Journal of Biogeography, 26, 825–841

Patterns in the structure of Asian and NorthAmerican desert small mammal communitiesDouglas A. Kelt, Kontantın Rogovin, Georgy Shenbrot and James H. Brown Department of

Wildlife, Fish, & Conservation Biology, University of California, Davis, CA 95616, U.S.A.,

Academy of Sciences of Russia, Institute of Animal Evolutionary Morphology and Ecology,

Leninskyi pr. 33, Moscow 117071, Russia, Ramon Science Center, Ben Gurion University of

the Negev, Mitpe Ramon, 80600, Israel, and Department of Biology, University of New

Mexico, Albuquerque, NM 87131, U.S.A.

AbstractAim We compared assemblages of small mammal communities from three major desert

regions on two continents in the northern hemisphere. Our objective was to compare these

with respect to three characteristics: (1) species richness and representation of trophic

groups; (2) the degree to which these assemblages exhibit nested community structure; and

(3) the extent to which competitive interactions appear to influence local community

assembly.

Location We studied small mammal communities from the deserts of North America (N=201 sites) and two regions in Central Asia (the Gobi Desert (N=97 sites) and the Turan

Desert Region (N=36 sites), including the Kara-Kum, Kyzyl-Kum, NE Daghestan, and

extreme western Kazakhstan Deserts).

Method To provide baseline data we characterized each desert region in terms of alpha,

beta, and gamma diversity, and in terms of the distribution of taxa across trophic and

locomotory groups. We evaluated nestedness of these communities using the Nestedness

Temperature Calculator developed by Atmar & Patterson (1993, 1995), and we evaluated

the role of competitive interactions in community assembly and applied a null model of

local assembly under varying degrees of competitive interaction (Kelt et al., 1995, 1996).

Results All three desert regions have low alpha diversity and high beta diversity. The total

number of species in each region varied, being highest in North America, and lowest in

the Turan Desert Region. The deserts studied all present evidence of significant nestedness,

but the mechanism underlying this structure appears different in North American and Asia.

In North America, simulations strongly implicate interspecific competition as a dominant

mechanism influencing community and assemblage structure. In contrast, data from Asian

desert rodent communities suggest that these are not strongly influenced by competition;

in fact, they have greater numbers of ecologically and morphologically similar species than

expected. These results appear to reflect strong habitat selection, with positive associations

among species that share similar habitat requirements in these communities. Our analyses

support earlier reports suggesting that predation and abiotic forces may have greater

influences on the assembly and organization of Asian desert rodent communities, whereas

interspecific competition dominates assembly processes in North America. Additionally, we

suggest that structuring mechanisms may be very different among the two Asian deserts

studied. Gobi assemblages appear structured by trophic and locomotory strategies. In

contrast, Turan Desert Region assemblages appear to be randomly structured with respect

to locomotory strategies. When trophic and locomotory categories are combined, however,

Turan species are positively and nonrandomly associated.

Correspondence: Douglas A. Kelt, Department of Wildlife, Fish, &Conservation Biology, University of California, Davis, CA 95616,U.S.A. e-mail: dakelt@ucdavis.edu

1999

826 Douglas A. Kelt et al.

Main conclusions Very different ecological dynamics evidently exist not only between

these continents, but within them as well. These small mammal faunas differ greatly in

terms of community structure, but also appear to differ in the underlying mechanisms by

which communities are assembled. The underlying role of history and geography are

strongly implicated as central features in understanding the evolution of mammalian faunas

in different deserts of the world.

KeywordsDesert rodents, North American deserts, Asian deserts, community assembly, nested subsets,

null model, competition, importance of history

composition. Additionally, the trophic structure of communitiesINTRODUCTIONis variable from desert to desert, and is only loosely predictable

Desert small mammals have long held the attention of by the pool of species that are available to a site. Trophiccommunity ecologists, and have served as models for characteristics of species in a given region appear stronglyunderstanding both patterns of community structure and the influenced by regional and phylogenetic history, and largely byproximate mechanisms underlying these patterns. This partly the characteristics of species that initially colonized thesereflects the relative simplicity of arid zones, with reduced regions (Kelt et al., 1996).habitat complexity and more apparent underlying resources, Most of these earlier studies have been based on the statisticalat least when compared with shrub or forest habitats. In North and graphical distribution of species and species characteristicsAmerica, numerous studies have suggested or demonstrated across multiple sites. Some authors have compared the numbercompetitive interactions among desert rodent species (e.g. of species per site, the number of trophic groups and theirRosenzweig & Winakur, 1969; Rosenzweig & Sterner, 1970; relative richness, and patterns of ecomorphological structureBrown, 1973, 1975; Rosenzweig, 1973; Rosenzweig et al., 1975; (Rogovin & Surov, 1990; Rogovin et al., 1991, 1994; Shenbrot,Schroder & Rosenzweig, 1975; Price, 1978, 1986; Munger & 1992; Rogovin & Shenbrot, 1993; Shenbrot et al., 1994), whileBrown, 1981; Freeman & Lemen, 1983; Frye, 1983; Brown & others have addressed the autecology of specific species (e.g.Munger, 1985; Brown, 1989a; Heske et al., 1994; Valone & Abramsky & Sellah, 1982; Schroder, 1987; Fox & Gullick,Brown, 1996), and interspecific competition, both direct and 1989; Kerley, 1989; Brown & Harney, 1993). A complementaryindirect, has been implicated as a dominant mechanism of approach to understanding the structure and assembly of largecommunity assembly there. Subsequent studies have assemblages of communities is to incorporate null modelsdocumented behavioural (Reichman, 1983; Reichman & Price, (Caswell, 1976; Strong et al., 1979; Harvey et al., 1983; Colwell1993), social (Jones, 1993), morphological (e.g. Rosenzweig & & Winkler, 1984; Kelt et al., 1995). Ecologists have used nullSterner, 1970; Bowers & Brown, 1982), and biogeographical models to address questions in a wide array of ecological and(Brown, 1975, 1987; Brown & Kurzius, 1987, 1989; Kelt and behavioural contexts (reviewed in Gotelli & Graves, 1995).Brown, in press a, b) correlates to local co-occurrence of These have greatly aided the search for effects of particularspecies. ecological processes by comparing observed structure to that

The patterns reported for North American rodent obtained assuming nonindependence among or within speciescommunities were so simple and intuitively appealing that they or between biotic and abiotic factors such as temperature,gradually reached the status of paradigms for the structure and precipitation, etc.assembly of desert small mammal communities. This generality Earlier (Kelt et al., 1996) we evaluated patterns of specieswas challenged by several authors (e.g. Mares, 1983; Morton, distribution and community structure in the desert small1985; Kerley, 1992; 1993a; 1993b; Morton et al., 1994; Rogovin mammal faunas of seven deserts on four continents. In theet al., 1994; Kelt et al., 1996), resulting in changes in our present study we employ two null models to expand beyondunderstanding of the patterns of community organization in pattern description and analysis to a more refined evaluationdifferent deserts. In particular, desert small mammal of the structure and assembly of desert small mammal

communities generally exhibit low alpha diversity (S=2–4 communities in North America and Asia. Specifically, we ask

species) and high beta diversity, whereas gamma diversity varies if nocturnal terrestrial small mammal communities in North

from desert to desert, although this is confounded by differences American and two Asian deserts are similarly structured, or if

in their geographical extent (Kelt et al., 1996). Species in there are broad differences in the type of structure characterizing

most desert regions are distributed in a Gleasonian manner, these faunas. The first null model that we apply evaluates the

responding to the spatial distribution of those variables that degree to which communities are nested subsets of more species-

determine their individual niches (Brown & Kurzius, 1987; rich communities (Patterson & Atmar, 1986). Significant

Morton et al., 1994; Shenbrot et al., 1994). As a result, local nestedness is thought to reflect underlying hierarchical

relationships among the species in a region (Patterson &communities are fluidly structured with respect to species

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

North American and Asian desert rodents 827

Brown, 1991). Such factors as differential immigration and/or several endemic taxa, they also share a number of species and

genera (e.g. Mares, 1993a). Direct comparisons of these twoextinction, hierarchical competitive relationships, or differential

habitat relationships might result in a nested pattern. The hierarchical schemes are difficult at best. However, the Turanian

and Mongolian regions served as the two main centres ofsecond model implicitly assumes competitive interactions to

occur between ecologically similar species. This model radiation for Central Asian rodent faunas, and these are

separated physically by the expansive (c. 2000 km east tocompares an observed assemblage of communities against

simulated assemblages produced by random assembly (i.e. no west) Kazakhstan Desert (Heptner, 1945). As a result, higher

taxonomic comparisons (e.g. genera, families) demonstrateinteractions among species), thereby testing the null hypothesis

of noninteractive assembly. Additionally, this model allows for much greater differences among Asian deserts than among

North American deserts. Because these Asian deserts arean estimate of the mean strength of interaction among species,

as well as the power of this estimate (Kelt et al., 1995). We physically separated from each other by the Kazakhstan Desert

and high mountain ranges (e.g. the Altay and Sayan Mountains),discuss the results of these analyses in the context of earlier

studies on the worlds desert small mammal communities, and are physiognomically and climatically very different (Walter &

Box, 1983), remain relatively little studied (relative to Norththe role that history has played in structuring contemporary

communities. American deserts; e.g. Genoways & Brown, 1993), and exhibit

substantial faunal differences above the level of species, we

feel it is more informative to analyse the Turan and GobiMETHODS

regions separately.

For both data sets, only nocturnal and terrestrial speciesTerminology

under 500 g in body weight were included; because we are not

certain of the efficiency with which sciurids, gophers, andTo avoid confusion, we define a community as the set of species

that occur at a local site. In this paper we will use the term soricines were sampled, these groups have been excluded from

these analyses. This results in only minor changes to theassemblage to refer to a set of communities. Thus, for example,

we will refer to the assemblage of small mammal communities database used by Kelt et al. (1996); a single site in the Gobi

Desert has been removed from the present analysis because itfrom the Gobi Desert. Taxonomy follows Wilson & Reeder

(1993). possessed only Rhombomys opimus (Lichtenstein 1823), a large

diurnal prairie dog-like gerbilline rodent.

DataAnalyses

Data for North American deserts were taken from Brown &

Kurzius (1987) as corrected in Morton et al. (1994), and consist As background for interpretation of subsequent analyses we

compared the three desert regions in terms of the speciesof a presence/absence matrix of forty-one species at 201 sites

broadly distributed throughout the Great Basin, Sonoran, composition within functional groups occurring at local sites,

as well as those in both habitat and regional species pools,Chihuahuan, and Mojave Deserts of the western United States

(Table 1). All sites experienced a minimum of 100 trap-nights using a one-way Model II analysis of variance, and employed

Scheffe’s multiple-comparison test to assess which desertsof effort; in long-term studies only data from the first year

were used. Details on the locations and characteristics of these deviated significantly. We then employed two statistical

randomizations to evaluate internal structure.sites are given in Brown & Kurzius (1987).

Similar data for Asian deserts have been taken from Kelt

et al. (1996), and were originally reported by Shenbrot et al. Null model of nested structure

A series of communities is considered to be nested when each(1994) and Rogovin & Shenbrot (1995). These data include

twenty species from ninety-seven sites in Mongolia, and fifteen species is present in all communities richer than the most

depauperate community in which that species occurs (Pattersonspecies from thirty-six sites in the Turan Desert Region, which

here is considered to include the Kyzyl-Kum, Kara-Kum, & Atmar, 1986). Atmar & Patterson (1995) have recently

expanded the generality and availability of this concept withDaghestan and extreme western Kazakstan Deserts (Table 1).

Many Asian species did not readily enter live traps, so this the Nestedness Temperature Calculator, which resolves several

problematic issues in earlier nestedness metrics (e.g. their earliermethod was supplemented with visual surveys and capturing

animals with hand-held nets. metric, N, emphasizes unexpected presences more than

absences, all absences are given equal weight, the metric isAlthough we have grouped all North American deserts in a

single category while evaluating the two Asian deserts dependent upon matrix size and therefore not comparable

across data sets; for details, see Atmar & Patterson, 1993; Kelt,separately, it might be argued that faunal differences between

North American deserts are nearly as extreme as those between 1997). In their approach, an assemblage of communities is

compared to that expected under maximum nestedness. TheAsian deserts, if not equally so, and that these also should

be analysed separately. Hagmeier (1966) segregated North unexpected presence or absence of a species is similar to

information surprise, and may be thought of conceptuallyAmerican deserts into four separate ‘super-provinces’, while

the Turan and Gobi Deserts are considered distinct “provinces” as increased disorder or entropy. The system’s ‘characteristic

temperature’ is a thermodynamic-like metric reflecting thewithin the same (Turano-Gobian) ‘subrealm’ (G. Shenbrot,

unpublished observations). Although both Asian deserts possess degree to which communities deviate from nestedness. A

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

828 Douglas A. Kelt et al.

Table 1 List of species included in the

analyses. Differences between this listing and

that in Kelt et al. (1996) reflect different

objectives. For example, Eolagurus przwalskii

(Buchner 1889) and Meriones meridianus

(Pallas 1773) are diurnal and are not included

here. Microtus limnophilis (Buchner 1889)

and Phodopus campbelli (Thomas 1905) were

not collected at any sites and were not

included in our earlier analysis. These species

do contribute to the species pool for these

sites, however, and are therefore included

here for use in the null model. Taxonomy

follows Wilson & Reeder (1993). Codes are:

B=bipedal, Q=quadrupedal, F=folivore,

G=granivore, I=insectivore, M=mixed

folivore/omnivore, O=omnivore.

Mode of locomotion Trophic code

North America

Heteromyidae, Dipodomyinae

Dipodomys deserti Stephens, 1887 B G

Dipodomys merriami Mearns, 1890 B G

Dipodomys microps (Merriam, 1904) B F

Dipodomys ordii Woodhouse, 1853 B G

Dipodomys panamintinus (Merriam, 1894) B G

Dipodomys spectabilis Merriam, 1890 B G

Microdipodops megacephalus Merriam, 1981 Q G

Microdipodops pallidus Merriam, 1901 Q G

Heteromyidae, Perognathinae

Chaetodipus baileyi Merriam, 1894 Q G

Chaetodipus fallax Merriam, 1889 Q G

Chaetodipus formosus Merriam, 1889 Q G

Chaetodipus hispidus Baird, 1858 Q G

Chaetodipus intermedius Merriam, 1889 Q G

Chaetodipus nelsoni Merriam, 1894 Q G

Chaetodipus penicillatus Woodhouse, 1852 Q G

Perognathus amplus Osgood, 1990 Q G

Perognathus flavus Baird, 1855 Q G

Perognathus longimembris (Coues, 1875) Q G

Perognathus parvus (Peale, 1848) Q G

Muridae, Arvicolinae

Lemmiscus curtatus (Cope, 1868) Q F

Microtus longicaudus (Merriam, 1888) Q F

Microtus montanus (Peale, 1848) Q F

Muridae, Cricetinae

Baiomys taylori (Thomas, 1887) Q O

Neotoma albigula Hartley, 1894 Q F

Neotoma lepida Thomas, 1893 Q F

Neotoma micropus Baird, 1885 Q F

Onychomys leucogaster (Wied-Neuwied, 1841) Q I

Onychomys torridus (Coues, 1874) Q I

Peromyscus boylii (Baird, 1855) Q O

Peromyscus crinitus (Merriam, 1891) Q O

Peromyscus eremicus (Baird, 1858) Q O

Peromyscus leucopus (Rafinesque, 1818) Q O

Peromyscus maniculatus (Wagner, 1845) Q O

Peromyscus pectoralis Osgood, 1904 Q O

Peromyscus truei (Shufeldt, 1885) Q O

Reithrodontomys fulvescens J. A. Allen. 1894 Q O

Reithrodontomys megalotus (Baird, 1858) Q G

Reithrodontomys montanus (Baird, 1855 Q O

Sigmodon arizonae Mearns, 1890 Q F

Sigmodon hispidus Say and Ord, 1825 Q F

Sigmodon ochrogaster Bailey, 1902 Q F

Gobi Desert

Dipodidae, Allactaginae

Allactaga balikunica Hsia and Fang, 1964 B O

Allotaga bullata Allen, 1925 B O

Allactaga sibirica (Forster, 1778) B O

Pygeretmus pumilio (Kerr, 1792) B F

Dipodidae, Cardiocraniinae

Cardiocranius paradoxus Satunin, 1903 B G

Salpingotus crassicauda Vinogradov, 1924 B G

Salpingotus kozlovi Vinogradon, 1922 B I

Dipodidae, Dipodinae

Dipus sagitta (Pallas, 1773) B M

Stylodipus andrewsii Allen, 1925 B M

continued

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

North American and Asian desert rodents 829

Table 1 continued.

Mode of locomotion Trophic code

Dipodidae, Euchoreutinae

Euchoreutes naso Sclater, 1891 B I

Muridae, Arvicolinae

Microtus limnophilus Buchner, 1889 Q F

Muridae, Cricetinae

Allocricetulus curtatus Allen, 1925 Q I

Cricetulus migratorius (Pallas 1773) Q O

Cricetulus sokolovi Orlov and Malygin, 1988 Q O

Phodopus campbelli (Thomas, 1905) Q G

Phodopus roborovskii (Satunin, 1903) Q G

Muridae, Gerbillinae

Meriones meridianus (Pallas, 1773) B M

Muridae, murinae

Mus musculus Linnaeus, 1758 Q O

Turan Desert Region

Dipodidae, Allactaginae

Allactaga elater (Lichtenstein, 1828) B M

Allactaga major (Kerr, 1792) B O

Allactaga severtzovi Vinogradov, 1925 B O

Allactodipus bobrinskii Kolesnikov, 1937 Q F

Pygeretmus platiurus (Lichtenstein, 1823) B F

Pygeretmus pumilio (Kerr, 1792) B F

Dipodidae, Dipodinae

Dipus sagitta (Pallas, 1773) B M

B MJaculus blanfordi turcmenicus Vinogradov and Bondar, 1949

Stylodipus telem (Lichtenstein, 1823) B M

Dipodidae, Paradipodinae

Paradipus ctenodactylus (Vinogradov, 1929) B F

Muridae, Cricetidae

Allocricetulus eversmani (Brabdt, 1895) Q I

Cricetulus migratorius (Pallas, 1773) Q O

Muridae, Gerbillinae

Meriones libycus Lichtenstein, 1823 Q M

Meriones meridianus (Pallas, 1773) Q M

Meriones tamariscinus (Pallas, 1773) Q M

perfectly ‘cold’ assemblage would be completely nested, Fox’s model assumes that competitive interactions occur

between species that use their environment in a similar manner.whereas increasingly non-nested assemblages would include

greater numbers of unexpected presences and absences, and Such species share some ecologically important attributes, and

comprise the set of species in a given functional group (seewould therefore have a higher ‘temperature’ (see Atmar &

Patterson (1993) for methodological details). Kelt et al., 1995 for details). According to the null model,

communities should develop such that each new species isWe calculated the ‘temperatures’ of North American and

Asian desert rodent communities, as well as the probability drawn from a different functional group, until all functional

groups are represented, at which point the procedure isthat these are random subsets of the overall pools of species

available. Earlier, Patterson & Brown (1991) analysed the repeated. Functional groups often are based on diet, such that

species are characterized as omnivores, folivores, carnivores,nestedness of the granivore component of the North American

data set, and reported these to be significantly nested. etc. In a community with these three functional groups, the

first species to enter could be a member of any functionalSubsequently, Kelt and Brown (in press a) expanded these

analyses to include nongranivorous species. Here, we extend group. The second species, however, would be expected to

come from one of the two functional groups that are not yetthese analyses to include Asian deserts, and to compare the

patterns there with those in North America. represented, and the third species would be a member of the

final functional group. Therefore, a three species community

would be predicted to possess one omnivore, one folivore, andNull model of competitive assembly

Fox (1987, 1989) presented a null model to describe the assembly one carnivore (we could denote this as [1,1,1]). A fourth species

could be drawn from any of the functional groups, and so on.of communities of locally interacting species. Readers familiar

with this model, especially as advanced by Kelt et al. (1995), On the other hand, the model would not predict a three species

community in which two species were from one functionalmay wish to skip this section.

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

830 Douglas A. Kelt et al.

group, one species was from a second functional group, and Lotka–Volterra models (e.g. Roughgarden, 1979), as it does

not reflect direct interaction between individuals. Rather, h isthe third functional group was not represented (denoted as [2,

1,0]). Such a configuration of species would be unexpected, as the reduction in the probability of establishment by a species

that has immigrated to a site. Assembly proceeds by thecompetitive interactions would be greatest among the two

species sharing functional group membership, and one of these immigration and establishment of species. Immigration is

entirely dependent upon the pool of species that are availablewould be expected to be competitively excluded from the

community, until a member of the third functional group was to enter a community. Species are drawn from functional groups

according to the relative size of these groups, and species arepresent. At this point, there would be an equal competitive

influence within any of the three functional groups, so a fourth drawn without replacement, so that no species may be entered

twice. Immigration probabilities are recalculated after eachspecies could come from any group. Unexpected combinations

of species, such as the [2,1,0] configuration just described, are successful establishment.

The probability of establishment of a species depends uponreferred to as unfavoured states. In contrast, configurations

conforming to the model are referred to as favoured states. the composition of the community. Each species present in the

community decreases the probability of successfulExamples of favoured states include [1,1,1], [2,1,1], or [3,2,2].

Favoured states are those in which the difference between the establishment of another species in the same functional group;

the strength of this interaction is h, and theoretically may rangenumber of species in any two categories is not greater than

one. from –x to 1. Positive values of h reflect negative associations

among species, whereas negative values represent positiveFox’s (1987) model has three principal features (see Kelt

et al., 1995). First, all species (both those present at observed associations. When h =1 an incoming species is barred from

establishing. In the null model of random assembly, interactionssites and those comprising the pool of species that could

enter these sites) are placed in functional groups. Second, are zero, and h =0. Two factors that may produce positive

associations among species (h < 0) are habitat choice orcommunities at all sites are scored as being favoured or

unfavoured, and a tally (Tobs) of the number of sites in a geographical structuring of species pools. Unfortunately,

because h theoretically extends to –x it is difficult to evaluatefavoured configuration is computed. Third, Tobs is compared

to the expected distribution of T under the null hypothesis of the strength of such associations (e.g. is h =–1 ‘strong’?; how

much ‘stronger’ is h=−3?), except to document that these areno competitive interaction. This distribution is obtained via a

Monte Carlo simulation. For each site, a simulated community nonrandom (e.g. h≠h).

Species pools were prepared from the literature and ouris assembled by randomly drawing species from the species

pool for that site until the number of species in the simulated personal knowledge of the ecological and geographical

distribution of these species. For North America and Asia, thecommunity equals that in the observed community. The

simulated community is scored as being in a favoured or species pools consisted of those species whose geographical

ranges overlapped a particular site. Thus, each site had aunfavoured state, and the process is repeated for the remaining

sites in the assemblage. The number of simulated communities potentially unique pool of potential immigrants. Because this

was generated strictly by the geographical distribution ofthat are favoured is recorded (yielding one estimate of the

value of T), and the entire process is repeated. Each such species, we refer to this as the geographical species pool (GSP).

Because two of us (KR and GS) collected all of the Asian datacycle represents one iteration of the model, and produces an

additional estimate of T. This is repeated many times (here, ourselves we also have detailed information on the particular

habitats at each Asian site, and were therefore able to developn=2000) to produce a frequency distribution of the number

of favoured states expected for this assemblage of communities. a second set of species pools based on the known habitat

tolerances of these species; we refer to this as the habitat speciesThe observed community is then compared to this distribution.

If the observed number of favoured states lies in the upper pool (HSP). Because the North American data set was largely

extracted from the literature we could not develop a series ofcritical region (aupper) of this distribution then the observed

assemblage is considered to be significantly more structured HSPs that we believed was consistent across all sites.

Finally, we developed species pools based on two differentthan expected by random assortment, and competition is

inferred. Alternatively, the observed number of favoured states functional categories. Extensive research on North American

desert rodents has suggested that both locomotory modemay lie in the lower critical region (alower) of the distribution,

indicating that the observed assemblage is significantly more (bipedal vs. quadrupedal) and trophic habits are important

factors influencing community structure. Additionally, theseclumped than expected by random assortment, and some sort

of positive association between species is inferred. Because factors – especially locomotory mode – have been invoked to

help explain structure in Asian desert rodent communities (e.g.either alternative is possible, we employ a two-tailed test with

a=2.5%. Rogovin & Surov, 1990; Shenbrot, 1992; Rogovin & Shenbrot,

1993, 1995; Shenbrot et al., 1994; Shenbrot & Rogovin, 1995).Kelt et al. (1995) extended Fox’s model by incorporating

explicit alternative hypotheses through the introduction of an Thus, we prepared GSPs and HSPs with each of these

characteristics, as well as with both. Trophically based species‘association coefficient’ (h,=‘interaction coefficient’ of Kelt

et al., 1995) that produces a decrease in the probability of a pools segregated species as omnivores, folivores, carnivores, or

granivores. Because many Asian species are both granivorousspecies successfully establishing in a community if another

species in the same functional group is already present. Note and folivorous (Shenbrot et al., 1994), we have included a fifth

trophic category that we call mixed granivorous-folivorous. Inthat this is not the same interaction coefficient used in classic

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

North American and Asian desert rodents 831

a separate analysis (Kelt et al., 1996) we allocated such species constitutes an estimate of the power with which we can state

that observed communities are significantly different from theequally to both granivorous and folivorous categories. Because

the model used in this paper requires that functional groups null. Note that because the frequency distributions are discrete

(not continuous) we cannot provide power at exactly a=0.05;contain integer quantities of species, however, we could not

consider a species to be 50% granivorous and 50% folivorous. rather, we bracket this a level with adjacent categories.

In North America, many microtines undergo seasonal shifts in

their dietary strategies, focusing largely on green vegetationRESULTS

when available (winter and spring), but switching to seeds and

other items during summer and fall. This is fundamentally In order to provide sufficient background for interpreting the

results of simulations, we compared these three desert regionsdifferent from the mixed diets of such Asian taxa as Allactaga

(F. Cuvier 1837), Dipus Zimmermann 1780, Jaculus Erxleben in terms of local species richness (within and across functional

groups) and we evaluated patterns of species co-occurrence1777, Meriones Illiger 1811, and Stylodipus Allen 1925, which

forage on both food items whenever available. Therefore we and species combinations (sensu Brown & Kurzius, 1987;

Morton et al., 1994; Kelt et al., 1996).have retained this category, although we recognize that some

readers may find it awkward. To evaluate the sensitivity of

these analyses to such allocations, however, we conducted anLocal communities and regional species pools

additional set of analyses in which we grouped mixed folivores/

omnivores with omnivores. Locomotory based species pools The number of species present at local sites varies considerably

across these deserts. The total number of species present issegregated species according to whether they were bipedal or

quadrupedal. Finally, these categories were combined to form lower in the Turan Desert Region than in the Gobi or in North

America, but the distribution of trophic and locomotory typescombination species pools, in which species were segregated as

bipedal granivores, quadrupedal folivores, bipedal omnivores, is more complex (Table 2; Figs 1 and 2). North America

and the Gobi Desert have high numbers of insectivores andquadrupedal carnivores, etc.

The probability of a species successfully immigrating into omnivores, whereas the Turan Desert Region shares high

representation of folivores with North America, and has thea site, and subsequently establishing itself there, may be

calculated as: fewest number of granivores of these three regions. The Turan

Desert Region also possesses more mixed omnivore/folivores

than the Gobi. North America and the Gobi Desert haveP(yj= i | Xj−1)=

(ni−Xi, j−1)(1−h)Xi, j−1

;ki=1 [(ni−Xi, j−1)(1−h)Xi, j−1]

(1) significantly more bipedal species, and significantly fewer

quadrupedal species, than the Turan Desert Region.

The number of species in the geographical species pools for

these sites also varies greatly from desert to desert (Table 3).where i and j index functional groups and species, respectively,

k is the number of functional groups, ni is the number of North American sites have the largest geographical species

pools, and include significantly more folivorous, omnivorous,species in functional group n, yj is a random variable, indicating

the functional group to which the jth species will be placed, and granivorous species than Asian deserts. This result may

be confounded by the much larger area of the North AmericanXj−1 is a vector of local species composition after the j−1th

species has entered the community, and Xi, j−1 is a scalar which deserts (Kelt et al., 1996), or by the subdivision of North

American deserts by the many mountain ranges that occurgives the number of species in functional group i after the

jth species has entered. The numerator gives the combined there North America is intermediate in richness of insectivorous

species, although exclusion of shrews (Insectivora, Soricidae)probability of a species immigrating and becoming established.

The denominator normalizes these probabilities so that they may have influenced this result. North America had significantly

fewer bipedal species, and significantly more quadrupedalsum to one (Kelt et al., 1995).

Finally, by varying the value of h we can determine the species, than either Asian region. The Gobi Desert has the

greatest number of bipedal species, and of insectivorous species,value of h that produces a distribution of expected number of

favoured states that best agrees with the observed number of and the fewest folivorous species. The Turan Desert Region

has the fewest insectivores, omnivores, and granivores, and anfavoured states. We refer to this value as h, and it constitutes

a maximum likelihood estimate of the mean strength of intermediate number of folivores. This region had a greater

number of mixed omnivore/folivores than the Gobi Desert.nonindependence across all sites and all species. While this is

clearly a general value with limited ability to predict the Finally, the numbers of species whose habitats are congruent

with the characteristics of a given site (species in the habitatstrength of interaction at any given site, or with any pair of

species, its existence allows us to estimate the statistical power species pools) was only evaluated for Asian sites. There, sites

in the Gobi Desert were ecologically suitable to more speciesof our model. Power is the probability that a hypothesis is

incorrect and therefore, should be refuted. If we assume that than were sites in the Turan Desert Region. Habitat pools in

the Gobi Desert contained more bipedal species, and moreh reflects the real strength of interaction among these species,

then the proportion of the distribution of expected values insectivorous and omnivorous species, than the Turan Desert

Region, whereas these pools in the latter region, in turn,(number of favoured states) produced with h=h that lies in

the 2.5% critical region (two-tailed test) of the distribution of possessed more quadrupedal species and more folivorous and

mixed omnivorous/folivorous taxa (Table 4).expected values produced with h=0 (the null hypothesis)

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

832 Douglas A. Kelt et al.

Table 2 Results of analysis of variance on the number of species found at sites. Desert regions with the same letters were not significantly

different in a posteriori tests (A denotes greater richness than B, etc.).

Trophic categories d.f. SS F P Gobi Turan Desert North America

Region

All species

Model 2 23.16 3.42 0.0339 A,B B A

Error 331 1121.54

Folivores

Model 2 10.18 16.67 0.0001 B A A

Error 331 101.00

Insectivores

Model 2 4.31 7.41 0.0007 A B A

Error 331 96.29

Omnivores

Model 2 8.93 8.14 0.0004 A B A

Error 331 181.66

Granivores

Model 2 243.22 91.36 0.0001 B C A

Error 331 440.61

Mixed Folivores/Omnivores

Model 2 194.39 408.49 0.0001 B A C

Error 331 78.76

Bipedal species

Model 2 45.75 10.90 0.0001 A B A

Error 331 694.31

Quadrupedal species

Model 2 7.52 4.11 0.0173 B A B

Error 331 302.76

Species co-occurrences for more restrictive data sets by Fox & Brown (1993) and Kelt

& Brown (in press a). When species were characterized byIn both North America and Asia, species co-occurred with a

both trophic (granivore, omnivore, folivore, insectivore) andlarge number of other species and in many different

locomotory (bipedal v. quadrupedal) features, thesecombinations (Fig. 3). Species generally occurred most

communities appeared highly structured (Table 6). This patternfrequently with a small number of species, and progressively

also held true for analyses using only locomotory mode toless frequently with a large number of species.

characterize species. When only trophic features were

incorporated, however, no structure was indicated (Table 6).Community nestedness Moreover, species at these sites exhibit strong negative

associations. Using locomotory categories, h=0.41 (powerNorth American sites exhibited moderately high system> 99%), whereas using trophic and locomotory categories‘temperatures’, but these were significantly lower (morecombined, h=0.36 (power=89–92%; Table 6).structured) than that expected by random (Table 5), indicating

Very different results were obtained for analyses on Asianthat these communities are significantly more nested thandeserts. With sixteen species and ninety-seven sites in the Gobiexpected by random assortment of species. Most Asian desertsDesert, analyses based on trophic or locomotory modes, andexhibit similar patterns. All three regions of the Gobi Desert,using either regional or habitat species pools, demonstratedand the Gobi as a whole, are significantly more nested thansignificant deviation from random assortment. However, in allexpected. The deserts of the Turan Desert Region are alsocases the results of simulations indicated that the distributionsignificantly more nested than expected assuming randomof species in functional groups in these communities wasdistribution of species. The only exception to this pattern issignificantly more clumped than expected by random; that is,the geographically restricted Daghestan+ Kazakhstan region.there were fewer favoured states in the real world than inThe lack of a significantly nested structure there may reflectassemblages of simulated communities. This indicates thatreduced power to discriminate, resulting from the relativelysimilar species (those occupying the same functional group)small pool of species (S=9 species) and small number of sitesco-occurred more frequently than expected by chance. The(N=14).power of these analyses was very high (Table 6), indicating

that we are highly likely to be correct in stating that these areNull assembly model

non-randomly assembled communities. The high power of

these analyses also argues strongly for positive, rather thanOur analysis of forty-one species of small mammal at 201 sites

throughout arid North America confirmed the results presented negative, associations among these species. When trophic and

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

North American and Asian desert rodents 833

Figure 1 Number of species found at each site (mean +1 SD), as

well as in the habitat and regional species pools, across all species

and two functional groups based on animals’ mode of locomotion.

Habitat species pools were not determined for North American sites.

It is evident that the greater regional diversity in North America

reflects a radiation of bipedal species (Dipodomys). However, this

increase is not observed at the local level, where observed diversity

of bipedal species is similar across all deserts. Because these figures

represent the mean number of species that occur at a site, or whose

habitat or geographical affinities correspond to those at a given site, Figure 2 Number of species found (mean +1 SD) in functionalthe number of species in the habitat pool may exceed that for the groups based on diet. Habitat species pools were not determined forgeographical pool at a given site (e.g. quadrupedal species in the North American sites. North American sites do not have any mixedTuran Desert Region). omnivores/folivore species, whereas the Turan Desert Region lacks

granivores entirely. The regional diversification of granivores in

North America is also expressed locally, with more species per site

than in the Gobi Desert. North American pools also contain morelocomotory modes were combined, we found no evidence of

omnivores and folivores than Asian deserts, but these do not resultnon-random assortment.

in greatly elevated diversity of these groups locally. Because theseFinally, using fifteen species at thirty-six sites in the Turan figures represent the mean number of species that occur at a site, or

Desert Region, most analyses indicated that these sites are not whose habitat or geographical affinities correspond to those at asignificantly different from random. When we analysed either given site, the number of species in the habitat pool may exceed that

species pools using combined functional groups, our analyses for the geographical pool at a given site (e.g. folivores and

granivores in the Gobi Desert).suggested that these assemblages were significantly less favoured

than random collections of species, as in the analyses for the

Gobi Desert. Similar to analyses for the Gobi Desert, species

pools that demonstrated significant deviation from non- different from random association (Table 6, values in brackets).

For the other seven simulations that were repeated, however,interactive assembly were very powerful (Table 6), and provide

strong support for a conclusion that species sharing functional both h and the probability that these communities were

randomly assembled declined when these trophic categoriesgroup membership are positively associated. In contrast, non-

significant analyses also possessed relatively low statistical were combined (Table 6), indicating increasingly positive

associations and statistical significance. Thus, segregatingpower.

In order to evaluate the influence of the ‘mixed omnivore/ ‘mixed omnivorous/folivorous’ species provided a relatively

conservative analysis for comparison with patterns from Northfolivore’ category on these results we conducted a separate

simulation in which we combined this category with other America.

omnivorous species. In the Turan Desert Region, using the

habitat species pool and analysing structure with the combinedDISCUSSION

trophic and locomotory categories resulted in an increase in hfrom less than −2.0 to −0.4 (a reduction in the degree of Similarities and differences between North American and both

Asian desert rodent communities have been demonstrated inpositive associations), and these results were not significantly

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

834 Douglas A. Kelt et al.

Table 3 Results of analysis of variance on the number of species occurring in regional species pools. Desert regions with the same letters were

not dignificantly different in a posteriori tests (A denotes greater richness than B, etc).

d.f. SS F P Gobi Turan Desert North America

Region

All species

Model 2 3957.08 179.59 0.0001 B B A

Error 331 3646.52

Folivores

Model 2 764.11 365.79 0.0001 C B A

Error 331 345.72

Insectivores

Model 2 74.67 224.87 0.0001 A C B

Error 331 54.95

Omnivores

Model 2 427.11 80.09 0.0001 B C A

Error 331 882.63

Granivores

Model 2 2273.50 428.91 0.0001 B C A

Error 331 1070.20

Mixed Folivores/Omnivores

Model 2 1014.55 7830.31 0.0001 B A C

Error 331 21.44

Bipedal species

Model 2 1069.03 520.19 0.0001 A B C

Error 331 340.11

Quadrupedal species

Model 3 9001.44 540.85 0.0001 B B A

Error 331 2754.43

Table 4 Results of analysis of variance on the number of species occurring in habitat species pools (North American deserts excluded from this

analysis). Letters (A, B) indicate which region had greater richness, with A denoting higher richness than B.

d.f. SS F P Gobi Turan Desert

Region

All species

Model 1 20.64 6.66 0.0109 A B

Error 131 405.75

Folivores

Model 1 20.51 81.89 0.0001 B A

Error 131 32.80

Insectivores

Model 1 32.72 116.45 0.0001 A B

Error 131 36.80

Omnivores

Model 1 3.36 6.53 0.0117 A B

Error 131 67.46

Mixed Folivores/Omnivores

Model 1 159.77 431.22 0.0001 B A

Error 131 48.53

Bipedal species

Model 1 91.87 39.80 0.0001 A B

Error 131 302.39

Quadrupedal species

Model 1 25.42 25.60 0.0001 B A

Error 131 130.10

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

North American and Asian desert rodents 835

Figure 3 Left panels give the numbers of

sites occupied (solid bars) by the most

commonly encountered species in three

deserts, and numbers of different

combinations involving those species (hatched

bars) as a function of the number of

coexisting species per site. Right panels give

the frequency with which each species occurs

with other species. Species presented are the

most commonly encountered species in North

American deserts (Dipodomys merriami) and

the two most commonly encountered species

in the Gobi Desert (Dipus sagitta and

Allactaga sibirica) and the Turan Desert

Region (Meriones meridianus and Allactaga

elator).

Table 5 Results of an analysis on the degree of nestedness in communities from three desert regions. Presented for the original data are the

number of species in the region and the number of sites analysed, the observed system temperature (see text), and the percentage fill of the data

matrix. Simulation results, based on of 500 randomizations, include the probability of the observed system temperature (and the number of

standard deviations between the observed and the mean of the 500 simulations), and the mean (‘characteristic’) temperature and standard

deviation of 500 simulated assemblages.

Characteristic

System temperature

Number of

Desert species/sites Temp Fill P(TΖTobs) Mean SD

Southern Gobi 11/32 26.72° 28.6% 9.48e-5 (–3.74 r) 51.18° 6.54°Western Gobi 12/29 39.34° 31.0% 1.30e-2 (–2.235 r) 53.80° 6.49°Eastern Gobi 10/36 38.18° 39.7% 7.10e-4 (–3.20 r) 56.16° 5.63°Gobi desert 16/97 32.39° 22.6% 2.71e-12 (–6.90 r) 55.42° 3.33°Daghestan + Kazakhstan 9/14 48.60° 34.9% 0.655 (+0.43 r) 44.63° 9.17°Kyzyl-Kum 10/22 38.82° 34.8% 0.0105 (–1.26 r) 48.77° 7.93°Turan Desert Region 15/36 31.87° 22.4% 0.00462 (–2.61 r) 47.88° 6.14°North America 41/201 13.84° 9.7% 4.32e-47 (–15.84 r) 36.44° 1.42°

terms of species diversity, guild structure, and ecomorphology areal extent of our sampling regions (greatest in North America,

smallest in the Turan Region; see Kelt et al., 1996), this should(Rogovin & Surov, 1990; Shenbrot et al., 1994; Rogovin &

Shenbrot, 1995; Kelt et al., 1996). As treated here, North not influence the simulations conducted here, with the single

exception of nestedness in the Daghestan+Kazakhstan portionAmerican deserts possess twice as many terrestrial nocturnal

small mammal species (S=41) as either the Gobi Desert (S= of the Turan Desert Region (see below). The composition of

these deserts were also very different (Figs 1 and 2). Species20) or the Turan Desert Region (S=15). Although this

difference may be partially confounded by differences in the pools for North American sites were dominated by quadrupedal

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

836 Douglas A. Kelt et al.

Table 6 Results of 2000 Monte Carlo simulations. Values of hˆ in boldface print are significantly different from 0. Power is given for critical

values bracketing a=0.05 (see text). Values in brackets are for analyses in which mixed omnivores/folivores were combined with omnivores;

power was not evaluated for these simulations.

Gobi Desert Turan Desert Region North America

P(h=0) h Power P(h=0) h Power P(h=0) h Power

Regional species pool

Trophic categories 0.0215 –0.45 0.5560–0.6490 0.1555 –0.51 0.1820–0.3165 0.0925 –0.30 0.4530–0.5385

[0.0024 c. –0.55 0.0005 < –2.00]

Locomotion categories 0.0005 –0.69 0.9445–0.9615 0.3605 –0.11 0.0755–0.1495 0.0005 0.41 0.9925–0.9950

Combination categories 0.1760 –0.27 0.2410–0.3240 0.0005 –1.90 0.8400–0.9195 0.0001 0.36 0.8865–0.9185

[0.0040 c. –0.95 0.0070 c. –1.9]

Habitat species pool

Trophic categories 0.0025 –0.97 0.8780–0.9280 0.3400 –0.44 0.1780–0.2950

[0.0035 –0.8 0.0195 –2.00]

Locomotion categories 0.0005 –0.75 0.9950–0.9900 0.0580 –0.52 0.2550–0.3925

Combination categories 0.5645 –0.07 0.0810–0.1215 0.0050 –2.20 0.8135–0.8945

[0.0015 c. –0.95 0.3290 –0.40]

species with folivorous, omnivorous, or granivorous diets, 1986; Brown & Harney, 1993; Kelt & Brown, in press a), North

American communities provide strong evidence for competitivewhereas the pools for Gobi Desert sites were dominated by

nonfolivorous species, with a slightly greater representation of interactions. Notably, use of trophic categories alone did not

demonstrate significant structuring, but when the locomotorybipedal than quadrupedal species. Species pools for sites in the

Turan Desert Region were moderately biased towards bipedal category was used, or when both categories were combined,

associations were significantly negative (e.g. h > 0). In contrast,species, and were dominated by folivorous or omnivorous

species (or mixed folivorous/omnivorous species); this region Asian assemblages provide no evidence for a significant role

of interspecific competition in community organization, inlacked granivores entirely. In spite of these functional

differences, alpha diversity remained low in all deserts (Fig. 1), agreement with other pattern-based analyses using these data

or subsets of them (Rogovin & Surov, 1990; Shenbrot et al.,with three to four species typically occurring at a site. Beta

diversity also remained high, as demonstrated by the great 1994).

However, the results of simulations for the two Asianvariability in the number and combinations of species with

which the most widespread species occurred (Fig. 3). deserts were also very different from each other. Assemblages

from the Turan Desert Region exhibited random associationsMost analyses provided strong indications of nested

structure (Table 5), but there were notably different patterns of species when analysed with trophic and locomotory

categories, but were significantly positively associated whenin North America and Asia. Perhaps most apparent is that

both the temperature and fill of the North American deserts these categories were combined (Table 6). When we combined

omnivorous and mixed omnivore/folivore species, however,was much lower than that of the two Asian deserts studied.

The reduced fill is a consequence of combining all North significantly positive associations characterized all analyses

except those based on locomotory categories. Finally, analysesAmerican deserts into one analysis, so that less of the species

pool occurs at any one site. The statistics effect of this, however, on the Gobi Desert demonstrated positive associations among

species, with respect to both trophic and locomotoryis to reduce the characteristic temperature for North American

deserts. That this is still significantly greater than the observed functional groupings, but the combined groupings (both

trophic and locomotory) were significant only when omnivorestemperature (Table 5) further underscores the difference

between North American and Asian faunas. Additionally, when were combined with mixed omnivore/folivore species

(Table 6).analysed separately each North American desert retains a cooler

temperature than any Asian desert studied (Great Basin, 13.55°, Thus, these simulations suggest either that the processes or

dynamics underlying the assembly of small mammalMojave, 23.16°, Sonoran, 13.59°, Chihuahuan, 19.37°; all are

significantly cooler than expected, P < < 10–9). Thus, it is likely communities in these deserts are very different, or that similar

dynamics are occurring, but that these are either very context-that nestedness in North American and Asian assemblages is

a result either of different underlying mechanisms or of dependent or are controlled by different parameter values, such

that the final result appears very different. For example, bothdifferential influence of similar mechanisms. For reasons

discussed below, we believe that different processes operate to Abrams (1990) and Leibold (1996) have suggested that, under

appropriate conditions, competitive interactions may lead toproduce nestedness in North American and Asian deserts.

Results of an iterative model of local assembly suggested the coexistence of ecologically similar species. In any case, the

three desert regions studied appear to constitute a structuralmajor differences in patterns of community assembly in these

deserts. On the basis of much empirical data (reviewed in Price, continuum from significantly dispersed (North America) to

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

North American and Asian desert rodents 837

random or moderately clumped (Turan) to significantly commonly to explain the local coexistence of species in North

American deserts (see also Brown & Harney, 1993). In aclumped (Gobi), at least with respect to this null model and

these functional groups. The factors underlying such variation community of three rodent species in the Negev Desert, Brown

et al. (1994) demonstrated that habitat heterogeneity, promotedmerit consideration, although only further research will allow

us to fully evaluate the intrinsic dynamics producing these by daily winds that re-distributed sand and food resources,

when combined with differing foraging tactics of the threepatterns.

species, appeared to explain the coexistence of these common

species.Factors producing positive associations

Four factors that could produce positive associations among

species are local endemism, shared distributional strategies, Factors leading to nestedness in non-insularshared geographical origins, and shared habitat (Gilpin & communitiesDiamond, 1982). Endemism is not a factor in these continental

faunas, since many species occur over most or all of the Although our analyses support the overriding importance of

competition in structuring North American desert communities,regions concerned. By shared distributional strategies, Gilpin

& Diamond (1982) were referring to their tramp-supertramp competition is only one type of interaction involved in

structuring ecological communities, and the relative strengthcontinuum, which reflects the habitat specificity and dispersal

abilities of each species. However, this is not relevant to of this may vary from place to place and from time to time

(e.g. Kotler & Holt, 1989). Nested faunal structure neithercontinental rodent communities in which the geographical

ranges of most species span the entire desert, as is the case in depends upon, nor implies the action of, competition among

the constituent species (Patterson & Brown, 1991), although itboth the Gobi Desert and the Turan Desert Region.

Although Asian desert rodent assemblages consist of species is consistent with a mechanism of competition (Kelt & Brown,

in press a).with different geographical origins (e.g. Asian dipodid (jerboas)

and cricetid rodents likely radiated within northern Asia, Our analyses for central Asian communities offer no evidence

that competition structures these communities, agreeing withwhereas gerbilline rodents (jirds and gerbils) evolved in the

Sahara-Sindian Desert region; Pavlinov et al., 1991; Shenbrot studies on ecomorphology (Rogovin & Surov, 1990), niche

packing (Shenbrot et al., 1994), and geographical ecology ofet al., 1995), they have occupied similar regions of temperate

Asia since the late Miocene (Pavlinov et al., 1995), and such a these deserts (Rogovin & Shenbrot, 1995). In fact, our results

suggest that species in these communities either arelong period of co-occurrence may contribute to the positive

associations we observed. However, North American deserts noninteractive or are positively interactive, with species being

found more frequently with similar species than expectedand taxa have been present since the late Miocene or early

Pleistocene, and desert adapted rodents in the Heteromyidae at random. Yet, these assemblages are highly nested. The

mechanisms underlying nestedness in Asian assemblages appearmay have a longer history than Asian desert rodent taxa (for

fuller discussions of the history of these deserts and faunas, to have a very different basis than in North America.

Patterson & Brown (1991) proposed three necessarysee Shenbrot et al., 1994; Kelt et al., 1996). We believe that it

is not so much the ages of these deserts and lineages, but conditions for the development of nested subset structure: (1)

a common biogeographic history (2) generally similarthe geomorphology and eco-climatic history that has differed

between them, that has led to the differences that we observe contemporary environments, and (3) hierarchical organization

of niche relationships. These factors all appear to be operationaltoday. We elaborate upon this below.

Shenbrot et al. (1994) argued that Asian desert rodents were both in North America and in central Asia, but the mechanisms

that operate locally to produce the observed communitiesorganized into spatial guilds that were separated primarily by

soil and vegetative characteristics. Thus, some species are found appear to be very different in these geographically isolated

desert regions.only on rocky soils (e.g. Allactodipus bobrinskii Kolesnikov

1937), whereas other species occur primarily or exclusively on The overwhelming evidence for the importance of

competition in North American desert rodent communities,sandy soils (e.g. Salpingotus crassicaudata Vinogradov 1922,

and S. kozlovi Vinogradov 1922, in the western and eastern and the fact that these are much more strongly nested structure

than are Asian communities, supports an argument thatGobi, respectively). Such habitat guilds possess species that co-

occur more frequently than a random assortment would interspecific competition, rather than area-dependent

extinction, is responsible for the observed nestedness. However,suggest, and we believe that the positive associations that we

observe in Asian deserts are largely a reflection of the similar this is not the case for Asian communities. We believe that the

differences observed are at least partly due to the different ageshabitat requirements of the functional group members. When

communities are assembled in our simulations, individual and geomorphologies of North American and Asian deserts,

and the different ages and origins of the mammal lineages inhabitat associations are ignored, and functional groups are

more evenly represented than in real communities. these deserts. In this study we have minimized the variability

in these factors by studying desert regions that share severalAt a smaller spatial scale, variation in microhabitat use (e.g.

Rosenzweig & Winakur, 1969; Brown, 1975), spatial variation features which have been implicated to influence desert

evolution and contemporary structure across a global scale.in resource abundance (Brown, 1989b), and seasonal shifts in

foraging efficiencies (Brown, 1989b) have been invoked most Earlier we reviewed the arguments for historical influences on

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841

838 Douglas A. Kelt et al.

local structure (Kelt et al., 1996), but it is worth reiterating may have sorted the species pools for Asian communities, and

that species presently constituting this fauna represent thosebriefly to place these communities in a broader perspective.

species that were successful competitors and/or those species

that shifted their niche requirements so as to minimizeThe hand of history – again

competitive interactions. This would result in highly structured

regional faunas, predetermining some degree of structure locallyThe deserts considered in this manuscript are all relatively high

latitude deserts, occurring >35–76°N. Additionally, they share (the ‘Narcissus’ effect of Colwell & Winkler, 1984). The greater

age and persistence of Asian deserts has allowed much moresimilar patterns of precipitation and of variation in precipitation

(Kelt et al., 1996). Thus, differences in community structure time to sort out competitive regimes. This hypothesis

emphasizes the pivotal influence of historical processes incannot be explained in terms of strategies for dealing with

extended droughts, as has been postulated for Australia structuring contemporary ecological communities (e.g. Connell,

1980; papers in Ricklefs & Schluter, 1993), and may in fact be(Morton, 1985, 1993; Morton et al., 1994; Kelt et al., 1996).

However, differences are also apparent between these correct (see, e.g. Diamond, 1986; Van Devender, 1986; for

similar arguments). Unfortunately, it is essentially ad hoc, anddeserts. The Gobi and Turan Desert Regions both occur at

relatively high elevations within large continents, whereas is probably not testable in this system (see Connell, 1980;

Lawton, 1984; Strong, 1984).North American deserts vary over a broad range of elevations,

and range from the coast of Mexico to the interior of the

continent. Asian deserts are thought to have originated in the CONCLUSIONCretaceous, but did not become widespread until the Miocene

Our analyses document important similarities and significant(Sinitzin, 1962), whereas North American deserts likelydifferences between the three desert regions studied, andoriginated sometime between the late Miocene and theunderscore the importance of regional and historical influencesPleistocene (Axelrod, 1958; Webb, 1977; Van Devender &on the structure of local communities. Competitive interactionsSpaulding, 1979; Wells, 1979; Thompson & Mead, 1982; Riddle,appear to strongly influence the assembly of rodent faunas in1995). Asian deserts are extensive and present little topographicNorth American deserts. In contrast, our simulations providerelief, whereas those in North America are embedded withinno indication that competition is a central feature in thea Basin and Range topography that provides a much greaterassembly of temperate Asian deserts. We agree with earliervariety of habitats, as well as barriers to gene flow that haveauthors that the distribution and extent of major habitatvaried in severity with the elevational advance and retreat offeatures appear to have greater influence on the assembly offorests during pluvial/interpluvial cycles. These barriers likelysmall mammal communities there (see also, Rogovin & Surov,have favoured isolation of local populations (e.g. Schmidly1990; Shenbrot et al., 1994). While this idea is not novel toet al., 1993), providing greater opportunities for speciationthis paper, ours is the first attempt to model such regionallythan in the more topographically homogeneous Asian deserts.specific assembly dynamics. Additionally, this study indicatesAdditionally, and a consequence of the Basin and Rangethat there may be important differences in the mechanisms bytopography plus the pluvial/interpluvial history of Northwhich communities in the Gobi and Turan Desert Region areAmerica, species found in forested regions of North Americaorganized. The hand of history likely has played a role inhave evolved in proximity to arid regions, and some taxaforming these assemblages, but how this has differentially(e.g. Reithrodontomys Giglioli 1874, Peromyscus Gloger 1841,influenced Gobi and Turan faunas is not clear. If temperateNeotoma Say & Ord 1825) have since colonized desertAsian desert assemblages are structured to some extent by thecommunities. Finally, the pluvial/interpluvial phases justmacrohabitats that are available, then further emphasis on thementioned have resulted in significant temporal changes in thedynamics within these macrohabitats might be expected toareal extent of North American deserts, whereas Asian desertselucidate fundamental structuring forces at the level of habitatare thought to have persisted as arid regions since theirpatches not visible at the broader scale of the present analyses.formation (Sinitzin, 1962). Thus, North American deserts

probably are younger than Asian deserts, but more importantly,

are more heterogeneous (both spatially and temporally), and ACKNOWLEDGMENTShave had greater and more recent opportunities for radiation

We thank Barry Fox, Rick Ostfeld, Bruce Patterson, Maryof the taxa present.Price, and three anonymous reviewers for numerous commentsWe do not wish to imply that Asian rodents do not compete.that improved and clarified the presentation.Competitive interactions likely have helped mold the current

suites of coexisting species, via mechanisms such as

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BIOSKETCHES

Douglas A. Kelt (Ph.D., University of New Mexico), is an Assistant Professor at the University of California, Davis. His research

interests emphasize macroecology, biogeography, and community ecology, primarily of desert and Neotropical small mammals.

Konstantin A. Rogovin (Ph.D., Moscow State University), is Principal Investigator at the Severtzov Institute of Ecology and

Evolution, Russian Academy of Science. He has studied the ecology and behaviour of rodents in Central Asia deserts for many

years, and is currently studying terrestrial vertebrates in Kalmykia (Russian Federation).

Georgy Shenbrot (Ph.D., Moscow State University) is a Researcher at the Ramon Science Center, Ben-Gurion University of the

Negev. He has over 25 years of experience in desert rodent community ecology and taxonomy, and is currently writing a book on

‘Spatial ecology of desert rodent communities’.

James H. Brown (Ph.D., University of Michigan) is a Regents’ Professor of Biology at the University of New Mexico, and past

President of the American Society of Mammalogists, the American Society of Naturalists, and the Ecological Society of America.

Current research projects include long-term experimental studies in the Chihuahuan Desert, investigations into macroecological

patterns of biotic structure, the influence of body size and biological scaling on biodiversity, and applied efforts to preserve

ranching livelihoods and biodiversity in the American South-west.

Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841