Spatial organization of a rodent community in the Western Rajasthan desert (India

Spatial organization of a rodent communityin the Western Rajasthan desert (India)

by K.A. ROGOVIN1, G.I. SHENBROT2, A.V. SUROV1, and M. IDRIS3

'Institute of Animal Evolutionary Morphology and Ecology, USSR Academy of Sciences, Leninskypr. 33, Moscow 117071, Russia

-Ramon Science Center, Ben-Gurion University of the Negev, Mitipe Ramon, 80600, Israel

3Central Arid Zone Research institute, Jodhpur, India

Summary. - We studied the distribution of niches in the resource space and niche patterns ofa 9-rodent-species community of the Thar desert (Western Rajasthan, India) using discriminantfunction analysis. We measured eighteen quantitative parameters of the environment (soil structureand vegetation) at 375 sample plots within 15 1-ha grids. The first two canonical axes of resourcespace account for 95 % of the variance ; the first axis accounting for 89.6 % of the variance repre-sents a general landscape gradient from sand desert with sparse shrub vegetation to sandy - claysoils with dense annual grass vegetation of semi-savannah type. The second axis reflects a gradientof increasing gravel in the substrata with decreasing crown volumes of perennial vegetation. Thedistribution of niches in the resource space reflects the complex structure of the environment.Considerable opposition of a typical desert psammophilous Gerbillus gleadowi to all other speciesis a characteristic of geographical location and young age of Rajasthan sand desert. There were nosignificant correlations between niche position (distance from species centroid to the center of theresource space) and maximal population density or biomass. Also, there were no significant corre-lations between the niche breadth and the niche position due to the absence of species with quitebroad niches. The rodent biomass increased along the first, and, to a lesser extent along the secondaxis of the resource space and peaked in the most productive semi-savannah areas. Rodent speciesdiversity also increased in parallel fashion, but a more complex, hump-shaped pattern of variationwas noted. This could be viewed as a result of a complex structure of the environment and histori-cal events in origin of the fauna.

Resume. - Nous avons etudie la repartition des niches dans l'espace des ressources et lescaractoristiques des niches dans une communauto de 9 esp&ces de rongeurs du desert de Thar(Rajasthan occidental, Inde) par des analyses de fonction discriminante. Nous avons mesure 18parametres quantitatifs de Tenvironnement (structure du sol et vegetation) ä 375 points d'ochan-tillonnage sur 15 grilles de 1 hectare. Les deux premiers axes canoniques de l'espace des res-sources representent 95 % de la variance; le premier axe (89,6 % de la variance) represente ungradient de paysages, depuis le desert sableux avec une vegetation eparse de buissons jusqu'ä des

Mammalia, t. 58, n° 2,1994 :243 - 260.

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244 MAMMALIA

sols argilo-sableux avec une vegetation herbeuse annuelle de type semi-savane. Le second axereflete un gradient d'accroissement du gravier dans les sols accompagne d'une decroissance duvolume des couronnes de vegetation permanente. La repartition des niches dans l'espace des res-sources reflete la structure complexe de l'environnement. La grande difference qui existe entre lerongeur desertique typiquement psammophile, Gerbillus gleadowi, et toutes les autres especes,tient ä la localisation geographique du desert sableux de Rajasthan et a son jeune age. II n'y aaucune correlation significative entre la position de la niche de 1'espece (distance depuis le cen-tro'ide de 1'espece jusqu'au centre de l'espace des ressources) et sä densite de population et sä bio-masse maximales. II n'y a pas non plus de correlation significative entre l'amplitude et la positionde la niche ce qui est du ä 1'absence d'espece ä niche tres large. La biomasse de rongeurs s'accroitle long du premier axe et, dans une moindre mesure, le long du second axe de l'espace des res-sources, et eile culmine dans les regions de semi-savane les plus productives. La diversite desespeces de rongeurs s'accroit parallelement, mais selon un schema de variation en cloche pluscomplexe. Ceci doit etre conside're comme le resultat d'une structure complexe de l'environnementet des evenements historiques qui sont ä l'origine de cette faune.

INTRODUCTION

Among the approaches to the study of terrestrial vertebrate communities, examina-tion of their spatial structure remains one of the most popular. The significant role ofspatial displacement in desert rodent communities has been demonstrated many times(Brown and Lieberman 1973; Meserve 1976, 1981 ; Nel 1978; Stamp and Ohmart1978). Revealed patterns in spatial niches displacement can indicate likelihood of thehypotheses concerning mechanisms enabling species coexistence. On the other hand,comparison of community spatial organization in different regions with similar environ-ment such as different deserts of the world can help in understanding which patterns areuniversal and which are determined by community taxonomical composition, evolutio-nary history, and so on.

There are a number of ordination techniques (Jongman et al. 1987 ; Morrison et al1992) revealing spatial community structure. Among different ordination techniques thebest one for the description of the niche placement in an ecological space is (in our opi-nion) discriminant function analysis because (a) species densities and environmentalvariables are considered simultaneously and (b) the subjects of the analysis are not onlyspecies means for habitat variables but also their dispersions that is important for theestimation of niche breadth. The result of applying discriminant analysis is the construc-tion of a model of the ecological space, the main property of which is the maximal nichedivision. The axes of this space are linear combinations of the original environmentalvariables and usually they may be easily interpreted as composite environmental gra-dients. This approach was theoretically developed by R.H. Green (1971, 1974) and wassuccessfully applied to a number of different vertebrate communities (Cody and Walter1976; M'Closkey 1976; Dueser and Shugart 1979; Morris 1979; Morrison andAnthony 1989; Rogovin et al. 1991 ; Shenbrot et al. 1991 ; Shenbrot 1992). Althoughsome applications of discriminant analysis deserve criticism (Carnes and Slade 1982;Van Home and Ford 1982) this method remains theoretically sound for the descriptionof the niche structure of communities.

Data collected during short time period can represent only temporal section of com-munity. The processes of density dependent habitat selection (Rosenzweig 1987, 1991)may drastically change some aspects of community organization on temporal scale (Jak-sic 1993). However, many general patterns of desert rodent spatial community organiza-


RODENT COMMUNITY IN RAJASTHAN 245

tion such as general patterns of niche placement in ecological space, level of alfa-diver-sity, etc. stay more or less stable during temporal density fluctuations (Shenbrot andRogovin, unpubl. data). Thus, short-term based data may be usable for some kinds ofbetween-community comparisons.

If the partition of ecological space between coexisting species is one of the impor-tant factors in the structuring of the communities, then there are good reasons to believethat not only niche placement, but also some general characters of the community, suchas overall biomass and species diversity, should display meaningful patterns in themodel of ecological space that is produced by discriminant analysis. Analysis of correla-tions between the abundance of species and the main niche parameters, known as « nichepattern » (Shugart and Patten 1972), have been developed within this framework. Untilnow niche patterns have been examined for a limited set of communities. There areinsufficient data for making any definite conclusions about the generality of such rela-tionships (Shugart and Patten 1972 ; Dueser and Shugart 1979 ; Seagle and McCracken1986 ; Rogovin et al 1991 ; Shenbrot et al. 1991 ; Shenbrot 1992).

The main objective of this paper was to study specific features of the spatial organi-zation, niche patterns and species diversity in the community of rodents in the Thardesert of Western Rajasthan (India) on the basis of discriminant function analysis and tocompare the results with those previously analyzed using the same methods (Rogovin etal. 1991 ; Shenbrot 1992). Comparisons of the communities occurring in a more or lesssimilar environment, but possessing different histories and genetics, can provide infor-mation about general directions in the evolution of community organization, as well asidentify the specific features of each community. Though a usage of discriminant func-tion analysis primarily was associated with the theory of habitat selection as a result ofinterspecific competition, we bear in mind possibility of another reasons (such as mor-phological locomotory adaptations) which may lead to niche displacement in ecologicalspace. The discussion in a framework of competition theory is a traditional tribute, butwe hope that only comparative approach and likeness in results obtained for differentcommunities at a structural level of analysis can help in evaluation of productive causalhypothesis.

MATERIAL AND METHODS

Study area.Field studies were conducted in the eastern part of the Thar desert between 26° and

28° NL, and between 73° and 75° EL in the Jodhpur and Sikar districts of Western Rajas-than. Rajasthan desert is a land-locked tropical desert situated on the eastern fringe of the« Saharo-Rajasthanian » belt. Its age, estimated by archaeological data, is about 5.000 -10.000 years (Krishnan 1952 ; Wadia 1960). Rainfall varies from 80 mm to 425 mm peryear. Ninety percent of it falls during the monsoon season, from July to September. Theaverage minimum temperature for January varies through localities from 6.9° to 9.2°C.The average maximum temperature for July varies from 39.8° to 40.6°C (Pramanik andHariharan 1952).

Native desert habitats of Rajasthan have been classified into sandy, gravel or rocky(Prakash 1962, 1964). Among these three types, sands are the most widespread. Sandsare represented by sand plains and dunes. Sand plains are formed by low sand hills andridges with sparse shrubs and more or less dense annual herb and grass vegetation


246 MAMMALIA

consisting primarily of Leptadenia pyrotechnica, Cenchrus setigerus, Tephrosia purpu-rea, Crotalaria burhia, Eleusine compressa and Aristida sp. The most common trees andshrubs are Acacia tortilis, Prosopis cineraria, Capparis decidua. Total vegetative coverin sandy plains could be as much as 80 %. Stable sand dunes and semi-stable dunes (bar-khans) are stabilized by Tephrosia purpurea, Aerva pseudo-tomentosa, Crotalariaburhia and Citrullus sp. Total vegetative cover of sans dunes varies from 2 to 65 %.

Rocky habitat, based on malani rhyolite and sandstone rocks, is the most sparselyvegetated habitat type in Jodhpur district; clumps of Euphorbia caducifolia, Ziziphusnummularia, Commiphora wightii predominate. The foothills are characterized byvarious types of substrata, including sandy soil with dry steam beds and patches of flatrocky areas. Vegetation is richer and includes trees and big bushes of Acacia, Prosopis,Ziziphus and Capparis, as well as lower-lying vegetation (Tephrosia, Barleria andherbs). Total vegetative cover of rocky habitats ranges from 5 to 35 %.

Gravel habitats in Jodhpur district near Beriganga, typical for the entire region, arerepresented by very gentle deluvial slopes, consisting of sandy soil with high amounts ofgravel material. The clay particle content in the soil increases slowly from the top to thebottom of the valley, achieving its maximum (16 % per sample) in small sandy-clay hol-lows. Vegetation on slopes is sparse and consists of Euphorbia caducifolia, Fagonia cre-tica, and Calotropis procera and Capparis decidua with increasing abundance in hol-lows.

Species composition.

The terrestrial desert rodent fauna comprises 14 species :1. Funambulus pennanti Wroughton, 1905 ; 2. Gerbillus nanus Blanford, 1875 ; 3.

G. gleadowi Murray, 1886 ; 4. Tatera indica Hardwicke, 1807 ; 5. Meriones hurrianaeJerdon, 1867 ; 6. Bandicota bengalensis Grey, 1835 ; 7. Rattus cutchicus Wroughton,1912 ; 8. Millardia meltada Gray, 1837 ; 9. M. gleadowi Murray, 1885 ; 10. Mus muscu-lus Linnaeus, 1758 ; 11. M. booduga Gray, 1837 ; 12. M. cervicolor Hodgson, 1845 ; 13.M. platythrix Bennett, 1842 ; 14. Golunda ellioti Gray, 1837.

Most of these species occur consistently in different years in the desert habitats(Prakash 1962, 1964, 1981 ; Prakash and Jain 1970; Prakash and Rana 1970 and str).During our field research we found all but four of them, but none of these missing spe-cies can be considered typical desert forms. Bandicota bengalensis and Millardia mel-tada are most typical of ruderal habitats, Mus cervicolor prefers rocks, and Golundaellioti was so rarely captured that it is difficult to make any decision on its preferences.

Data collection.

Data were collected from October to December, 1989. 15 square 1-ha grids wereestablished to represent repeated samples of all habitat types at study sites. Each of thesegrids was divided into 25 small sample plots (20 X 20 m), whose centers were markedwith 50 cm steel stakes. Records of rodents were obtained by trapping with 25 by 7 by 7cm Sherman traps. Peanut butter was used as bait. One trap was set within 5 m of thecenter of each sample plot. Trapped rodents were marked by toe-clipping. Traps werechecked twice each night and once during the day. Some species of rodents which werenot trapped in Sherman traps were caught with a net night (Gerbillus nanus) or countedvisually in the mornings (Funambulus pennanti, Meriones hurrianae). For these animals,only points of initial contact were used in our analyses. Each grid was checked over aperiod of three nights. Densities of rodent species were estimated by direct enumeration



of animals caught on each grid. Accuracy of estimates was verified by recapture and alsoby visual observations during rechecking of the grid. Three nights were sufficient to parkmore than 80 % of the rodents on each grid.

Soil samples (0.5 kg) were taken from the center of each plot for laboratory analysisof mechanical composition. The number of shrubs (by species) in each plot was countedin a circle, 5 m in radius, to determine vegetation cover. Volume by height layers withineach plot, and the height and diameter of crowns of shrubs (for 30 shrubs of each speciesin the plot) were also measured. The abundance of grasses and herbs was evaluated byclipping all specimens in 0.25 m small sample plots (4 sample plots in each plot). 18parameters were used for the subsequent analysis (Table 1).

TABLE 1. - Variables included in the observation vectors for multiple discriminant function analysis.

Mnemonic Variable UnitCGRRCK

GRWCLY

AAGAAF

PGC

CCT

OSC

CSLCCBCZBCLB

CEB

PPV1PPV2

PPV3PPV4

Cover of the soil by gravel and rocksContent of rocks in the soilContent of gravel in the soilContent of clay in the soilAbundance of annual grassAbundance of annual forbsPerennial grass coverCrown cover of Prosopis and Acacia treesOverall shrub coverCover of shrubs family LeguminosaeCover of Capparis sp. bushesCover of Ziziphus sp. bushesCover of Licium sp. bushesCover of Euphorbia sp. bushesPerennial plant crown volume at the level 0-25 cmPerennial plant crown volume at the level 25-50 cmPerennial plant crown volume at the level 50-100 cmPerennial plant crown volume at the level 100-200 cm

no/sq.m.no/sq.m.

Data analysis.

Multivariate discriminant (canonical) analysis was used to describe dispersion ofspecies niches in ecological space for the community. Size of the original sample plot(20 X 20 m) was chosen to simplify the method of data collection. In principle the opti-mal size of the sample plot should be in close relation to the range of activity of the spe-cimens. The technique of « sliding square » (Shenbrot 1992) was used in part to avoidedge effects and because the original sample plots were small (20 X 20 m). Accordingly,


248 MAMMALIA

a single date unit comprised a square plot (40 X 40 m), consisting of four adjacent origi-nal sample plots. This square was moved along the sample grid in 20 m steps (the lengthof the original sample plot). New values for the environmental variables for the bigsquare plots were obtained by averaging over each of the four original sample plots.Thus, each of the original sample plots was weighted accordingly to its position withinthe grid : interior plots was counted four times, plots in external rows and columns of thegrid were counted twice, and corner plots - only once.

Before the analysis the initial data were log-transformed to normalize them. Indicesof the niche breadth and position (distance from the centroid of a particular species to thecentroid of the community) were calculated for each species by the results of canonicalvariate analysis. Unlike Dueser and Shugart (1979), niche breadth was measured as thestandard deviation of observation points of a given species from the centroid of this spe-cies in the space of the canonical axes (Carnes and Slade 1982). To avoid the effect ofdifferences in the abundance of species on calculation of the niche position, these werecalculated not as the distances of species centroids from the initial coordinates in canoni-cal space, but as distances of species centroids from the centroid of the whole commu-nity. Community centroids were calculated as arithmetic mean coordinates of the speciescentroids.

Niche overlap was evaluated by comparing the observation points of pairs of spe-cies using stepwise discriminant analysis. As a measure of niche overlap we used theoverlap in the probability of displacement along the discriminant axis, i.e. the probabilityof erroneous assignments (Green 1974 ; Cody and Walter 1976).

To evaluate the degree of unevenness in niche displacement in resource space, amatrix of niche overlap values was analyzed, employing the method of « nearest neigh-bor analysis » (Inger and Col well 1977). As a measure of species abundance, the obser-ved maximum population density (or biomass) was used. Such direct measures are lessinfluenced by possible artifacts of estimation than commonly used estimation indices(Dueser and Shugart 1979).

We estimated rodent biomass distribution along discriminant axes by directly plot-ting recorded rodent biomass against discriminant scores for each sample unit, but suchan approach was impossible for estimating rodent species diversity because of the smallarea of the sample units and the short period of census. To evaluate the patterns of rodentspecies diversity along the first two discriminant axes, we evaluated the probability esti-mates of the numbers for each species in each point of discriminant space. This approachutilizes observational data on the maximum species density and assumes a two-dimensio-nal normal distribution (Shenbrot 1992). The parameters of two-dimensional normal dis-tributions (coordinates of species centroids, standard deviations by each axis, coefficientsof correlation) were obtained from the discriminant analysis. Species diversity was calcu-lated for each point of discriminant space using the Shannon index (H = - Σ PI log Phwhere Pj is relative abundance, /-the species).

Because of limitations of the discriminant analysis technique (Williams and Titus1988), only species recorded on more than 20 sample plots were used in the main part ofour analysis. Discriminant axes were calculated based on this subset of common andabundant species. Rare species were later included in the analysis by calculating of thecoordinates of their observation records in the discriminant space obtained earlier. Thus,estimations of niche parameters for rare species are very rough as a result of both smallnumber of captures and non-optimal properties of discriminant space for determinationof the niche position of these species. We did not use Mus musculus in the analysisbecause we had only one record for this species.



RESULTS

Patterns of species abundance and habitat use.

On the 15 1-ha grids we recorded 9 Funambulus pennanti, 53 Gerbillus nanus, 205G. gleadowi, 222 Tatera indica, 317 Meriones hurrianae, 2 Rattus cutchicus, 9 Millar-dia gleadowi, l Mus musculus, 11 M. platythrix, and 3 M. booduga.

The distribution of rodents by five habitat types is presented in Table 2. T. indicaoccurred in all habitats. M. hurrianae and G. nanus occurred in four habitats. G. glea-dowi and M. booduga inhabited three habitat types. Five species occurred in only one ortwo habitat types.

The overall abundance of all species of rodents was highest on sand soils, lowest onsandy-clay hollows, and moderate on gravel plains and rocky foothills. The overallrodent species biomass was lowest on sandy-clay hollows and highest on sand plains.

Four species of gerbils were numerically dominant in these communities. Maxi-mum population density was 18 ind./ha for G. nanus, 74 ind./ha for G. gleadowi, 44ind./ha for T. indica, and 65 ind./ha for M. hurrianae. The two species of Gerbillus useddifferent types of habitats, but this was a characteristic for the other two gerbil species.

TABLE 2. - Density (no/hectare) of rodent species and overall rodent biomass (kg/hectare).

Rodent species

1. Funambulus pennati

2. Gerbillus nanus3. GerbillujLgleadoyi4. Tatera indica

5. Meriones hurrianae

6. Rattus cutchicus7. Millardia gleadowi

8. Mus musculus

9. Mus booduga

10. Mus platythrix

Overall densityOverall biomass

Habitat types

Sanddunes

-

-

64.00.33.7

-

-

-

0.3-

68.31.86

Sandplains

1.0

0.51.7

26.3

33.5-

-

-

0.2-

63.25.60

Gravelplains

-

12.7

1.015.0

16.3-

3.0

-

0.3-

48.33.15

Rockfoothills

1.3

4.5-

5.028.0

1.0-

1.0-

5.045.32.97

Sandy-clayhollows

-

3.0-

8.0------

11.00.92

Resource partitioning and general characteristics of communities.Division of species use of ecological space within the community occurs mainly

along the first two canonical axes, which account for about 95 % of the variance. Thecontribution of the first axis exceeds that of the second by more than 16 times.


250 MAMMALIA

Both these axes reflects a complex environmental gradients (Table 3). The first axisrepresents a general landscape gradient from sand soils with well-developed small shrubvegetation to sandy-clay soils with dense annual grass vegetation. The second axisreflects an increase in shrub crown volumes of the three upper layers and in the abun-dance of Capparis decidua, with a parallel decrease of gravel and stone material in soilcover and gravel in the upper layer of the soil.

TABLE 3. - Summary of multiple stepwise discriminant analysis of the niches for the subset of 7 common andabundant species of Rajasthan rodent community. *** P < 0.01. DPI and DF2 are the first two possiblediscriminant functions. Mnemonics are from Table 1.

Information

Eigenvalue

Chi-square test for significance of doscriminant function

Degrees of freedom

Percentage of discriminant information

Linearcorrelation(r) betweeneachdiscriminantfunction(DF1.DF2)and eachhabitatvariable intheobservationvector

(*-variablesnot in theanalysisafter step17)

AAGCLY

* CSL

OSC* GRW

PPV3

PPV2

PPV4

CGRCCB

* AAF

RCKCZBPPV1

CLBCCTPGCCEB

DPI

5.654

5277***

90

89.6

0.758

0.709

-0.458

-0.432

0.186

0.193

0.199

0.196

0.124

0.247

-0.282

0.137

0.088

0.179

-0.047

0.103

0.184

0.082

DF2

0.341

1247***

70

5.4

-0.018

0.013

0.189

0.325

-0.840

0.654

0.582

0.576

-0.575

0.468

0.299

-0.470

-0.048

0.508

0.136

-0.243

-0.178

-0.197

The placement of 75-percent confidence ellipses of common and abundant speciesand niche centroids of rare species along resource axes is shown in Fig. 1. Significantisolation of G. gleadowi is distinguished by the first axis. There are no other distinctclusters along the first axis. G. gleadowi is in opposition to all other species because ofits distinct association with sand dunes. Most other species have high values of overlapalong this axis.

The displacement of these species takes place mainly along environmental gradientrepresented by the second axis of the ecological space. This gradient separates two moreor less distinct groups of rodents, one group inhabiting the desert gravel plains and foo-



thills and another group inhabiting shrub and forb plains of semisavannah type. Conge-neric species tended to be well-separated along environmental gradients represented bythe first two discriminant functions.

Niche overlap of all species under consideration are given in Table 4. The majorityof pairs of species are characterized by low values of niche overlap. The maximum valueof overlap is only 42 % (between T. indica and M. hurrianae) and mean overlap between

-7.5-6.5-5.5-4.5-3.5-2.5-1.5-0.50.5 1.5 2.5 3.5 4.5

C\2

2.6

1.6

0.6

-0.4

-1.4

-2.4

-3.4

Gg.

2.6

1.6

0.6

-0.4

-1.4

-2.4

-3.4

-4.4-4.4-7.5-6.5-5.5-4.5-3.5-2.5-1.5-0.50.5 1.5 2.5 3.5 4.5

DF1Fig. 1. - Seventy-five percent confidence ellipses for the species observations on first two discri-

minant axes (DPI and DF2) for the common and abundant species of Rajasthan rodent com-munity : FP - Funambulus pennanti, GN - Gerbillus nanus, GG - G. gleadowi, TI - Tateraindica, MH - Meriones hurrianae, MG - Millardia gleadowi, MP - Mus platythrix. Asterisksmark niche centroids for rare species : RC - Rattus cutchicus, MB - Mus booduga.

TABLE 4. - Niche overlap values for the 9-species community of Rajasthan desert. Abbreviations: FP -Funambulus pennanti, GN - Gerbillus nanus, GG - G. gleadowi, TI - Tatera indica, MH - Meriones hur-rianae, RC - Rattus cutchicus, MG - Millardia gleadowi, MB - Mus booduga, MP - M. platythrix. Erra-tum : RC, last columm, read 004.

FP

GN

GGTI

MH

RC

MG

MB

GN

0.000

GG

0.0000.000

TI

0.2520.114

0.000

MH

0.0320.0610.001

0.423

RC

0.001

0.0000.0000.0000.001

MG

0.0000.0000.0000.0450.002

0.000

MB

0.0000.0000.0000.001

0.001

0.0000.042

MP

0.0070.0420.0000.0050.006

0.0000.0000.000


252 MAMMALIA

nearest neighbours is about 13 % (Fig. 2a). In 33 out of 36 cases overlaps of spatialniches are less than 10 %, and in 32 cases they are less than 5 %. Taking into account allpossible species pairs we can not find any distinct guilds of niches (single species remotefrom all other species can not be considered as a guild). This is seen in the continuousdecrease in standard deviations of niche overlaps in groups evaluated by the use of the« nearest neighbour method » (Fig. 2b).

The main parameters of rodent niches and the relationships between them are givenin Tables 5 and 6. In the subset of common and abundant species (gerbils) the widestniche exceeds that of narrowest one 2.5 times, whereas for the whole set of species thisratio equal 8.1. All other niche parameters vary significantly between species. Both thesubset of common and abundant species and the whole set of species have only one posi-tive significant correlation between maximum population density and maximum popula-tion biomass. There are no significant relationships between other niche parameters.

0.2

0.15-

360.1-SS0.05-

3 4 5 6RANK OF NEARNESS

3 4 5 6RANK OF NEARNESS

Fig. 2. - The results of the nearest neighbor analysis : dependence of mean value (a) and standarddeviation (b) of overlap on the degree of the ecological similarity of species for Rajasthanrodent community.

TABLE 5. - Mean weight and main niche parameters of rodent species: W - mean weight (g), S - nichebreadth, D - niche position, A - maximal density (sp/ha), B - maximal biomass (kg/ha). Rare species aremarked by asterisk.

Rodent species

Funarnbulus pennatiGerbillus nanus

Gerbillus gleadowiTatera indicaMeriones hurrianaeRattus cutchicus *Millardia gleadowiMus booduga *Mus olatvthrix

W

89

11

24

110

78

25

24

8

13

S

6.34

6.94

3.01

7.72

6.21

7.44

1.94

10.63

15.64

D

1.98

1.79

4.85

1.62

1.59

3.92

2.46

2.06

2.61

A

2

18

74

44

65

2

3

1

3

B

0.18

0.20

1.78

4.84

5.07

0.05

0.05

0.01

0.04



TABLE 6. - Interrelationships (linear correlation - r) between main niche parameters in the subset of commonand abundant species (right upper part) and in the whole set of species (left lower part) of rodent com-munity : S - niche breadth, D - niche position, A - maximal density (sp/ha), B - maximal biomass (g/ha).* P < 0.01. NS - not significant.

SD

A

B

S

-0.24 NS

-0.39 NS

-0.17NS

D

-0.25 NS

0.20 NS

-0.28 NS

A

-0.33 NS

0.38 NS

0.78*

B

-0.09 NS

-0.25 NS

0.74*

The patterns of rodent biomass and species diversity along the first two discriminantaxes are presented in Figures 3 and 4. There is a noticeable increase of rodent biomassalong the first discriminant axis corresponding an increase of productivity of annual grassvegetation. This increase is monotonous but not linear (it best fits to regression equationz = a + fox*, where a = - 1.58, b = + 7.69, R2 = 0.31, F = 527.98, P < 0.001). Along thesecond axis changes in biomass are not as pronounced and represented by several peaks.Maximal values correspond to well-developed perennial vegetation of the three upperlayers. Species diversity has a clear hump-shaped pattern along the first discriminant axis(Fig. 4) which represents an productivity gradient (it best fit to regression equationz = a + bx - ex*, where a = 4.54, b = + 5.93, c = 1.01, R2 = 0.75, F = 6180.44, P < 0.001).

Fig. 3. - Patterns of rodent biomass variation within the space of the first two discriminant axes(DF1 - gradient from sand soils with well developed small shrub vegetation to clay-sandysoils with dens vegetation of annual herbs. DF2 - an increase of shrub crown volumes ofthree upper layers with a parallel decrease of gravel and stone material in soil cover gravel inthe upper layer of the soil).


254 MAMMALIA

Fig. 4. - Patterns of rodent species diversity variation within the space of the first two discriminantaxes (DPI - gradient from sand soils with well developed small shrub vegetation to clay-sandy soils with dense vegetation of annual grass. DF2 - an increase of shrub crown volumesof three upper layers with a parallel decrease of gravel and stone material in soil cover andgravel in the upper layer of the soil).

DISCUSSION

Although there are now a number of publications describing the structure of diffe-rent animal communities using the discriminant analysis techniques, direct comparisonof the results is difficult because of differences in the field data gathering protocols andin the details of their statistical treatment. Data collected and processed by identicalmethods entirely comparable with those presented here exist for a Chihuahuan desertrodent community (Rogovin et al. 1991) and for a Middle Asian rodent community(Shenbrot 1992).

In plotting an ecological space using the method of discriminant analysis a relativescale of the space variables reflects the contribution of these variables to the partitioningof resources by the species of the particular community (Green 1971, 1974). The struc-ture of the resource space determines in a broad sense a degree of adaptive radiation. Inold faunas with saturated communities the diversity of rodent species corresponds to thediversity of represented habitats. We found such relationship in communities in the Oldand New World deserts (Rogovin et al. 1991 ; Shenbrot 1992). In the desert rodent com-munity of Middle Asia the main spatial structuring factors form three independentgroups, in which the mechanical composition of soil is most important, productivity is of



second importance, and the abundance of shelters is of third importance (Shenbrot1992). The same patterns, with the exception of productivity, were also shown for ano-ther type of rodent community in a North American desert (Rogovin et al. 1991). Howe-ver, the contribution of particular structural factors to the axes of resource space and thusthe causal mechanisms of its structuring in different communities are fundamentally dif-ferent. The first axis of the resource space in both cases corresponds mainly to a gradientfrom sand to clay soils. In the Chihuahuan Desert rodent community (Mexico), thesecond important axis is the gradient of the content of rocks and gravel, which is ofessentially no significance in the Kyzylkum desert (Uzbekistan). The presence of theadditional axis of resource space in the Chihuahua Desert, reflecting the gradient of roc-kiness of the substratum, and the absence of this factor in Middle Asia, may be explainedby topographic and geomorphological differences between these regions, discussedelsewhere (Shenbrot et al. 1991). The importance of a specific axis of plant productivityis also the characteristic of Middle Asia. This feature can be explained by the climaticregime of the temperate Turan desert, with is strong seasonal fluctuations in temperature,precipitation and aspects of vegetation. In the Chihuahuan Desert community, environ-mental variables characterizing productivity are partially included in the different discri-minant axes. This also takes place in the Rajasthan desert rodent community consideredhere, but the structure of resource space of this community also differs in many featuresfrom both communities mentioned above. The main spatial structuring factors in theRajasthan desert form independent discriminant axes in which the soil and vegetationparameters are in strong correlation. The three first axes of resource space also characte-rize the specific structure of the landscapes of Rajasthan, but the degree of conformitywith the visible diversity of habitats is not very evident.

There are two distinct formations of Thar desert (sand dunes and desert rocks) andplain formations characterized by gradual transformation from semisavannah type oflandscape with dense vegetation in low layers to desert landcapes with sparse and pat-chily distributed vegetation. Physiognomically the Thar desert looks sandy. Even mostloamy soils include a high percent of sand. The huge contribution of the first axis ofresource space towards explaining the total variance could be explained by such specificstructure of the environment and also by historical events in rodent fauna formation. TheThar desert rodent fauna is very young. The age of the desert is about 5.000 - 10.000years (Krishnan 1952 ; Wadia 1960). Most real sand desert landscapes (stable and semis-table sand dunes with sparse vegetation, and also moving sands) are the results of humanactivity during historical time. The Indus river is a powerful barrier which prevents thedispersal of species from the Great Asian Deserts to the east. There are no typical formsof desert rodents such as jerboas (Dipodidae) in Rajasthan. Among four species of ger-bils, only G. gleadowi and G. nanus represent real desert fauna, and among nine speciesof murid rodents only M. gleadowi appears to be a desert-adapted species. The Indiangerbil (M. hurrianae), whose range coincides completely with the desert zone (North-western and Western India, Afghanistan and Iran) can not be viewed as a desert specia-list. M. hwrianae inhabits a broad number of arid and semiarid habitats with savannahtype of vegetation, though in comparison with T. indica it prefers a more open environ-ment (Prakash 1981). This is also confirmed by the results of our analysis. But in factboth species live together in most types of habitats and divide niche space mainly alongtemporal and trophic axes. Most of the murid species as well as T. indica and F. pennantiinhabit tropical semiarid and even humid zones of India. In fact only G. gleadowi can beviewed as an endemic species of the Thar desert (7 % of the fauna). Four species repre-sent a complex of typical desert rodents (29 % of the fauna). The last 10 species (71 %of the fauna) are represented by the forms of other faunistic complexes.


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The low level of niche overlap in the Rajasthan desert rodent community is similarto that found in the Chihuahuan and Middle Asian communities. At the same time, spa-tial organization of these communities differs substantially in the patterns of internalstructure. While the niches of the Chihuahuan species are distributed more or less evenlyin ecological space (Rogovin et al. 1991), in Middle Asia there are two distinct spatialguilds - psammophilous and sclerophilous species. These differences can be explainedby differences in the age of sand landscapes (see discussion in Shenbrot et al. 1991) ormay be connected with the origin of the faunas : the North American desert fauna ismore or less homogeneous genetically, whereas the Middle Asian desert fauna consistsof two distinct components - psammophiles of the local Turan origin and sclerophilesmixed from elements of Iranian, Kazakhstanian, and Central Asian origin (Heptner1945). In the Rajasthan desert, with its young aged and mixed fauna, niches are not aswell-dispersed. The absence of an ascending pattern in the beginning of the curve forstandard deviation of niche overlap (Fig. 2b) can be interpreted as a characteristic ofuneven distribution in the resource space of niches of ecologically related (nearest neigh-bour) species. In contrast, a tendency for an even distribution of nearest neighbour spe-cies was found in the saturated community of Mapimi in Chihuahuan Desert (Rogovin etal. 1991).

One of the pronounced peculiarities of the Middle Asian desert rodent communityis the greater extent of spatial overlap between congeneric species than between speciesof different genera (Shenbrot 1992). In the Rajasthan rodent community as well as in theChihuahuan community (Rogovin et al. 1991) the opposite ratio of congeneric to bet-ween-generic spatial overlaps takes place. In Chihuahuan desert a number of speciesfrom congeneric pairs (from the Dipodomys, Perognathus, and Peromyscus genera)which are not highly separated spatially have very similar feeding adaptations, but differin sizes, whereas in the case of Middle Asian community pairs of congeneric co-occur-ring species have different feeding adaptations and a weight ratio about 1 : 2 (Meriones)or even more - 1 : 3 (Allactaga). In Rajasthan, the weight ratio of the pair of separatelyliving Gerbillus species is 1 : 2.2, but in the pair of murid congeners it is only 1 : 1.6(Mus). In pairs of gerbillid species with high niche overlap indices weight ratio variesfrom 1 : 1.4 (Meriones - Tatera) to 1 : 10 (Tatera - G. nanus). In coexisting murid spe-cies weight ratio varies from 1 : 2 in M. platythrix - R. cutchicus to 1 : 3 in M. booduga -M. gleadowi. Such a wide variety of size ratios show no evidence to regular distributionof species by size in the Rajasthan community, though among murids which seems to beall omnivorous and nocturnal such tendency is more real. The size of community seemstoo small for any statistical conclusions.

The analysis of niche patterns (interrelationships between main niche parameters)shows no significant correlations between niche breadth and niche position, and betweeneach of these parameters and maximal population density and biomass in the Rajasthandesert.

The result of niche pattern analysis depends on the method of plotting a resourcespace, and on the method of determining the center of a community (Shenbrot et al.1991). Using the analysis of canonical variates and considering a community center tolie at the origin of the coordinates in discriminant space, the niche position characterizesthe degree of remoteness of the microhabitat of a given species from the microhabitat ofthe most numerous species (Cannes and Slade 1982 ; Van Home and Ford 1982). By themethod used in this paper, when the center of the community was determined as the cen-ter of gravity of all species centroids, the niche position characterizes the degree ofremoteness of the microhabitat of the given species from the central areas of physicalgradients in the community ecological space. The relationships between the indices of



breadth and the position of the niche can be quite different for the same communityusing these two methods. A strong correlation between niche breadth and niche positioncan be a statistical artifact, if the community center coincides with the origin of the coor-dinates in discriminant space (Dueser and Shugart 1978). In the method used here, astrong correlation should be observed only in the case when niches become more narrowthe further they lie from the center of an ecological space. Such a strong negative corre-lation was found in the community of the Chihuahuan Desert of North America (Rogo-vin et al. 1991), and reflected a high level of continuity in niche distribution. The cen-troids of broadest niches were close to the center of ecological space, and the nichebreadth decreases gradually with increase of the distance from the center. In contrast,there was no significant negative correlation in the rodent community with pronouncedguild structure from Middle Asia (Shenbrot 1992). Though the guild structure in theRajasthan community analyzed here is not pronounced, we have no really broad nicheswhich should be located near the community center any way, but only medium sizedniches. In all cases mentioned above a really narrow niches were located at a peripheralzone of the ecological space. It is not clear if there are any prohibitions for narrow nichesto be located near the center. To answer this question we need to compare moreexamples of genetically independent communities with different spatial structures.

Negative correlations between niche position and maximal species abundance havebeen shown earlier for several types of communities (Dueser and Shugart 1979 ; Seagleand McCracken 1986; Shenbrot 1992). In some cases, the strong correlation betweenniche position (or niche breadth) and maximal species abundance or biomass is an arti-fact of calculation (Van Home and Ford 1982). In the present case, our methods of cal-culation have eliminated this artifact. The presence of this correlation as well as the cor-relation between maximal density and biomass in the Middle Asian desert rodentcommunity (Shenbrot 1992), and the absence of it in Chihuahuan (Rogovin et al. 1991)and in Rajasthan illustrates the fundamental differences in the organization of these com-munities.

Finally, the coincidence in part in general maxima of rodent biomass and speciesdiversity along the two main axes of the ecological space revealed here is not suprising,being determined by the close interrelationship between overall productivity and diver-sity of the environment. The existence of some fluctuations in the basically low diver-sity associated with conditions of typical desert landscape (left side of the model inFigure 4) are the result of a combination of only one abundant species (G. gleadowi)with some other species which occur in real desert more or less accidentally, and belongto another faunistic complex. Formally speaking our model fits the productivity hypo-thesis (Connel and Orians 1964). Accordingly to this hypothesis species richness in acommunity is determined by the quantity of energy flowing through it. The more foodproduced, the greater the diversity. This hypothesis is supported by some observationsand experiments (Pianka 1967 ; Brown and Davidson 1977 ; Kirchner 1977) but is incontradiction with other ones (Rosenzweig 1971 ; Silvertown 1980). In accordance withanother group of models (Grime 1979 ; Huston 1979 ; Tilman 1982) richness increasesasymmetrically along an productivity gradient with a pronounced humped-shaped pat-tern. This models has been applied to rodents (Abramsky and Rosenzweig 1984 ; Owen1988), and the hump-shaped diversity-productivity curves were obtained through theanalysis of mammalian distribution patterns (Abramsky 1988 ; Owen 1990). In our casethe low productive desert habitats have low rodent species diversity. The diversityincreases rapidly in a complex productive habitats, though in most productive areas ofsemisavannah type it tend partly to decrease. The last pattern is in contrast to biomass-productivity relationship in these areas. The revealed tendency to hump-shaped relation-


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ship of rodent species diversity and productivity of the environment along main axes ofecological space does not very much coincide with Tilman's (1982) curvilinear modeland with similar hump-shaped relations obtained for desert rodents mentioned above. Inaccordance with that model diversity (richness) increases rapidly in a narrow bandacross very low resource level and peaks at moderately low resource values. Asresources continue to increase, richness decreases slowly, trailing off with a long tail. Inour case the influence of a mixed origin and a young age of Rajasthan rodent faunaseems significant. It is a characteristic that high species diversity falls to the microhabi-tats not typical for the real desert.

ACKNOWLEDGEMENTS

This work was part of a joint program for research in the ecology of desert zones between theCentral Arid Zone Research Institute (Jodhpur, India) and the Institute of Animal EvolutionaryMorphology and Ecology of the Russian Academy of Sciences (Moscow, Russia), and was sup-ported by the Department of Science and Technology of India and the Russian Academy ofSciences. We thank Dr. I. Prakash and Dr. B.D. Rana for their aid on this project. We also thankProf. Z. Abramsky of Ben-Gurion University of the Negev for his helpful comments on the manus-cript.

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Spatial organization of a rodent community in the Western Rajasthan desert (India

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