Download Patterns in the structure of Asian and North American desert small

Journal of Biogeography, 26, 825–841
Patterns in the structure of Asian and North
American desert small mammal communities
Douglas A. Kelt, Kontantı́n 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.
Abstract
Aim 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: [email protected]
 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.
Keywords
Desert rodents, North American deserts, Asian deserts, community assembly, nested subsets,
null model, competition, importance of history
INTRODUCTION
Desert small mammals have long held the attention of
community ecologists, and have served as models for
understanding both patterns of community structure and the
proximate mechanisms underlying these patterns. This partly
reflects the relative simplicity of arid zones, with reduced
habitat complexity and more apparent underlying resources,
at least when compared with shrub or forest habitats. In North
America, numerous studies have suggested or demonstrated
competitive interactions among desert rodent species (e.g.
Rosenzweig & Winakur, 1969; Rosenzweig & Sterner, 1970;
Brown, 1973, 1975; Rosenzweig, 1973; Rosenzweig et al., 1975;
Schroder & Rosenzweig, 1975; Price, 1978, 1986; Munger &
Brown, 1981; Freeman & Lemen, 1983; Frye, 1983; Brown &
Munger, 1985; Brown, 1989a; Heske et al., 1994; Valone &
Brown, 1996), and interspecific competition, both direct and
indirect, has been implicated as a dominant mechanism of
community assembly there. Subsequent studies have
documented behavioural (Reichman, 1983; Reichman & Price,
1993), social (Jones, 1993), morphological (e.g. Rosenzweig &
Sterner, 1970; Bowers & Brown, 1982), and biogeographical
(Brown, 1975, 1987; Brown & Kurzius, 1987, 1989; Kelt and
Brown, in press a, b) correlates to local co-occurrence of
species.
The patterns reported for North American rodent
communities were so simple and intuitively appealing that they
gradually reached the status of paradigms for the structure and
assembly of desert small mammal communities. This generality
was challenged by several authors (e.g. Mares, 1983; Morton,
1985; Kerley, 1992; 1993a; 1993b; Morton et al., 1994; Rogovin
et al., 1994; Kelt et al., 1996), resulting in changes in our
understanding of the patterns of community organization in
different deserts. In particular, desert small mammal
communities generally exhibit low alpha diversity (S=2–4
species) and high beta diversity, whereas gamma diversity varies
from desert to desert, although this is confounded by differences
in their geographical extent (Kelt et al., 1996). Species in
most desert regions are distributed in a Gleasonian manner,
responding to the spatial distribution of those variables that
determine their individual niches (Brown & Kurzius, 1987;
Morton et al., 1994; Shenbrot et al., 1994). As a result, local
communities are fluidly structured with respect to species
composition. Additionally, the trophic structure of communities
is variable from desert to desert, and is only loosely predictable
by the pool of species that are available to a site. Trophic
characteristics of species in a given region appear strongly
influenced by regional and phylogenetic history, and largely by
the characteristics of species that initially colonized these
regions (Kelt et al., 1996).
Most of these earlier studies have been based on the statistical
and graphical distribution of species and species characteristics
across multiple sites. Some authors have compared the number
of species per site, the number of trophic groups and their
relative richness, and patterns of ecomorphological structure
(Rogovin & Surov, 1990; Rogovin et al., 1991, 1994; Shenbrot,
1992; Rogovin & Shenbrot, 1993; Shenbrot et al., 1994), while
others have addressed the autecology of specific species (e.g.
Abramsky & Sellah, 1982; Schroder, 1987; Fox & Gullick,
1989; Kerley, 1989; Brown & Harney, 1993). A complementary
approach to understanding the structure and assembly of large
assemblages of communities is to incorporate null models
(Caswell, 1976; Strong et al., 1979; Harvey et al., 1983; Colwell
& Winkler, 1984; Kelt et al., 1995). Ecologists have used null
models to address questions in a wide array of ecological and
behavioural contexts (reviewed in Gotelli & Graves, 1995).
These have greatly aided the search for effects of particular
ecological processes by comparing observed structure to that
obtained assuming nonindependence among or within species
or between biotic and abiotic factors such as temperature,
precipitation, etc.
Earlier (Kelt et al., 1996) we evaluated patterns of species
distribution and community structure in the desert small
mammal faunas of seven deserts on four continents. In the
present study we employ two null models to expand beyond
pattern description and analysis to a more refined evaluation
of the structure and assembly of desert small mammal
communities in North America and Asia. Specifically, we ask
if nocturnal terrestrial small mammal communities in North
American and two Asian deserts are similarly structured, or if
there are broad differences in the type of structure characterizing
these faunas. The first null model that we apply evaluates the
degree to which communities are nested subsets of more speciesrich communities (Patterson & Atmar, 1986). Significant
nestedness is thought to reflect underlying hierarchical
relationships among the species in a region (Patterson &
 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
extinction, hierarchical competitive relationships, or differential
habitat relationships might result in a nested pattern. The
second model implicitly assumes competitive interactions to
occur between ecologically similar species. This model
compares an observed assemblage of communities against
simulated assemblages produced by random assembly (i.e. no
interactions among species), thereby testing the null hypothesis
of noninteractive assembly. Additionally, this model allows for
an estimate of the mean strength of interaction among species,
as well as the power of this estimate (Kelt et al., 1995). We
discuss the results of these analyses in the context of earlier
studies on the worlds desert small mammal communities, and
the role that history has played in structuring contemporary
communities.
METHODS
Terminology
To 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
assemblage to refer to a set of communities. Thus, for example,
we will refer to the assemblage of small mammal communities
from the Gobi Desert. Taxonomy follows Wilson & Reeder
(1993).
several endemic taxa, they also share a number of species and
genera (e.g. Mares, 1993a). Direct comparisons of these two
hierarchical schemes are difficult at best. However, the Turanian
and Mongolian regions served as the two main centres of
radiation for Central Asian rodent faunas, and these are
separated physically by the expansive (c. 2000 km east to
west) Kazakhstan Desert (Heptner, 1945). As a result, higher
taxonomic comparisons (e.g. genera, families) demonstrate
much greater differences among Asian deserts than among
North American deserts. Because these Asian deserts are
physically separated from each other by the Kazakhstan Desert
and high mountain ranges (e.g. the Altay and Sayan Mountains),
are physiognomically and climatically very different (Walter &
Box, 1983), remain relatively little studied (relative to North
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 Gobi
regions separately.
For both data sets, only nocturnal and terrestrial species
under 500 g in body weight were included; because we are not
certain of the efficiency with which sciurids, gophers, and
soricines were sampled, these groups have been excluded from
these analyses. This results in only minor changes to the
database used by Kelt et al. (1996); a single site in the Gobi
Desert has been removed from the present analysis because it
possessed only Rhombomys opimus (Lichtenstein 1823), a large
diurnal prairie dog-like gerbilline rodent.
Data
Data for North American deserts were taken from Brown &
Kurzius (1987) as corrected in Morton et al. (1994), and consist
of a presence/absence matrix of forty-one species at 201 sites
broadly distributed throughout the Great Basin, Sonoran,
Chihuahuan, and Mojave Deserts of the western United States
(Table 1). All sites experienced a minimum of 100 trap-nights
of effort; in long-term studies only data from the first year
were used. Details on the locations and characteristics of these
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.
(1994) and Rogovin & Shenbrot (1995). These data include
twenty species from ninety-seven sites in Mongolia, and fifteen
species from thirty-six sites in the Turan Desert Region, which
here is considered to include the Kyzyl-Kum, Kara-Kum,
Daghestan and extreme western Kazakstan Deserts (Table 1).
Many Asian species did not readily enter live traps, so this
method was supplemented with visual surveys and capturing
animals with hand-held nets.
Although we have grouped all North American deserts in a
single category while evaluating the two Asian deserts
separately, it might be argued that faunal differences between
North American deserts are nearly as extreme as those between
Asian deserts, if not equally so, and that these also should
be analysed separately. Hagmeier (1966) segregated North
American deserts into four separate ‘super-provinces’, while
the Turan and Gobi Deserts are considered distinct “provinces”
within the same (Turano-Gobian) ‘subrealm’ (G. Shenbrot,
unpublished observations). Although both Asian deserts possess
 Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841
Analyses
As background for interpretation of subsequent analyses we
compared the three desert regions in terms of the species
composition within functional groups occurring at local sites,
as well as those in both habitat and regional species pools,
using a one-way Model II analysis of variance, and employed
Scheffe’s multiple-comparison test to assess which deserts
deviated significantly. We then employed two statistical
randomizations to evaluate internal structure.
Null model of nested structure
A series of communities is considered to be nested when each
species is present in all communities richer than the most
depauperate community in which that species occurs (Patterson
& Atmar, 1986). Atmar & Patterson (1995) have recently
expanded the generality and availability of this concept with
the Nestedness Temperature Calculator, which resolves several
problematic issues in earlier nestedness metrics (e.g. their earlier
metric, N, emphasizes unexpected presences more than
absences, all absences are given equal weight, the metric is
dependent upon matrix size and therefore not comparable
across data sets; for details, see Atmar & Patterson, 1993; Kelt,
1997). In their approach, an assemblage of communities is
compared to that expected under maximum nestedness. The
unexpected presence or absence of a species is similar to
information surprise, and may be thought of conceptually
as increased disorder or entropy. The system’s ‘characteristic
temperature’ is a thermodynamic-like metric reflecting the
degree to which communities deviate from nestedness. A
828 Douglas A. Kelt et al.
Mode of locomotion Trophic code
North America
Heteromyidae, Dipodomyinae
Dipodomys deserti Stephens, 1887
Dipodomys merriami Mearns, 1890
Dipodomys microps (Merriam, 1904)
Dipodomys ordii Woodhouse, 1853
Dipodomys panamintinus (Merriam, 1894)
Dipodomys spectabilis Merriam, 1890
Microdipodops megacephalus Merriam, 1981
Microdipodops pallidus Merriam, 1901
Heteromyidae, Perognathinae
Chaetodipus baileyi Merriam, 1894
Chaetodipus fallax Merriam, 1889
Chaetodipus formosus Merriam, 1889
Chaetodipus hispidus Baird, 1858
Chaetodipus intermedius Merriam, 1889
Chaetodipus nelsoni Merriam, 1894
Chaetodipus penicillatus Woodhouse, 1852
Perognathus amplus Osgood, 1990
Perognathus flavus Baird, 1855
Perognathus longimembris (Coues, 1875)
Perognathus parvus (Peale, 1848)
Muridae, Arvicolinae
Lemmiscus curtatus (Cope, 1868)
Microtus longicaudus (Merriam, 1888)
Microtus montanus (Peale, 1848)
Muridae, Cricetinae
Baiomys taylori (Thomas, 1887)
Neotoma albigula Hartley, 1894
Neotoma lepida Thomas, 1893
Neotoma micropus Baird, 1885
Onychomys leucogaster (Wied-Neuwied, 1841)
Onychomys torridus (Coues, 1874)
Peromyscus boylii (Baird, 1855)
Peromyscus crinitus (Merriam, 1891)
Peromyscus eremicus (Baird, 1858)
Peromyscus leucopus (Rafinesque, 1818)
Peromyscus maniculatus (Wagner, 1845)
Peromyscus pectoralis Osgood, 1904
Peromyscus truei (Shufeldt, 1885)
Reithrodontomys fulvescens J. A. Allen. 1894
Reithrodontomys megalotus (Baird, 1858)
Reithrodontomys montanus (Baird, 1855
Sigmodon arizonae Mearns, 1890
Sigmodon hispidus Say and Ord, 1825
Sigmodon ochrogaster Bailey, 1902
Gobi Desert
Dipodidae, Allactaginae
Allactaga balikunica Hsia and Fang, 1964
Allotaga bullata Allen, 1925
Allactaga sibirica (Forster, 1778)
Pygeretmus pumilio (Kerr, 1792)
Dipodidae, Cardiocraniinae
Cardiocranius paradoxus Satunin, 1903
Salpingotus crassicauda Vinogradov, 1924
Salpingotus kozlovi Vinogradon, 1922
Dipodidae, Dipodinae
Dipus sagitta (Pallas, 1773)
Stylodipus andrewsii Allen, 1925
B
B
B
B
B
B
Q
Q
G
G
F
G
G
G
G
G
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
G
G
G
G
G
G
G
G
G
G
G
Q
Q
Q
F
F
F
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q
O
F
F
F
I
I
O
O
O
O
O
O
O
O
G
O
F
F
F
B
B
B
B
O
O
O
F
B
B
B
G
G
I
B
B
M
M
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
(Büchner 1889) and Meriones meridianus
(Pallas 1773) are diurnal and are not included
here. Microtus limnophilis (Büchner 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.
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
Muridae, Arvicolinae
Microtus limnophilus Büchner, 1889
Muridae, Cricetinae
Allocricetulus curtatus Allen, 1925
Cricetulus migratorius (Pallas 1773)
Cricetulus sokolovi Orlov and Malygin, 1988
Phodopus campbelli (Thomas, 1905)
Phodopus roborovskii (Satunin, 1903)
Muridae, Gerbillinae
Meriones meridianus (Pallas, 1773)
Muridae, murinae
Mus musculus Linnaeus, 1758
Turan Desert Region
Dipodidae, Allactaginae
Allactaga elater (Lichtenstein, 1828)
Allactaga major (Kerr, 1792)
Allactaga severtzovi Vinogradov, 1925
Allactodipus bobrinskii Kolesnikov, 1937
Pygeretmus platiurus (Lichtenstein, 1823)
Pygeretmus pumilio (Kerr, 1792)
Dipodidae, Dipodinae
Dipus sagitta (Pallas, 1773)
Jaculus blanfordi turcmenicus Vinogradov and Bondar, 1949
Stylodipus telem (Lichtenstein, 1823)
Dipodidae, Paradipodinae
Paradipus ctenodactylus (Vinogradov, 1929)
Muridae, Cricetidae
Allocricetulus eversmani (Brabdt, 1895)
Cricetulus migratorius (Pallas, 1773)
Muridae, Gerbillinae
Meriones libycus Lichtenstein, 1823
Meriones meridianus (Pallas, 1773)
Meriones tamariscinus (Pallas, 1773)
perfectly ‘cold’ assemblage would be completely nested,
whereas increasingly non-nested assemblages would include
greater numbers of unexpected presences and absences, and
would therefore have a higher ‘temperature’ (see Atmar &
Patterson (1993) for methodological details).
We calculated the ‘temperatures’ of North American and
Asian desert rodent communities, as well as the probability
that these are random subsets of the overall pools of species
available. Earlier, Patterson & Brown (1991) analysed the
nestedness of the granivore component of the North American
data set, and reported these to be significantly nested.
Subsequently, Kelt and Brown (in press a) expanded these
analyses to include nongranivorous species. Here, we extend
these analyses to include Asian deserts, and to compare the
patterns there with those in North America.
Null model of competitive assembly
Fox (1987, 1989) presented a null model to describe the assembly
of communities of locally interacting species. Readers familiar
with this model, especially as advanced by Kelt et al. (1995),
may wish to skip this section.
 Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841
B
I
Q
F
Q
Q
Q
Q
Q
I
O
O
G
G
B
M
Q
O
B
B
B
Q
B
B
M
O
O
F
F
F
B
B
B
M
M
M
B
F
Q
Q
I
O
Q
Q
Q
M
M
M
Fox’s model assumes that competitive interactions occur
between species that use their environment in a similar manner.
Such species share some ecologically important attributes, and
comprise the set of species in a given functional group (see
Kelt et al., 1995 for details). According to the null model,
communities should develop such that each new species is
drawn from a different functional group, until all functional
groups are represented, at which point the procedure is
repeated. Functional groups often are based on diet, such that
species are characterized as omnivores, folivores, carnivores,
etc. In a community with these three functional groups, the
first species to enter could be a member of any functional
group. The second species, however, would be expected to
come from one of the two functional groups that are not yet
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, and
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.
On the other hand, the model would not predict a three species
community in which two species were from one functional
830 Douglas A. Kelt et al.
group, one species was from a second functional group, and
the third functional group was not represented (denoted as [2,
1,0]). Such a configuration of species would be unexpected, as
competitive interactions would be greatest among the two
species sharing functional group membership, and one of these
would be expected to be competitively excluded from the
community, until a member of the third functional group was
present. At this point, there would be an equal competitive
influence within any of the three functional groups, so a fourth
species could come from any group. Unexpected combinations
of species, such as the [2,1,0] configuration just described, are
referred to as unfavoured states. In contrast, configurations
conforming to the model are referred to as favoured states.
Examples of favoured states include [1,1,1], [2,1,1], or [3,2,2].
Favoured states are those in which the difference between the
number of species in any two categories is not greater than
one.
Fox’s (1987) model has three principal features (see Kelt
et al., 1995). First, all species (both those present at observed
sites and those comprising the pool of species that could
enter these sites) are placed in functional groups. Second,
communities at all sites are scored as being favoured or
unfavoured, and a tally (Tobs) of the number of sites in a
favoured configuration is computed. Third, Tobs is compared
to the expected distribution of T under the null hypothesis of
no competitive interaction. This distribution is obtained via a
Monte Carlo simulation. For each site, a simulated community
is assembled by randomly drawing species from the species
pool for that site until the number of species in the simulated
community equals that in the observed community. The
simulated community is scored as being in a favoured or
unfavoured state, and the process is repeated for the remaining
sites in the assemblage. The number of simulated communities
that are favoured is recorded (yielding one estimate of the
value of T), and the entire process is repeated. Each such
cycle represents one iteration of the model, and produces an
additional estimate of T. This is repeated many times (here,
n=2000) to produce a frequency distribution of the number
of favoured states expected for this assemblage of communities.
The observed community is then compared to this distribution.
If the observed number of favoured states lies in the upper
critical region (aupper) of this distribution then the observed
assemblage is considered to be significantly more structured
than expected by random assortment, and competition is
inferred. Alternatively, the observed number of favoured states
may lie in the lower critical region (alower) of the distribution,
indicating that the observed assemblage is significantly more
clumped than expected by random assortment, and some sort
of positive association between species is inferred. Because
either alternative is possible, we employ a two-tailed test with
a=2.5%.
Kelt et al. (1995) extended Fox’s model by incorporating
explicit alternative hypotheses through the introduction of an
‘association coefficient’ (h,=‘interaction coefficient’ of Kelt
et al., 1995) that produces a decrease in the probability of a
species successfully establishing in a community if another
species in the same functional group is already present. Note
that this is not the same interaction coefficient used in classic
Lotka–Volterra models (e.g. Roughgarden, 1979), as it does
not reflect direct interaction between individuals. Rather, h is
the reduction in the probability of establishment by a species
that has immigrated to a site. Assembly proceeds by the
immigration and establishment of species. Immigration is
entirely dependent upon the pool of species that are available
to enter a community. Species are drawn from functional groups
according to the relative size of these groups, and species are
drawn without replacement, so that no species may be entered
twice. Immigration probabilities are recalculated after each
successful establishment.
The probability of establishment of a species depends upon
the composition of the community. Each species present in the
community decreases the probability of successful
establishment of another species in the same functional group;
the strength of this interaction is h, and theoretically may range
from –x to 1. Positive values of h reflect negative associations
among species, whereas negative values represent positive
associations. When h =1 an incoming species is barred from
establishing. In the null model of random assembly, interactions
are zero, and h =0. Two factors that may produce positive
associations among species (h < 0) are habitat choice or
geographical structuring of species pools. Unfortunately,
because h theoretically extends to –x it is difficult to evaluate
the strength of such associations (e.g. is h =–1 ‘strong’?; how
much ‘stronger’ is h=−3?), except to document that these are
nonrandom (e.g. h≠h).
Species pools were prepared from the literature and our
personal knowledge of the ecological and geographical
distribution of these species. For North America and Asia, the
species pools consisted of those species whose geographical
ranges overlapped a particular site. Thus, each site had a
potentially unique pool of potential immigrants. Because this
was generated strictly by the geographical distribution of
species, we refer to this as the geographical species pool (GSP).
Because two of us (KR and GS) collected all of the Asian data
ourselves we also have detailed information on the particular
habitats at each Asian site, and were therefore able to develop
a second set of species pools based on the known habitat
tolerances of these species; we refer to this as the habitat species
pool (HSP). Because the North American data set was largely
extracted from the literature we could not develop a series of
HSPs that we believed was consistent across all sites.
Finally, we developed species pools based on two different
functional categories. Extensive research on North American
desert rodents has suggested that both locomotory mode
(bipedal vs. quadrupedal) and trophic habits are important
factors influencing community structure. Additionally, these
factors – especially locomotory mode – have been invoked to
help explain structure in Asian desert rodent communities (e.g.
Rogovin & Surov, 1990; Shenbrot, 1992; Rogovin & Shenbrot,
1993, 1995; Shenbrot et al., 1994; Shenbrot & Rogovin, 1995).
Thus, we prepared GSPs and HSPs with each of these
characteristics, as well as with both. Trophically based species
pools segregated species as omnivores, folivores, carnivores, or
granivores. Because many Asian species are both granivorous
and folivorous (Shenbrot et al., 1994), we have included a fifth
trophic category that we call mixed granivorous-folivorous. In
 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
equally to both granivorous and folivorous categories. Because
the model used in this paper requires that functional groups
contain integer quantities of species, however, we could not
consider a species to be 50% granivorous and 50% folivorous.
In North America, many microtines undergo seasonal shifts in
their dietary strategies, focusing largely on green vegetation
when available (winter and spring), but switching to seeds and
other items during summer and fall. This is fundamentally
different from the mixed diets of such Asian taxa as Allactaga
(F. Cuvier 1837), Dipus Zimmermann 1780, Jaculus Erxleben
1777, Meriones Illiger 1811, and Stylodipus Allen 1925, which
forage on both food items whenever available. Therefore we
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 an
additional set of analyses in which we grouped mixed folivores/
omnivores with omnivores. Locomotory based species pools
segregated species according to whether they were bipedal or
quadrupedal. Finally, these categories were combined to form
combination species pools, in which species were segregated as
bipedal granivores, quadrupedal folivores, bipedal omnivores,
quadrupedal carnivores, etc.
The probability of a species successfully immigrating into
a site, and subsequently establishing itself there, may be
calculated as:
P(yj = i | Xj−1) =
(ni−Xi, j−1)(1−h)Xi, j−1
[(ni−Xi, j−1)(1−h)Xi, j−1]
k
i=1
(1)
where i and j index functional groups and species, respectively,
k is the number of functional groups, ni is the number of
species in functional group n, yj is a random variable, indicating
the functional group to which the jth species will be placed,
Xj−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
gives the number of species in functional group i after the
jth species has entered. The numerator gives the combined
probability of a species immigrating and becoming established.
The denominator normalizes these probabilities so that they
sum to one (Kelt et al., 1995).
Finally, by varying the value of h we can determine the
value of h that produces a distribution of expected number of
favoured states that best agrees with the observed number of
favoured states. We refer to this value as ĥ, and it constitutes
a maximum likelihood estimate of the mean strength of
nonindependence across all sites and all species. While this is
clearly a general value with limited ability to predict the
strength of interaction at any given site, or with any pair of
species, its existence allows us to estimate the statistical power
of our model. Power is the probability that a hypothesis is
incorrect and therefore, should be refuted. If we assume that
ĥ reflects the real strength of interaction among these species,
then the proportion of the distribution of expected values
(number of favoured states) produced with h=ĥ that lies in
the 2.5% critical region (two-tailed test) of the distribution of
expected values produced with h=0 (the null hypothesis)
 Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841
constitutes an estimate of the power with which we can state
that observed communities are significantly different from the
null. Note that because the frequency distributions are discrete
(not continuous) we cannot provide power at exactly a=0.05;
rather, we bracket this a level with adjacent categories.
RESULTS
In order to provide sufficient background for interpreting the
results of simulations, we compared these three desert regions
in terms of local species richness (within and across functional
groups) and we evaluated patterns of species co-occurrence
and species combinations (sensu Brown & Kurzius, 1987;
Morton et al., 1994; Kelt et al., 1996).
Local communities and regional species pools
The number of species present at local sites varies considerably
across these deserts. The total number of species present is
lower in the Turan Desert Region than in the Gobi or in North
America, but the distribution of trophic and locomotory types
is more complex (Table 2; Figs 1 and 2). North America
and the Gobi Desert have high numbers of insectivores and
omnivores, whereas the Turan Desert Region shares high
representation of folivores with North America, and has the
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 have
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).
North American sites have the largest geographical species
pools, and include significantly more folivorous, omnivorous,
and granivorous species than Asian deserts. This result may
be confounded by the much larger area of the North American
deserts (Kelt et al., 1996), or by the subdivision of North
American deserts by the many mountain ranges that occur
there North America is intermediate in richness of insectivorous
species, although exclusion of shrews (Insectivora, Soricidae)
may have influenced this result. North America had significantly
fewer bipedal species, and significantly more quadrupedal
species, than either Asian region. The Gobi Desert has the
greatest number of bipedal species, and of insectivorous species,
and the fewest folivorous species. The Turan Desert Region
has the fewest insectivores, omnivores, and granivores, and an
intermediate number of folivores. This region had a greater
number of mixed omnivore/folivores than the Gobi Desert.
Finally, the numbers of species whose habitats are congruent
with the characteristics of a given site (species in the habitat
species pools) was only evaluated for Asian sites. There, sites
in the Gobi Desert were ecologically suitable to more species
than were sites in the Turan Desert Region. Habitat pools in
the Gobi Desert contained more bipedal species, and more
insectivorous and omnivorous species, than the Turan Desert
Region, whereas these pools in the latter region, in turn,
possessed more quadrupedal species and more folivorous and
mixed omnivorous/folivorous taxa (Table 4).
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
All species
Model
Error
Folivores
Model
Error
Insectivores
Model
Error
Omnivores
Model
Error
Granivores
Model
Error
Mixed Folivores/Omnivores
Model
Error
Bipedal species
Model
Error
Quadrupedal species
Model
Error
d.f.
SS
2
331
23.16
1121.54
2
331
F
P
Gobi
Turan Desert
Region
North America
3.42
0.0339
A,B
B
A
10.18
101.00
16.67
0.0001
B
A
A
2
331
4.31
96.29
7.41
0.0007
A
B
A
2
331
8.93
181.66
8.14
0.0004
A
B
A
2
331
243.22
440.61
91.36
0.0001
B
C
A
2
331
194.39
78.76
408.49
0.0001
B
A
C
2
331
45.75
694.31
10.90
0.0001
A
B
A
2
331
7.52
302.76
4.11
0.0173
B
A
B
Species co-occurrences
In both North America and Asia, species co-occurred with a
large number of other species and in many different
combinations (Fig. 3). Species generally occurred most
frequently with a small number of species, and progressively
less frequently with a large number of species.
Community nestedness
North American sites exhibited moderately high system
‘temperatures’, but these were significantly lower (more
structured) than that expected by random (Table 5), indicating
that these communities are significantly more nested than
expected by random assortment of species. Most Asian deserts
exhibit similar patterns. All three regions of the Gobi Desert,
and the Gobi as a whole, are significantly more nested than
expected. The deserts of the Turan Desert Region are also
significantly more nested than expected assuming random
distribution of species. The only exception to this pattern is
the geographically restricted Daghestan + Kazakhstan region.
The lack of a significantly nested structure there may reflect
reduced power to discriminate, resulting from the relatively
small pool of species (S=9 species) and small number of sites
(N=14).
Null assembly model
Our analysis of forty-one species of small mammal at 201 sites
throughout arid North America confirmed the results presented
for more restrictive data sets by Fox & Brown (1993) and Kelt
& Brown (in press a). When species were characterized by
both trophic (granivore, omnivore, folivore, insectivore) and
locomotory (bipedal v. quadrupedal) features, these
communities appeared highly structured (Table 6). This pattern
also held true for analyses using only locomotory mode to
characterize species. When only trophic features were
incorporated, however, no structure was indicated (Table 6).
Moreover, species at these sites exhibit strong negative
associations. Using locomotory categories, ĥ=0.41 (power
> 99%), whereas using trophic and locomotory categories
combined, ĥ=0.36 (power=89–92%; Table 6).
Very different results were obtained for analyses on Asian
deserts. With sixteen species and ninety-seven sites in the Gobi
Desert, analyses based on trophic or locomotory modes, and
using either regional or habitat species pools, demonstrated
significant deviation from random assortment. However, in all
cases the results of simulations indicated that the distribution
of species in functional groups in these communities was
significantly more clumped than expected by random; that is,
there were fewer favoured states in the real world than in
assemblages of simulated communities. This indicates that
similar species (those occupying the same functional group)
co-occurred more frequently than expected by chance. The
power of these analyses was very high (Table 6), indicating
that we are highly likely to be correct in stating that these are
non-randomly assembled communities. The high power of
these analyses also argues strongly for positive, rather than
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,
the number of species in the habitat pool may exceed that for the
geographical pool at a given site (e.g. quadrupedal species in the
Turan Desert Region).
locomotory modes were combined, we found no evidence of
non-random assortment.
Finally, using fifteen species at thirty-six sites in the Turan
Desert Region, most analyses indicated that these sites are not
significantly different from random. When we analysed either
species pools using combined functional groups, our analyses
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 noninteractive assembly were very powerful (Table 6), and provide
strong support for a conclusion that species sharing functional
group membership are positively associated. In contrast, nonsignificant analyses also possessed relatively low statistical
power.
In order to evaluate the influence of the ‘mixed omnivore/
folivore’ category on these results we conducted a separate
simulation in which we combined this category with other
omnivorous species. In the Turan Desert Region, using the
habitat species pool and analysing structure with the combined
trophic and locomotory categories resulted in an increase in ĥ
from less than −2.0 to −0.4 (a reduction in the degree of
positive associations), and these results were not significantly
 Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841
Figure 2 Number of species found (mean +1 SD) in functional
groups based on diet. Habitat species pools were not determined for
North American sites. North American sites do not have any mixed
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 more
omnivores and folivores than Asian deserts, but these do not result
in greatly elevated diversity of these groups locally. 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, the number of species in the habitat pool may exceed that
for the geographical pool at a given site (e.g. folivores and
granivores in the Gobi Desert).
different from random association (Table 6, values in brackets).
For the other seven simulations that were repeated, however,
both ĥ and the probability that these communities were
randomly assembled declined when these trophic categories
were combined (Table 6), indicating increasingly positive
associations and statistical significance. Thus, segregating
‘mixed omnivorous/folivorous’ species provided a relatively
conservative analysis for comparison with patterns from North
America.
DISCUSSION
Similarities and differences between North American and both
Asian desert rodent communities have been demonstrated in
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).
All species
Model
Error
Folivores
Model
Error
Insectivores
Model
Error
Omnivores
Model
Error
Granivores
Model
Error
Mixed Folivores/Omnivores
Model
Error
Bipedal species
Model
Error
Quadrupedal species
Model
Error
d.f.
SS
F
P
Gobi
Turan Desert
Region
North America
2
331
3957.08
3646.52
179.59
0.0001
B
B
A
2
331
764.11
345.72
365.79
0.0001
C
B
A
2
331
74.67
54.95
224.87
0.0001
A
C
B
2
331
427.11
882.63
80.09
0.0001
B
C
A
2
331
2273.50
1070.20
428.91
0.0001
B
C
A
2
331
1014.55
21.44
7830.31
0.0001
B
A
C
2
331
1069.03
340.11
520.19
0.0001
A
B
C
3
331
9001.44
2754.43
540.85
0.0001
B
B
A
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.
All species
Model
Error
Folivores
Model
Error
Insectivores
Model
Error
Omnivores
Model
Error
Mixed Folivores/Omnivores
Model
Error
Bipedal species
Model
Error
Quadrupedal species
Model
Error
d.f.
SS
1
131
20.64
405.75
1
131
F
P
Gobi
Turan Desert
Region
6.66
0.0109
A
B
20.51
32.80
81.89
0.0001
B
A
1
131
32.72
36.80
116.45
0.0001
A
B
1
131
3.36
67.46
6.53
0.0117
A
B
1
131
159.77
48.53
431.22
0.0001
B
A
1
131
91.87
302.39
39.80
0.0001
A
B
1
131
25.42
130.10
25.60
0.0001
B
A
 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
temperature
System
Desert
Southern Gobi
Western Gobi
Eastern Gobi
Gobi desert
Daghestan + Kazakhstan
Kyzyl-Kum
Turan Desert Region
North America
Number of
species/sites
Temp
Fill
11/32
12/29
10/36
16/97
9/14
10/22
15/36
41/201
26.72°
39.34°
38.18°
32.39°
48.60°
38.82°
31.87°
13.84°
28.6%
31.0%
39.7%
22.6%
34.9%
34.8%
22.4%
9.7%
terms of species diversity, guild structure, and ecomorphology
(Rogovin & Surov, 1990; Shenbrot et al., 1994; Rogovin &
Shenbrot, 1995; Kelt et al., 1996). As treated here, North
American deserts possess twice as many terrestrial nocturnal
small mammal species (S=41) as either the Gobi Desert (S=
20) or the Turan Desert Region (S=15). Although this
difference may be partially confounded by differences in the
 Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841
P(TΖTobs)
9.48e-5 (–3.74 r)
1.30e-2 (–2.235 r)
7.10e-4 (–3.20 r)
2.71e-12 (–6.90 r)
0.655 (+0.43 r)
0.0105 (–1.26 r)
0.00462 (–2.61 r)
4.32e-47 (–15.84 r)
Mean
SD
51.18°
53.80°
56.16°
55.42°
44.63°
48.77°
47.88°
36.44°
6.54°
6.49°
5.63°
3.33°
9.17°
7.93°
6.14°
1.42°
areal extent of our sampling regions (greatest in North America,
smallest in the Turan Region; see Kelt et al., 1996), this should
not influence the simulations conducted here, with the single
exception of nestedness in the Daghestan + Kazakhstan portion
of the Turan Desert Region (see below). The composition of
these deserts were also very different (Figs 1 and 2). Species
pools for North American sites were dominated by quadrupedal
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
Regional species pool
Trophic categories
Locomotion categories
Combination categories
Habitat species pool
Trophic categories
Locomotion categories
Combination categories
Turan Desert Region
North America
P(h=0)
ĥ
Power
P(h=0)
ĥ
Power
P(h=0)
ĥ
Power
0.0215
[0.0024
0.0005
0.1760
[0.0040
–0.45
c. –0.55
–0.69
–0.27
c. –0.95
0.5560–0.6490
0.0005
0.9445–0.9615
0.2410–0.3240
0.0070
0.1555
< –2.00]
0.3605
0.0005
c. –1.9]
–0.51
0.1820–0.3165
0.0925
–0.30
0.4530–0.5385
–0.11
–1.90
0.0755–0.1495
0.8400–0.9195
0.0005
0.0001
0.41
0.36
0.9925–0.9950
0.8865–0.9185
0.0025
[0.0035
0.0005
0.5645
[0.0015
–0.97
–0.8
–0.75
–0.07
c. –0.95
0.8780–0.9280
0.0195
0.9950–0.9900
0.0810–0.1215
0.3290
0.3400
–2.00]
0.0580
0.0050
–0.40]
–0.44
0.1780–0.2950
–0.52
–2.20
0.2550–0.3925
0.8135–0.8945
species with folivorous, omnivorous, or granivorous diets,
whereas the pools for Gobi Desert sites were dominated by
nonfolivorous species, with a slightly greater representation of
bipedal than quadrupedal species. Species pools for sites in the
Turan Desert Region were moderately biased towards bipedal
species, and were dominated by folivorous or omnivorous
species (or mixed folivorous/omnivorous species); this region
lacked granivores entirely. In spite of these functional
differences, alpha diversity remained low in all deserts (Fig. 1),
with three to four species typically occurring at a site. Beta
diversity also remained high, as demonstrated by the great
variability in the number and combinations of species with
which the most widespread species occurred (Fig. 3).
Most analyses provided strong indications of nested
structure (Table 5), but there were notably different patterns
in North America and Asia. Perhaps most apparent is that
both the temperature and fill of the North American deserts
was much lower than that of the two Asian deserts studied.
The reduced fill is a consequence of combining all North
American deserts into one analysis, so that less of the species
pool occurs at any one site. The statistics effect of this, however,
is to reduce the characteristic temperature for North American
deserts. That this is still significantly greater than the observed
temperature (Table 5) further underscores the difference
between North American and Asian faunas. Additionally, when
analysed separately each North American desert retains a cooler
temperature than any Asian desert studied (Great Basin, 13.55°,
Mojave, 23.16°, Sonoran, 13.59°, Chihuahuan, 19.37°; all are
significantly cooler than expected, P < < 10–9). Thus, it is likely
that nestedness in North American and Asian assemblages is
a result either of different underlying mechanisms or of
differential influence of similar mechanisms. For reasons
discussed below, we believe that different processes operate to
produce nestedness in North American and Asian deserts.
Results of an iterative model of local assembly suggested
major differences in patterns of community assembly in these
deserts. On the basis of much empirical data (reviewed in Price,
1986; Brown & Harney, 1993; Kelt & Brown, in press a), North
American communities provide strong evidence for competitive
interactions. Notably, use of trophic categories alone did not
demonstrate significant structuring, but when the locomotory
category was used, or when both categories were combined,
associations were significantly negative (e.g. h > 0). In contrast,
Asian assemblages provide no evidence for a significant role
of interspecific competition in community organization, in
agreement with other pattern-based analyses using these data
or subsets of them (Rogovin & Surov, 1990; Shenbrot et al.,
1994).
However, the results of simulations for the two Asian
deserts were also very different from each other. Assemblages
from the Turan Desert Region exhibited random associations
of species when analysed with trophic and locomotory
categories, but were significantly positively associated when
these categories were combined (Table 6). When we combined
omnivorous and mixed omnivore/folivore species, however,
significantly positive associations characterized all analyses
except those based on locomotory categories. Finally, analyses
on the Gobi Desert demonstrated positive associations among
species, with respect to both trophic and locomotory
functional groupings, but the combined groupings (both
trophic and locomotory) were significant only when omnivores
were combined with mixed omnivore/folivore species
(Table 6).
Thus, these simulations suggest either that the processes or
dynamics underlying the assembly of small mammal
communities in these deserts are very different, or that similar
dynamics are occurring, but that these are either very contextdependent or are controlled by different parameter values, such
that the final result appears very different. For example, both
Abrams (1990) and Leibold (1996) have suggested that, under
appropriate conditions, competitive interactions may lead to
the coexistence of ecologically similar species. In any case, the
three desert regions studied appear to constitute a structural
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
clumped (Gobi), at least with respect to this null model and
these functional groups. The factors underlying such variation
merit consideration, although only further research will allow
us to fully evaluate the intrinsic dynamics producing these
patterns.
Factors producing positive associations
Four factors that could produce positive associations among
species are local endemism, shared distributional strategies,
shared geographical origins, and shared habitat (Gilpin &
Diamond, 1982). Endemism is not a factor in these continental
faunas, since many species occur over most or all of the
regions concerned. By shared distributional strategies, Gilpin
& Diamond (1982) were referring to their tramp-supertramp
continuum, which reflects the habitat specificity and dispersal
abilities of each species. However, this is not relevant to
continental rodent communities in which the geographical
ranges of most species span the entire desert, as is the case in
both the Gobi Desert and the Turan Desert Region.
Although Asian desert rodent assemblages consist of species
with different geographical origins (e.g. Asian dipodid (jerboas)
and cricetid rodents likely radiated within northern Asia,
whereas gerbilline rodents (jirds and gerbils) evolved in the
Sahara-Sindian Desert region; Pavlinov et al., 1991; Shenbrot
et al., 1995), they have occupied similar regions of temperate
Asia since the late Miocene (Pavlinov et al., 1995), and such a
long period of co-occurrence may contribute to the positive
associations we observed. However, North American deserts
and taxa have been present since the late Miocene or early
Pleistocene, and desert adapted rodents in the Heteromyidae
may have a longer history than Asian desert rodent taxa (for
fuller discussions of the history of these deserts and faunas,
see Shenbrot et al., 1994; Kelt et al., 1996). We believe that it
is not so much the ages of these deserts and lineages, but
the geomorphology and eco-climatic history that has differed
between them, that has led to the differences that we observe
today. We elaborate upon this below.
Shenbrot et al. (1994) argued that Asian desert rodents were
organized into spatial guilds that were separated primarily by
soil and vegetative characteristics. Thus, some species are found
only on rocky soils (e.g. Allactodipus bobrinskii Kolesnikov
1937), whereas other species occur primarily or exclusively on
sandy soils (e.g. Salpingotus crassicaudata Vinogradov 1922,
and S. kozlovi Vinogradov 1922, in the western and eastern
Gobi, respectively). Such habitat guilds possess species that cooccur more frequently than a random assortment would
suggest, and we believe that the positive associations that we
observe in Asian deserts are largely a reflection of the similar
habitat requirements of the functional group members. When
communities are assembled in our simulations, individual
habitat associations are ignored, and functional groups are
more evenly represented than in real communities.
At a smaller spatial scale, variation in microhabitat use (e.g.
Rosenzweig & Winakur, 1969; Brown, 1975), spatial variation
in resource abundance (Brown, 1989b), and seasonal shifts in
foraging efficiencies (Brown, 1989b) have been invoked most
 Blackwell Science Ltd 1999, Journal of Biogeography, 26, 825–841
commonly to explain the local coexistence of species in North
American deserts (see also Brown & Harney, 1993). In a
community of three rodent species in the Negev Desert, Brown
et al. (1994) demonstrated that habitat heterogeneity, promoted
by daily winds that re-distributed sand and food resources,
when combined with differing foraging tactics of the three
species, appeared to explain the coexistence of these common
species.
Factors leading to nestedness in non-insular
communities
Although our analyses support the overriding importance of
competition in structuring North American desert communities,
competition is only one type of interaction involved in
structuring ecological communities, and the relative strength
of this may vary from place to place and from time to time
(e.g. Kotler & Holt, 1989). Nested faunal structure neither
depends upon, nor implies the action of, competition among
the constituent species (Patterson & Brown, 1991), although it
is consistent with a mechanism of competition (Kelt & Brown,
in press a).
Our analyses for central Asian communities offer no evidence
that competition structures these communities, agreeing with
studies on ecomorphology (Rogovin & Surov, 1990), niche
packing (Shenbrot et al., 1994), and geographical ecology of
these deserts (Rogovin & Shenbrot, 1995). In fact, our results
suggest that species in these communities either are
noninteractive or are positively interactive, with species being
found more frequently with similar species than expected
at random. Yet, these assemblages are highly nested. The
mechanisms underlying nestedness in Asian assemblages appear
to have a very different basis than in North America.
Patterson & Brown (1991) proposed three necessary
conditions for the development of nested subset structure: (1)
a common biogeographic history (2) generally similar
contemporary environments, and (3) hierarchical organization
of niche relationships. These factors all appear to be operational
both in North America and in central Asia, but the mechanisms
that operate locally to produce the observed communities
appear to be very different in these geographically isolated
desert regions.
The overwhelming evidence for the importance of
competition in North American desert rodent communities,
and the fact that these are much more strongly nested structure
than are Asian communities, supports an argument that
interspecific competition, rather than area-dependent
extinction, is responsible for the observed nestedness. However,
this is not the case for Asian communities. We believe that the
differences observed are at least partly due to the different ages
and geomorphologies of North American and Asian deserts,
and the different ages and origins of the mammal lineages in
these deserts. In this study we have minimized the variability
in these factors by studying desert regions that share several
features which have been implicated to influence desert
evolution and contemporary structure across a global scale.
Earlier we reviewed the arguments for historical influences on
838 Douglas A. Kelt et al.
local structure (Kelt et al., 1996), but it is worth reiterating
briefly to place these communities in a broader perspective.
The hand of history – again
The deserts considered in this manuscript are all relatively high
latitude deserts, occurring >35–76°N. Additionally, they share
similar patterns of precipitation and of variation in precipitation
(Kelt et al., 1996). Thus, differences in community structure
cannot be explained in terms of strategies for dealing with
extended droughts, as has been postulated for Australia
(Morton, 1985, 1993; Morton et al., 1994; Kelt et al., 1996).
However, differences are also apparent between these
deserts. The Gobi and Turan Desert Regions both occur at
relatively high elevations within large continents, whereas
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
Cretaceous, but did not become widespread until the Miocene
(Sinitzin, 1962), whereas North American deserts likely
originated sometime between the late Miocene and the
Pleistocene (Axelrod, 1958; Webb, 1977; Van Devender &
Spaulding, 1979; Wells, 1979; Thompson & Mead, 1982; Riddle,
1995). Asian deserts are extensive and present little topographic
relief, whereas those in North America are embedded within
a Basin and Range topography that provides a much greater
variety of habitats, as well as barriers to gene flow that have
varied in severity with the elevational advance and retreat of
forests during pluvial/interpluvial cycles. These barriers likely
have favoured isolation of local populations (e.g. Schmidly
et al., 1993), providing greater opportunities for speciation
than in the more topographically homogeneous Asian deserts.
Additionally, and a consequence of the Basin and Range
topography plus the pluvial/interpluvial history of North
America, species found in forested regions of North America
have evolved in proximity to arid regions, and some taxa
(e.g. Reithrodontomys Giglioli 1874, Peromyscus Gloger 1841,
Neotoma Say & Ord 1825) have since colonized desert
communities. Finally, the pluvial/interpluvial phases just
mentioned have resulted in significant temporal changes in the
areal extent of North American deserts, whereas Asian deserts
are thought to have persisted as arid regions since their
formation (Sinitzin, 1962). Thus, North American deserts
probably are younger than Asian deserts, but more importantly,
are more heterogeneous (both spatially and temporally), and
have had greater and more recent opportunities for radiation
of the taxa present.
We do not wish to imply that Asian rodents do not compete.
Competitive interactions likely have helped mold the current
suites of coexisting species, via mechanisms such as
microhabitat selection and use of different foraging tactics in
a heterogenous environment (e.g. Abramsky, 1989; Brown et al.,
1994; Rosenzweig & Abramsky, 1997; Rosenzweig et al., 1997).
However, the strength of competition, and the degree to which
it has been supplanted by other more visible mechanisms, may
have lowered the detectability of this interaction beyond the
resolution of this study (Abramsky, 1989).
It is tempting to speculate that past competitive interactions
may have sorted the species pools for Asian communities, and
that species presently constituting this fauna represent those
species that were successful competitors and/or those species
that shifted their niche requirements so as to minimize
competitive interactions. This would result in highly structured
regional faunas, predetermining some degree of structure locally
(the ‘Narcissus’ effect of Colwell & Winkler, 1984). The greater
age and persistence of Asian deserts has allowed much more
time to sort out competitive regimes. This hypothesis
emphasizes the pivotal influence of historical processes in
structuring contemporary ecological communities (e.g. Connell,
1980; papers in Ricklefs & Schluter, 1993), and may in fact be
correct (see, e.g. Diamond, 1986; Van Devender, 1986; for
similar arguments). Unfortunately, it is essentially ad hoc, and
is probably not testable in this system (see Connell, 1980;
Lawton, 1984; Strong, 1984).
CONCLUSION
Our analyses document important similarities and significant
differences between the three desert regions studied, and
underscore the importance of regional and historical influences
on the structure of local communities. Competitive interactions
appear to strongly influence the assembly of rodent faunas in
North American deserts. In contrast, our simulations provide
no indication that competition is a central feature in the
assembly of temperate Asian deserts. We agree with earlier
authors that the distribution and extent of major habitat
features appear to have greater influence on the assembly of
small mammal communities there (see also, Rogovin & Surov,
1990; Shenbrot et al., 1994). While this idea is not novel to
this paper, ours is the first attempt to model such regionally
specific assembly dynamics. Additionally, this study indicates
that there may be important differences in the mechanisms by
which communities in the Gobi and Turan Desert Region are
organized. The hand of history likely has played a role in
forming these assemblages, but how this has differentially
influenced Gobi and Turan faunas is not clear. If temperate
Asian desert assemblages are structured to some extent by the
macrohabitats that are available, then further emphasis on the
dynamics within these macrohabitats might be expected to
elucidate fundamental structuring forces at the level of habitat
patches not visible at the broader scale of the present analyses.
ACKNOWLEDGMENTS
We thank Barry Fox, Rick Ostfeld, Bruce Patterson, Mary
Price, and three anonymous reviewers for numerous comments
that improved and clarified the presentation.
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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.
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