure between oursystem and communities elsewhere.

Fig. 1. A map of Egypt showing the sampling localities

included in the present study.

Material and methods

Egypt lies in north-eastern Africa approximately 221to 321 N and 251 and 351E and covers an area of1,019,600 km2. Despite its large size, habitat hetero-geneity in Egypt is low, due to the vast coverage of ariddeserts that cover approximately 95-96% of the country.Riparian habitat surrounding the Nile Valley and theDelta make up much of the remainder. However, thecountry lies at the intersection of several distinctbiogeographic regions (Mediterranean, Irano-Turanian,Saharo-Sindian, and Afro-Tropical), which togetherwith topography and climate, create a regional mosaic(Goodman and Meininger 1989).

Systematic surveys of small mammals in Egypt wereundertaken in the middle of the last century by theUnited States Naval Medical Research Unit number 3(NAMRU-3), headed by Dr Harry Hoogstraal. Thesurveys were intended to identify and assess potentialreservoirs and vectors animal-borne diseases. Accord-ingly, they used a wide variety of collecting methods,including snap- and live-trapping, excavating burrows,shooting, and netting at night. Although sampling wasintensive in areas near human settlements, the surveyssampled a host of microhabitats and all availablemacrohabitats. Voucher specimens are deposited at theField Museum of Natural History (FMNH, Chicago)and the United States National Museum (USNM,Washington DC), and a definitive report on theirtaxonomy and distribution by Osborne and Helmy(1980). Altogether, 5839 specimens of rodents werecollected from 764 localities, including 30 speciesof rodents in 6 families; three species (Gerbillus

mackilligini, G. floweri and G. perpadillus) are endemicto Egypt (Musser and Carleton 2005).

To increase consistency and to enhance comparisonswith studies on other continents, we restricted theanalysis to collections made between 1946 and 1982and excluded all collecting events with fewer than fivespecimens. Furthermore, we restricted the temporalscope of a few sites that were sampled repeatedly intointervals of o5 years (increasing the number of sites by27); 335 collecting events were used to assemblepresence-absence and abundance (number of specimensper site) matrices using 4897 specimens from 29 speciesof four families of rodents. The assemblages range over308 sites (see Fig. 1, Table 1).

Basic graphical and analytical statistics were used toassess the distribution of species (and their numbers) among

sites and coexisting species. We tallied the number of speciespresent at a site as the local species richness at that site. Weused the number of sites where species occurred as ameasure of its site incidence. We averaged the number ofspecimens collected over the total number of sites whereeach species occurred to estimate its local abundance. Wemeasured species geographic range size in Egypt using acombination of all museum records and the records andmaps provided by Osborne and Helmy (1980). Diet classesand average body mass for each species were determinedfrom Osborne and Helmy (1980), Kelt et al. (1999) andShenbrot et al. (1999). We used Spearman rank correlationsto assess relationships between the variables and quadraticmodels to fit non-linear regressions to cumulative variables.

Results

There are 29 species of rodents, chiefly desert gerbilsand jirds, distributed within the 335 collecting events.The studied taxa vary greatly in their natural historiesand ecology. Table 1 summarizes the distribution,abundance, and natural history attributes for all speciesincluded in the analyses.

Incidence and geographic range

The 29 rodent species occupied the 308 sites indifferent manners. Most species were recorded in

Table 1. List of Egyptian rodent species analyzed. Diet codes are: C=carnivore, F=folivore, G=granivore, O=omnivore.

Taxon # of

sites

# of

specimens

# of

coexisting

species

# of

assemblages

diet Average

body mass

(g)

Family Dipodidae

Allactaga tetradactyla 12 53 14 12 G/F 52.27

Jaculus jaculus 77 267 26 64 G/F 56.92

Jaculus orientalis 27 159 18 23 G/F 134.5

Family Muridae

Acomys cahirinus 76 582 17 49 O 41.62

Acomys russatus 11 46 10 9 O 37

Arvicanthis niloticus 28 122 18 25 G 139.8

Mus musculus 70 386 24 60 O 15.03

Nesokia indica 11 58 7 10 O 244

Rattus rattus 35 135 18 30 O 137.3

Rattus norvegicus 17 63 14 16 O 259.3

Dipodillus campestris 26 198 19 25 G 31.65

Dipodillus dasyurus 15 69 10 16 G 22.8

Dipodillus mackillingini 1 2 1 1 G 17

Dipodillus simoni 11 27 14 11 G 17.4

Gerbillus amoenus 30 101 21 29 G 13.2

Gerbillus andersoni 38 225 24 30 G 29.3

Gerbillus floweri 12 71 14 11 G 23

Gerbillus gerbillus 131 779 27 73 G 21.83

Gerbillus henleyi 30 51 21 28 G 8.6

Gerbillus perpallidus 18 183 19 17 G 36.3

Gerbillus pyramidum 74 362 21 50 G 53.65

Meriones crassus 56 295 21 38 G/F 81.5

Meriones libycus 22 139 18 21 G/F 84

Meriones shawi 22 57 20 21 G/F 90.6

Pachyuromys duprasi 7 24 8 5 G 36.5

Psammomys obesus 42 314 19 29 F 128.77

Sekeetamys calurus 13 66 7 9 G/F 41.4

Family Gliridae

Eliomys melanurus 7 28 11 7 O 51.8

Family Spalacidae

Spalax ehrenbergi 5 35 13 5 F 113.8


relatively few sites. In fact, 4 species occurred in 10 orfewer sites, 9 species in 11-20 sites, and 7 species in 21-30sites. Only a single species occurred in more than 80 sites(39% of those surveyed); Gerbillus gerbillus wasrecorded at 131 sites (Table 2). Species geographicranges varied over two orders of magnitude, from11893 km2 (Dipodillus mackillingini) to 969868 km2

(Gerbillus gerbillus). 9 species (31% of all) wereconfined to a northern coastal strip of less than200 km wide, 6 of which (Allactaga tetradactyla,

Jaculus orientalis, Gerbillus floweri, G. perpallidus,

Meriones shawi and Spalax ehrenbergi) spread only upto 100 km inland to the south. Four species wereconfined to the Nile Valley and Delta area (Arvicanthis

niloticus, Nesokia indica, Rattus rattus andR. norvegicus), while four others had distributionsranging only over the eastern mountains of Sinai and

Red Sea (Acomys russatus, Dipodillus dasyurus,

Sekeetamys calurus, Eliomys melanurus).Site incidence was closely correlated with the geo-

graphic range size of species, with wide ranging speciesoccurring at more local sites (rs=0.875). On the otherhand, the local abundance of a species (as measured bythe average number of specimens in site samples)showed little relationship to its geographic range size(rs=0.116).

Assemblage size

The species richness of local assemblages ranged froma lone species at 73 sites to 11 species at 3 sites. Meanspecies richness per site was 2.76, with a mode of 2.Approximately 22% of the sites had a single species,

Table 2. Commonness and rarity in Egyptian rodents. Species considered to be habitat specialists are in bold-face. No species

combines high incidence and local abundance with generalized habitat requirements.

Local abundance (specimens per site)

o4 4-6 4 6

Incidence (number of sites) o30 Dipodillus mackillingini Acomys russatus Dipodillus campestris

Dipodillus simoni Nesokia indica Gerbillus perpallidusMeriones shawi Sekeetamys calurus Meriones libycus

Gerbillus amoenus Allactaga tetradactyla Spalax ehrenbergiGerbillus henleyi Arvicanthis niloticus

Pachyuromys duprasi Dipodillus dasyurusRattus norvegicus Eliomys melanurus

Gerbillus floweri

Jaculus orientalis

30-80 Rattus rattus Gerbillus andersoni Acomys cahirinus

Jaculus jaculus Gerbillus pyramidum Psammomys obesusMeriones crassus

Mus musculus

480 Gerbillus gerbillus


75% had three or fewer species, and only one in eight(12.3%) supported five or more species. The two mostabundant species, Gerbillus gerbillus and Acomys cahirinus,

comprised a third of the single-site records, respectivelyoccupying 18 sandy sites and 8 rocky sites. Sixteen otherspecies were also recorded singly.

Poorer assemblages tended to be represented byfewer total individuals (rs=0.92, n=335). The averagenumber of individuals in single-species assemblageswas 8.7, while successively larger assemblages (2, 3, 4,5, 6, 7, and 8-species assemblages) were representedby 10, 12.25, 20, 26, 38, 44, and 72 individuals onaverage respectively. Nine-, ten- and eleven-speciesassemblages were represented by 60, 48, and 50.7individuals, respectively. Covariance of assemblage sizeand sampling intensity does not extend to the averagenumber of individuals per species in these assemblages(rs=�0.059, n=11).

Species combinations and coexistence

Local assemblages were composed of many differentspecies combinations, 214 in all. Of these, 164 (77%)were unique to a single site. Sites with two-speciesassemblages (n=110) exhibited the greatest number ofspecies combinations (n=63), followed by sites withthree species (n=69), four species (n=42), and fivespecies (n=25) with total combinations of 55, 39, and 22respectively.

G. gerbillus was the most abundant and mostubiquitous species. Across 131 sites, it coexisted with27 other rodent species in 73 different species combina-

tions. Other widespread species (those recorded frommore than 50 sites) generally showed similar patterns ofdistribution. They tended to occur most frequently witha small number of species, and less frequently with alarge number of species. Most species occurred fre-quently in assemblages with 1-4 other species at a site(Fig. 2). The three commonest species are ecologicallyand taxonomically differentiated. The gerbil is aquadrupedal granivore weighing ca 22 g, the spinymouse a quadrupedal omnivore weighing 42 g and thejerboa is a bipedal herbivore weighing 54 g. Becausethese species have distinct habitat associations(G. gerbillus on sand, A. cahirinus on rocky substrates,while J. jaculus is a habitat generalist that avoids rockyhabitats), they are never present in the same assemblage.Co-occurrence of sibling and ecologically similar specieswas low. G. gerbillus co-occurred with G. andersoni,G. floweri or G. perpallidus in 15, 3, or 12 sites (out of131), A. cahirinus co-occurred with A. russatus in sevensites (out of 76), while J. jaculus co-occurred withJ. orientalis and Allactaga tetradactyla only in 7 and 6sites respectively (out of 77).

Most species ranged across assemblages involvingmany other rodent species. Only six species co-occurredwith 10 or fewer species across their ranges. Nearly 80%(23 of 29 species) co-occurred in various local assem-blages with 11 or more species, and 27% (8) occurredwith 20 or more species across their ranges.

The cumulative number of species with which aspecies coexisted was positively correlated with thenumber of localities at which it was recorded (rs=0.857Fig. 3a). A weaker positive correlation existsfor cumulative number of coexisting species and

131

50

3929 25 24

15 12 11 10 9 9 8 6 6 5 4 4 3 3 3 3 3 2 2 1 1 10

20

40

60

80

100

120

140

0

10

20

30

40

50

0Assemblage size

Gerbillus gerbillus


incidence

76

29

19 19 1813 11 10 9 9 8 7

4 3 3 2 1 10

10

20

30

40

50

60

70

80

05

1015202530

0Assemblage size

Acomys cahirinus


incidence

77

50

30

1916

13 11 11 10 10 10 9 9 7 6 6 5 5 5 4 3 3 2 22 1 10

10

20

30

40

50

60

70

80

90

0

5

10

15

20

25

30

0Assemblage size

Jaculus jaculus


incidence

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

Fig. 2. Co-occurrence frequencies for the three commonest rodents in Egypt. Left histograms give the total occurrences of the three

most commonly encountered species and their co-occurrences with other species. Right panels give frequencies of the assemblage

sizes they inhabit (black bars) and the numbers of different species combinations involved (hatched bars).


the species’ geographic range size (rs=0.673, Fig. 3b).Many species with low incidence and small geographicranges coexist with large numbers of other rodentspecies.

Patterns in body mass

Body mass distribution is remarkably uniform for thefauna as a whole, with one to four species representing

Fig. 3. Total number of species with which a species coexisted across all sites as a function of (a) its incidence and (b) its geographic

range size.


each 10-g mass interval up to 60 g (Fig. 4). No speciesweighed 60-80 g, but 3 weighed 80-100 g, 5 weighed100-150 g, and 2 weighed 150-300 g, meaning thedistribution effectively lacked a mode. Massdistributions for assemblages of different sizes wereless uniform. Assemblages up to five species in size werecharacterized by modes between 20-30 g and 50-60 g.

Body mass appeared to influence the geographicdistribution and abundance of species (Fig. 5). Plots ofboth geographic range size and abundance showed atriangular pattern, with the largest and smallest speciesbeing narrowly distributed and locally uncommon,while the largest ranges and abundances characterizedcertain small-to-medium-sized rodents. The non-linear

nature of this relationship precluded significantcorrelations (Fig. 5).

Comparisons among deserts

We compared patterns of assemblage size (number ofspecies per site across sites) with those previouslydescribed from North American and Australian desertrodent communities (Fig. 6). The comparison is basedon a data set developed by Brown and Kurzius (1987)for small mammals of the North American deserts andMorton et al. (1994) for the Australian small mammalcommunities (comparisons between the two data sets are

Fig. 4. Frequency distributions of numbers of species as a

function of body mass (on a logarithmic scale) for the entire

species pool.


presented in Morton et al. 1994). In North America,82% of sites had 2-6 species present, and 9% containedmore than 6 species; the median number of species at asite was 3. In Australia, the median number of specieswas 2, 64% of sites had 2-6 species, and 2% possessedmore than 6 species. In Egypt however, 75% of the siteshad 2-6 species, and 3.5% had more than 6 species. Thehighest number of species at a site was greater in NorthAmerica followed by Egypt with 11 species in three sites.One- and two-species assemblages were representedsignificantly more frequently in both Egypt andAustralia, skewing the distribution to the right (Fig. 6).

Based on a chi-square test of heterogeneity, thefrequency distributions of number of species per siteagainst the number of sites differed significantly betweenEgypt, North America and Australia (w2=162.4,Po0.0001, df=20). The Egyptian assemblages signifi-cantly differed from the North American and theAustralian data sets when tested separately (w2=69.74,Po0.0001, df=9 and w2=51.87, Po0.0001, df=10).

The three continents differed significantly with respectto assemblage size distributions, even when the Egyptiansites with less than 8 and 10 specimens were excluded(w2=128.65.1; 119.9, Po0.0001, df=20 respectively).The Egyptian collections remained significantly differentfrom the Australian when sites with greater than 8 and10 specimens are used (w2 =44.85; 47.66, Po0.0001,df=10 respectively), but this procedure eliminated thedifference between the Egyptian and the North Amer-ican sites (w2=37.36, Po0.0001, df=9; w2=21.9,P=0.009, df=9).

Species representation across trophic categories wasuneven. Granivores dominated Egyptian assemblages orthe pool? (59%), while folivorous (17%) and omnivor-ous species (24%) were more poorly represented(Fig. 7). Proportionality of species among the fourtrophic categories differed significantly between Egypt,

North America and Australia (w2=51.8, Po0.001).However, when the Australian sites were excluded,there was no significant difference between Egyptand North America (w2=2.32, P=0.51). Granivorousand folivorous species dominated both the Egyptianand North American sites, while carnivorous andomnivorous species were most frequent in theAustralia (Fig. 7).

Discussion

The present study represents the first attempt toinvestigate patterns of local assembly for rodents inEgypt. Species differed substantially in abundance,range sizes, phenotypic attributes, and geographicorigins (Corbet 1978; Osborne and Helmy 1980;Harrison and Bates 1991; Musser and Carleton 2005),which influenced local assemblage structure. Assem-blage patterns are also influenced by factors involvingEgypt’s location and its low productivity. Nevertheless,Egyptian rodent assemblages share many features withrodent faunas elsewhere.

Based on species incidences among sites, three groupsof species are apparent: ‘‘satellite’’ species which wererecorded from only a few, localized sites; modal speciesfound at an intermediate number of sites over largerareas, and ubiquitous ‘‘core’’ species recorded from ahigh number of sites. However, most species of rodentsin Egypt (45%) are localized (see Table 2). The higherproductivity and floral diversity allowed the northernparts of Egypt to host 24 species, nine of which (31%)had distribution ranges confined within 200 km of thecoast. As expected, the number of sites occupied by aspecies was positively and significantly correlated withits geographic range size, but there was no significantrelationship between local abundance and range size.The generality of the relationship, for abundant speciesto be widely distributed and rare ones to be restricted(Hanski 1982; Brown 1984, 1995; Gaston et al. 1997),makes us hesitant to discount it here.

Despite differences in composition of taxa, our resultsagree with previous studies in the pattern of low alphadiversity, with one to three species typically occurring ata site, and most species occurring at less than 9% of thesites (Morton et al. 1994; Kelt et al. 1996, 1999). As inother studies, beta diversity was high, reflecting the factthat most species occurred with many other species inmany different combinations (Figs. 2 and 3). Althoughmost local assemblages consisted of only one to threespecies, 214 different combinations were observed. Suchdiversity has been attributed to the spatial heterogeneityof physical and biotic resources in arid zones (Brown1987). However, low productivity and environmentalheterogeneity in the arid zones of Egypt may have led to

Fig. 5. Body mass of rodent species and (a) area of geographic range (in km2) and (b) abundance (measured as the average number

of specimens per site).


higher levels of interspecific interactions and low alphadiversities (Abramsky 1988; Kotler and Brown 1988).

Brown and Kurzius (1987) drew five implicationsfrom similar results: 1) Species distributions are highlyindividualistic, 2) composition of local biotas is highlyvariable, 3) rodents (especially rare species) respond toenvironmental change at a small spatial scale, 4) single,local samples of assemblages may not be representativeof assemblages over larger regions, and 5) opportunitiesfor coevolution between pairs of continental species arelimited. They suggested that such assemblage structure

should be expected among many different organisms incontinental faunas (Brown and Kurzius 1987, 1989).Morton et al. (1994) found these patterns applied todata from Australian small mammals.

Our results suggest that species incidence is highlydependent on habitat requirements. Several groups canbe recognized by their strict habitat preferences: rockdwellers (eg. Acomys spp., and Sekeetamys calurus),sand dwellers (Gerbillus gerbillus), inhabitants of denselyvegetated wadis (Psammomys obesus and Eliomys

melanurus), inhabitants of open gravel plains (Allactaga

Fig. 6. Distributions for numbers of sites as a function of

assemblage size in a site. Egyptian collections (for sites with a

minimum of 5 specimens) compared to the Australian and

North American sites.

Fig. 7. Comparisons of local trophic structure in Egypt, North

America and Australia. Note: for comparison purposes,

species categorized as granivorous/folivorous from Egypt were

weighted one-half and tallied between the two categories.


tetradactyla, Jaculus jaculus, J. orientalis and G. henleyi),inhabitants of riverines and human habitations (Rattus

spp., Nesokia indica, Arvicanthis niloticus) and habitatgeneralists (Meriones crassus and G. dasyurus), seeOsborne and Helmy (1980); Harrison and Bates(1991); Musser and Carleton (2005). Krasnov et al.(1996) showed that 12 species negev desert rodents werespatially segregated along three ordination axes inter-preted as (a) a gradient of soil hardness from rock tosand, (b) a gradient of relief from cliffs to flat plains and(c) a gradient of vegetation density. All 12 species areincluded in the present study. Kelt et al. (1999) arguedthat the extended history of co-occurrence betweenspecies of different geographic origins contributed totheir positive associations. Shenbrot et al. (1994) foundthat Asian desert rodents were organized into spatiallyreplacing guilds defined primarily by soil and vegetative

characteristics, with species specializing on specific soiltypes (e.g. Allactodipus bobrinskii on rocky soils,Salpingotus crassicaudata and S. kozlovi on sandy soilsin the Gobi desert). Positive associations observedin Asian deserts are largely attributed to similarhabitat requirements of the functional group members(Shenbrot et al. 1994). Asian desert rodent assemblagescontain species with different geographical origins(Pavlinov et al. 1991; Shenbrot et al. 1995), but theyhave occupied similar regions of temperate Asia sincethe late Miocene (Pavlinov et al. 1995).

The pattern of predictable combinations establishedfor other deserts holds for common-wide ranging speciesin Egypt (Patterson and Brown 1991). Because the totalnumber of combinations is high (214), the fraction ofsites shared with a species’ most frequent co-occurrenceis low. For example, G. gerbillus occurs in 131 sites, itco-occurs most frequently with J. jaculus, but the twoco-occur only in 50 sites (38% of those with G. gerbillus,see Fig. 2). Such persistent patterns in local and regionalassembly may well expose certain species to regularpatterns of coexistence and hence co-evolutionarypressures (Brown et al. 1994; Patterson and Brown1991). Jaculus jaculus’s ability to travel longer distancesand utilize rich resources in the open microhabitat aswell as its diet choice eased its coexistence with twogerbils in the Negev desert (Brown et al. 1994). Theobserved negative associations between sibling andecologically similar species are a direct outcome ofinterspecific competition on similar resources.

Egyptian rodents’ body masses exhibited a uniformdistribution of 1-4 species on an arithmetic scale (10 gintervals) and a normal-shaped distribution on alogarithmic scale (Fig. 4). This distribution persisted inassemblages of 2, 3, 4, 5, and 6 species (Fig. 4). Thispattern could be attributed to the spatial scale of thearea sampled which represents smaller subsets of alarger biogeographic area (Brown 1995), and confirmsthat coexisting species in local habitats are non-randomsubsets of the continental fauna (Brown and Nicoletto1991). Body masses of Egyptian rodents apparentlyconstrain both the area of geographic range of eachspecies as well as its abundance (Fig. 5). Specializationby the smallest and largest species may explain theirrestricted distribution, while G. gerbillus (�24 g), ap-pears as a ‘‘core’’ species exhibiting a generalist strategyover a large geographic range. In agreement withpatterns previously reported by Brown (1995), thehighest abundance is reached by a relatively small (butnot the smallest) mouse, in this case G. perpadillus withan average of 10 specimens per site. Abundance declineswith decreasing and increasing body mass, but the slopeof the lower-boundary line is steeper (Fig. 5b). Moststudies show a negative relationship between body sizeand population abundance (Gregory and Blackburn1995; Brown 1995; Blackburn et al. 1993).


Considering the geographic extent of Egypt inrelation to North American and Australian deserts,our study included a large number of sites and l specieswith intensive sampling in an extreme arid area. Unlikeother areas, our study was done against very limitedecological backgrounds from the literature. Althoughwe could not standardize the effort according to censusdata with a specific number of trap-nights per site,our data set was assembled from a single country-wide survey designed to detect all species at eachcollecting site.

Differences between Egyptian, North American andAustralian desert small mammal assemblages are demon-strated in terms of species richness and trophic structure(Figs. 6 and 7). Aridity and low habitat heterogeneity inboth Egypt and Australia skewed species richness to theright and increased the number of low-richness sites.North American deserts, on the other hand, enjoy moreprecipitation, a larger species pool and high habitatheterogeneity. These differences are reflected not only inassemblage size but also in abundance. Reports haveshown that trapping success in Australia’s arid regions isin the vicinity of 0-5% while reaching up to 50% inNorth American deserts (Morton 1979). In the deserts ofthe Middle East, trapping success is ofteno20%. Thegreater intensity of research on the ecology and structureof desert rodent assemblages of North America has alsoplayed a major role in increasing the species pool.Between 1987 and 1999, species richness within the same201 North American sites has increased from 29 to 41(Brown and Kurzius 1987; Kelt et al. 1999).

Similar to previous reports from deserts in thenorthern hemisphere (Kelt et al. 1996), species poolsfor both Egypt -and North America- are dominated bygranivorous species with quadrupedal modes of loco-motion (Kelt et al. 1999) whereas the pools for theAustralian sites were greatly dominated by carnivorousand omnivorous species (Fig. 7), with a slightly greaterrepresentation of bipedal than quadrupedal species. Theresemblance of feeding guilds between the Egyptianand North American trophic structure maybe due tolower diversity of granivorous birds and ants and/orhigher seed production in those desert regions, com-pared to higher diversities of birds and ants in theAustralian deserts (Morton 1979, 1985). Despite simi-larity in trophic structure between North American andEgyptian rodents, New World heteromyid rodentsacquired external cheek pouches, a key innovationfor desert herbivory that has led to greater relianceor predominance of granivory in the New World(Genoways and Brown 1993).

Following prior analyses of desert rodent faunas inthe Americas, Asia, and Australia, we attempted toincorporate patterns of species assembly from an over-looked region in Northern Africa into the desert rodentcommunity ecology. Egyptian rodent assemblages share

many features with rodent faunas elsewhere in parti-cular, low alpha diversity and substantial variability inspecies composition. Species distributions reflect uniquerequirements of physical and biotic environmentalvariables. The comparison of pattern-based approachesat large scales (such as areography and macroecology)with process-based empirical methods at local scales(like habitat associations and foraging theory) aremaximally useful and complementary in developingappropriate processes to biogeographic patterns.

Acknowledgements

A primary debt is owed those who participated in thefield surveys of Egypt and documented its faunas. Weare also grateful to the insightful analysts of desertrodent faunas in the Americas, Asia, and Australiawhose writings made our task much easier. M. AbuBaker is grateful for the many insights and suggestionsof Joel S. Brown. We thank M. R. Sanchez-Villagra andan anonymous reviewer for comments that greatlyimproved the manuscript.

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Documents

Patterns in the local assembly of Egyptian rodent faunas: Areography and species combinations