composition, diversity, and distribution of a chihuahuan desert ant community (mapimı́, méxico)

Journal of Arid Environments (2000) 44: 213–227doi:10.1006/jare.1999.0583, available online at http://www.idealibrary.com on

Composition, diversity, and distribution of a ChihuahuanDesert ant community (MapimmH , MeH xico)

Patricia Rojas & Carlos Fragoso

Soil Biology Department, Institute of Ecology. A.P. 63, C.P. 91000,Xalapa, Ver. Mexico

(Received 24 December 1998, accepted 16 August 1999)

The Mapimi Chihuahuan Desert ant community was studied in 11 differ-ent vegetation sites grouped as rich shrubs (5 sites), poor shrubs (3), andgrasslands (3). The community was composed of 32 species, with speciesrichness varying in the range of 11–26 species per site. Species richness showedsignificant intergroup differences; but neither abundance nor biomassshowed intergroup differences. The most abundant species were Foreliusmaccooki and F. pruinosus; biomass was dominated by the larger speciesPogonomyrmex rugosus and Aphaenogaster cockerelli. Five trophic guilds wererecognized, with dominance of omnivores and granivores. Omnivorousbiomass was higher in shrubs than in grasslands, but no differences wereobserved in granivores. The community was dominated by small and verysmall ants; and size patterns varied in function of guilds and vegetation.

( 2000 Academic Press

Keywords: ants; desert; community; environmental heterogeneity; Chihua-huan Desert; guilds; shrubs; grasslands

Introduction

Arid zones have fewer species of ants when compared to tropical terrestrial ecosystems.In deserts, however, these insects constitute a very important group in terms of abund-ance (Pisarski, 1978; MacKay, 1991). In North America most desert ant studies havefocused on seed-feeding ants (Bernstein, 1974; Whitford & Ettershank, 1975; Davidson,1977a, b; Byron et al., 1980; Rissing, 1988; Rissing & Pollock, 1989; Crist & Weins,1994; Morehead & Feener, 1998, etc.) in spite of the fact that: (1) this group generallyconstitutes less than half of the total number of species, and (2) there are non-granivorous ants (omnivores, predators, honeydew feeders, etc.) which also influenceother ecosystem proccesses and that could potentially be interacting with seed-feedingants.

Only a few desert studies have attempted to estimate all ant species (Schumacher& Whitford, 1976; Whitford, 1978a; Wisdom & Whitford, 1981; Whitford et al., 1999),and in these studies, the analysis has been limited to one or two vegetation types withineach locality. This approach has been maintained, in spite of the fact that deserts showa high vegetation heterogeneity (Polis, 1991).

Located within the southern Chihuahuan Desert, the Mapimi Biosphere Reservecomprises a diverse mosaic of vegetation types established along a geomorphological

0140}1963/00/020213#15 $35.00/0 ( 2000 Academic Press

Figure 1. Localization of Mapimi Biosphere Reserve in north Mexico.

214 P. ROJAS & C. FRAGOSO

landscape of relief and soils (Montan8 a, 1990). In this paper we describe the antcommunity (species richness, abundance, and trophic spectrum) of this region, consid-ering 11 types of vegetation. This region has been the subject of several ecologicalstudies (Montan8 a, 1988; Aguirre & Maury, 1990) and can be considered one of the bestknown regions of the Mexican Chihuahuan Desert.

Study area

Considered within the realm of the Chihuahuan Desert (Schmidt, 1979), the MapimiBiosphere Reserve is located in the northern part of the Mexican tableland and includesportions of the states of Coahuila, Chihuahua, and Durango (Fig. 1). The altitude variesbetween 1100 and 1470 m (MartmHnez & Morello, 1977). The climate is arid, with anaverage annual temperature of 20)83C, maximum temperature of 423C, and with anannual rainfall of 284 mm (Cornet, 1988). Besides rainfall inter-annual variations(typical of arid zones), a high inter-zone variation (spatial distribution of rainfall) occurs,even over short distances (Cornet, 1988).

The vegetation is considered to be a heterogeneous xerophytic shrub (sensuRzedowski, 1978). A high diversity of land-forms and soil types has produced multipleplant associations like grasslands, shrubs of Larrea tridentata, rosette and mixed shrubs,dunes, and other vegetation subtypes.

Sites

Sampling was performed at 11 sites distributed over an area of 115 km2. These siteswere chosen because they were representative of major vegetation subtypes. Sites weregrouped into two categories according to the dominant vegetation form: grasslands andshrubs. Plant cover and soil data were taken from vegetation and soil maps of the region(Breimer, 1988; Montan8 a, 1988). The percentage of cover for each species was taken asrelative to the strata in which they are found.

CHIHUAHUAN DESERT ANT COMMUNITY 215

Shrubs

(1) ‘Conglomerate Hill’ (CH): Larrea tridentata shrub over calcareous lithosols andregosols of conglomerate origin. The high woody stratum is dominated byFouquieria splendens (10–15% cover) and the low woody stratum by L. tridentata(5–30%). There is a herbaceous stratum (mainly grasses) with low cover values(less than 10%).

(2) ‘San Ignacio Hill’ (SIH): mixed shrub established on the slope of San Ignacio Hill,over calcareous lithosols and regosols of volcanic origin. The dominant plants are L.tridentata (10–25%), Opuntia microdasys (5–10%), and F. splendens (1–5%).

(3) ‘Shrub of Larrea’ (LS): shrub dominated by L. tridentata (25–50%) and Opuntiarastrera (5–10%). The soils are haplic and calcic xerosols, and yermosols withpebbly facies.

(4) ‘Mixed shrub’ (MS): a mixture of bushy plants dominated by L. tridentata(5–10%), Agave asperrima (5–10%), and F. splendens (1–5%) growing over cal-careous regosols and haplic xerosols with stony facies.

(5) ‘Magueyal’ (MA): shrub dominated by A. asperrima (10–25%), O. rastrera(10–25%) and L. tridentata (5–10%). Soil are calcareous regosols with lithic andstony facies.

(6) ‘Dunes’ (DU): low cover mixed shrub over sandy soils, characterized by thedominant species Dalea scoparia (10–25%) and Yucca elata (10–25%).

(7) ‘Mogote’ (MO): mixed shrub dominated by Hilaria mutica (25–50%), Sida leprosa(10–25%), Prosopis glandulosa, and Flourencia cernua (5–10%). Soils are gipsic andluvic yermosols and xerosols with saline and sodic facies. This shrub is foundalternating with bands of denudated areas (‘peladeros’).

(8) ‘Peladero’ (PE): The bare area contiguous to the ‘mogote’; has the same soil typeand presents few and low density plants.

Grasslands

(1) ‘Grassland of Hilaria’ (HG): vegetation dominated by Hilaria mutica (50–100%)with Prosopis glandulosa in the arboreous stratum (25–50%). The soils are luvicyermosols and xerosols with saline and sodic facies.

(2) ‘Grassland of Sporobolus’ (SG): vegetation dominated by Sporobolus airoides(10–25%); the arboreal stratum is represented by P. glandulosa (cover less than5%). Soils are luvic yermosols and cromic vertisols with sodic and saline facies.

(3) ‘Mixed grassland’ (MG): grassland characterized by the codominance of H. mutica,S. airoides, and Atriplex canescens with P. glandulosa in the arboreous stratum. Soilsare gipsic and luvic yermosols and xerosols.

Materials and methods

Sampling

Sampling was performed during 1987, using two methods: pitfall traps and handsampling. (1) Twenty-five pitfall traps were placed in each site over a period of 7 days(nights included) during the month of July. Each trap consisted of a metal container of300 ml filled with formalin and detergent, and buried at soil level. We used formalin,because this preservative has a lower evaporation rate than alcohol and it is more usefulfor long periods of trapping. Time of trapping was the minimum possible in order toplace and retire all traps for the 11 sites. Traps were placed in a square net of 40]40 m,each trap being separated by a distance of 10 m. (2) During the months of March, July,and November each site was hand sampled by collecting the ants directly from soil and


vegetation with the aid of forceps, artist’s brushes, and baits of squid, meat, peanutbutter, and marmalade.

Ants were stored in 70% ethanol. Abundance and biomass (dry mass) of each specieswas calculated for each site from pitfall sampling ( July; mixing the individuals from alltraps). Intersite species richness comparisons were made with presence/abscence dataobtained from both pitfall and hand sampling for March, July, and November.

Data analysis

Species richness (the number of species for a given site) was calculated with data fromboth sampling methods, in order to include all the species; March and November handsamples were also included, because in these months two species appeared that werenot recorded during July. Diversity (inverse of Simpson Index, D"1/&p2

i , where p isthe proportion of ith species, i varies from 1 to S, and S is the total number of species inthe site; Southwood, 1978) was calculated considering only pitfall traps.

Localities were hierarchically classified with species presence/absence data from bothsampling methods. The association matrix was built up using the Bray & Curtix index,a non-linear dissimilarity measure which excludes 0]0 matches and has proved toperform consistently well with different types of data (Belbin, 1986). Clustering ofsites was obtained with the UPGMA method (Gauch, 1982). We used PATN to runthis analysis (Belbin, 1986). Worker ants were classified into the following size groups(Wheeler & Wheeler, 1986): very small (less than 2 mm total length), small (2}4 mm),medium (4}6 mm), and large (over 6 mm). Based on previously published information(summarized in Rojas-FernaH ndez & Fragoso, 1994) and on our field observations,species were classified in five different trophic habits: omnivores, granivores,predators, honeydew-feeders, and fungus growers.

Results

Species richness and diversity

Thirty-two ant species belonging to 18 genera and four subfamilies were recorded inMapimi (Table 1). Data on the biology and geographical distribution of species can befound elsewhere (Rojas-FernaH ndez & Fragoso, 1994). The number of species per sitevaried from 11 (SG, grassland) to 26 (CH, shrub), with an average number per site of17)4 species (n"11).

Sites were separated into three groups (Table 2) on the basis of species richness: richshrubs (CH, LS, MS, SIH, MA), poor shrubs (DU, MO, PE), and grasslands (HG, SG,MG). Average values of species richness significantly varied between vegetation groups(t-test, p(0)05). Rich shrubs presented a higher value (29 spp., xN +21)6) followed bypoor shrubs (23 spp., xN "15), and grasslands (18 spp., xN "13). Within each group,differences between total number of species and average values indicates highinter-site species composition variation, particularly in poor shrubs (Tables 1 and 2).

Average site diversity was 3)4, with values varying between 1)5 (MO, poor shrub) and4)9 (CH, rich shrub). Significant differences were found only between rich shrubs(xN "4)16) and poor shrubs (xN"2)23) (p"0)04; Table 2).

Abundance and biomass

Seven species dominated the community, accounting for more than 75% of the totalabundance (Table 1). These species were Forelius maccooki (26)6%), F. pruinosus


(13)4%), Pheidole subdentata (8)9%), Pogonomyrmex rugosus (8)2%), Ph. psammophila(7)1%), Neivamyrmex melanocephalus (6)3%), and Aphaenogaster cockerelli (6)2%).

Inter-group comparisons were not significantly different (t-test, p(0)05), not-withstanding that different site values were observed (Table 2). Sites with higherand lower abundance values were found either in poor and rich shrub sites: SIH, DU forhigher values, and MA and PE for lower values (Table 2).

Biomass was dominated by four species which accounted for 85% of thetotal biomass: P. rugosus (49)5%), A. cockerelli (25)16%), P. apache (5)43%), andP. maricopa (5)02%) (Table 1). Although no significant differences were foundbetween grasslands, rich, and poor shrubs, some sites presented very differentvalues that were more related to the size of dominant species than to the numberof individuals. For example, the highest biomass values corresponded to Mogote,a site in which two of the largest species dominate (P. rugosus and A. cockerelli) acountingfor 87% of total abundance (Tables 1 and 2). Lowest values, on the other hand,were found in Peladero, a site dominated by two small ants of less than 3 mmlength (F. pruinosus and Ph. subdentata), and which together contributed 68% of totalabundance.

The most abundant species F. pruinosus and F. maccooki presented, across all vegeta-tion types, an inverse pattern of abundance (Table 1). Their similar size, trophic roleand relatedness suggest the occurrence of some kind of competitive interaction.

Distribution of species

Ant species showed the following patterns of distribution among sites (Table 1):

(1) Species found in all or almost all sites (generalists): Ph. subdentata, F. maccooki (11sites); Ph. hyatti, Ph. psammophila, Dorymyrmex insanus (10 sites); P. rugosus, F.pruinosus (9 sites).

(2) Species found in one or very few sites (specialists): N. swainsoni, Trachymyrmexseptentrionalis, Brachymyrmex depilis (1 site); N. leonardi, Solenopsis (Diplorhoptrum)sp., Leptothorax sp. (two sites); P. desertorum, Cyphomyrmex wheeleri (three sites).

(3) Species limited to shrub sites: Acromyrmex versicolor, Tetramorium spinosum.(4) Species not inhabiting grasslands: Myrmecocystus depilis, M. placodops, A.

cockerelli.

Using species presence/absence data (obtained from records of Table 1), vegetationtypes were hierarchically classified. Three groups were recognized at a dissimilarityvalue of 0)32 (Bray-Curtis index) (Fig. 2). The first group included seven of the eightshrub sites (CH, SIH, LS, MS, MA, PE, DU), grouped together because all of themshare 10 specific species (M. placodops, M. depilis, Ph. subdentata, Ph. hyatti, D. insanus,F. maccooki, Ph. psammophila, Crematogaster depilis, F. pruinosus, and A. cockerelli), andlack Solenopsis (Diplorhoptrum) sp. and Leptothorax sp. The second group included onlyone shrub site (MO) and did not show any exclusive species. The third group includedall grasslands sites and was characterized by the absence of 14 species, the presence ofseven common species, and the presence of two exclusive species (Leptothorax sp. andSolenopsis (Diplorhoptrum) sp.). This arrangement partially agrees with the speciesrichness group separation (Fig. 2).

Trophic guilds

The ant community of Mapimi was dominated by omnivorous (14 species) andgranivorous (9 species) guilds, which together accounted for 72% of the total number of

Table 1. Abundance (ind per 25 pitfall traps) and biomass (mg per 25 pitfall traps; in brackets) of Mapimi ants. Species are grouped accordingto feeding habits. Asterisks indicate species collected only by hand sampling; L"large, M"medium, S"small, VS"very small; see text for

size ranks. Authorities of species can be found in Bolton (1995)

Rich scrubs Poor scrubs Grasslands

Species CH SIH LS MS MA DU PE MO MG HG SG TOTALFungus growers

AcromyrmexversicolorM 1(1)9) 8(15)4) 5(9)6) 0* 0 0 0 0 0 0 0 14(27)

TrachymyrmexseptentrionalisM 0 0 0 0 0 6(3)4) 0 0 0 0 0 6(3)4)

Honeydew feedersBrachymyrmex

depilisVS 5()2) 0 0 0 0 0 0 0 0 0 0 5()2)Myrmecocystus

depilisM 6(1)5) 6(1)5) 25(6)2) 11(2)7) 7(1)7) 35(8)7) 19(4)7) 13(3)2) 0 0 0 122(30)5)Myrmecocystus

placodopsM 1()2) 24(6) 14(3)5) 6(1)5) 2()5) 19(4)7) 14(3)5) 3()7) 0 0 0 83(20)7)Predators

NeivamyrmexleonardiS 4()1) 0 0 0 0 0 0 0 0 0 2()08) 6()2)

NeivamyrmexmelanocephalusM 0* 0 0 0 0 0 1()3) 2()6) 423(138) 43(13)9) 48(15)6) 517(168)

NeivamyrmexswainsoniM 0 0* 0 0 0 0 0 0 0 0 0 0

Pheidole hyattiS 4(1)1) 14(4) 1()2) 0* 1()2) 0* 3()8) 9(2)5) 0 4(1)1) 10(2)8) 46(13)1)Granivores

Pheidole cockerelliVS 0* 0 0* 1()06) 0 25(1)5) 0 0 1()06) 3()1) 183(11)4) 213(13)3)Pheidole

crassicornisVS 0 6()4) 0* 0 2()1) 0 9()6) 3()2) 10()7) 25(1)7) 11()7) 66(4)7)Pheidole

psammophilaVS 1()1) 48(6)7) 6()8) 4()5) 21(2)9) 0* 2()2) 0 51(7)1) 11(1)5) 445(62)4) 589(82)5)Pheidole

subdentataVS 53(2)9) 62(3)8) 142(8)8) 24(1)5) 16(1) 304(19) 39(2)4) 18(1)1) 7()4) 42(2)6) 28(1)7) 735(45)9)Pogonomyrmex

apacheL 3 (5)4) 148(270) 0* 0* 3(5)4) 0 0 3(5)4) 0 0 0 157(286)Pogonomyrmex

desertorumM 0 0 0* 1(1)9) 0 0* 0 0 0 0 0 1(1)9)

218P

.R

OJA

S&

C.

FR

AG

OS

O

PogonomyrmeximberbiculusS 16(9)1) 13(7)4) 0* 3(1)7) 2(1)1) 0 0* 0 0 0 0 34(19)4)

PogonomyrmexmaricopaL 7(13)5) 0 74(143) 8(15)4) 8(15)4) 39(75)3) 1(1)9) 0 0 0 0 137(264)

PogonomyrmexrugosusL 17(65)9) 0 20(77)6) 31(120)2) 9(34)9) 0 2(7)7) 528(2048) 2(7)7) 28(108)6) 35(135)8) 672(2607)

OmnivoresAphaenogaster

cockerelliL 256(665) 59(153)4) 47(122)2) 33(85)8) 51(132)6) 9(23)4) 9(23)4) 46(119)6) 0 0 0 510(1326)Camponotus sp.L 0* 38(65)1) 0 0 0 0 0 2(3)4) 2(3)4) 1(1)7) 0 43(73)7)Dorymyrmex

insanusS 52(4)6) 1()08) 0* 8()7) 5()4) 3()2) 2()1) 10()9) 78(7) 4()3) 0 163(14)6)Crematogaster

depilisS 185(39) 4()8) 15(3)1) 32(6)7) 4()8) 8(1)6) 2()4) 0 0 0 0 250(52)7)Cyphomyrmex

wheeleriS 13()5) 19()7) 0 0 0 0 0 0 0* 0 0 32(1)2)Forelius

foetidusS 13()5) 1323(54)3) 165(6)7) 163(6)7) 91(3)7) 0* 7()2) 9()3) 188(7)7) 174(7)1) 59(2)4) 2192(90)Forelius

pruinosusS 88(5)2) 23(1)3) 1()06) 0* 1()06) 873(51)9) 114(6)7) 0 0 0ı 1()06) 1101(65)5)Leptothorax sp.S 0 0 0 0 0 0 0 0 1()04) 5()2) 0 6()2)Monomorium

minimumVS 0* 2()08) 0* 0 4()1) 0* 0 4()1) 11()4) 0 0 21()8)Paratrechina

melanderiVS 0* 19(1)2) 0 0 0 0 0 0 93(6)3) 80(5)4) 0 192(13)Paratrechina sp.S 0* 18()7) 0 1()04) 6()2) 0 0 8()3) 2()08) 0 0 35(1)4)Solenopsis

aureaS 0* 167(14)1) 0* 23(1)9) 6()5) 3()2) 0 0 0 0 0 199(16)8)Solenopsis (D.)

sp.VS 0 0 0 0 0 0 0 0 0* 0 3()1) 3()1)Tetramorium

spinosumS 10(3)7) 46(17)2) 12(4)5) 1()3) 3(1)1) 0 0 0 0 0 0 72(27)

Total 735(820)4) 2048(624) 527(386)1) 350(247)6) 242(202)6) 1324(190) 224(53) 658(2186) 869(178)5) 420(144)2) 825(233) 8222(5266)

CH

IHU

AH

UA

ND

ES

ER

TA

NT

CO

MM

UN

ITY

219

Table 2. Abundance, biomass, species richness, and diversity of Mapimi ant communities (pft"pitfall traps)

Site Abundance Biomass Richness Diversity Richness(ind per 25 pft) (mg per 25 pft) (spp per 25 pft) (25 pft) (species per 25pft#

handsampling)

Rich ShrubsCH 735 820)4 19 4)9 26SIH 2048 624)1 21 2)3 22LS 527 386)1 13 4)8 21MS 350 247)6 16 3)9 20MA 242 202)6 19 4)9 19xN 780)4 456)2 17)6 4)16 21)6S)D) 732)8 261)5 3)1 1)1 2)7

Poor ShrubsDU 1324 190 11 2)0 16PE 224 53 14 3)2 15MO 658 2186 14 1)5 14xN 735)3 809)7 13 2)23 15S.D. 554 1194)1 1)7 0)9 1

GrasslandsMG 869 178)5 13 3)2 15HG 420 144)2 12 4)2 13SG 825 233 11 2)8 11xN 704)6 185)2 12)3 6 13S.D. 247)5 44)7 1 0)7 2

TotalxN 747)4 478)7 14)8 3)4 17)4S.D. 538)1 5609)8 3)4 1)2 4)5

220P

.R

OJA

S&

C.

FR

AG

OS

O

Figure 2. Classification of Mapimi vegetation types in function of ant species composition.Symbols of each site indicate the species richness grouping: rich shrubs, poor shrubs, and

grasslands. Dashed line indicates the disimmilarity value at which three groups are recognized.


species. The other guilds found in the community included predators (4 species),honeydew feeders (3 species), and fungus growers (2 species). When species of each sitewere assigned to their corresponding guild, the following patterns were recognized(Fig. 3):

Figure 3. Trophic guilds representation in Mapimi ant communities. (a) Number of species, (b)relative abundance, and (c) relative biomass. Omnivores, granivores, predators, honey-dew feeders, fungi growers.


(1) In four of the five rich shrubs (CH, SIH, LS, and MS) were the five guildspresent.

(2) All grassland sites presented only three guilds: granivores, omnivores, and pred-ators. The lack of fungus culturing and honeydew feeder guilds could be related tospecies environmental constraints and vegetation peculiarities. In the first case, thefungus grower species Trachymyrmex septentrionalis has been reported to showa preference for nesting in very sandy soils (Wheeler, 1907; Weber, 1956a, b),a type of substrate not found in any of the grasslands; accordingly, in Mapimi thisspecies was only found in dunes. In the case of vegetation peculiarities, the lack ofhoneydew feeders and the fungus grower Acromyrmex versicolor could be due to theabsence of certain plant species. Our personal observations indicate that A. versicolorrecollect composite seeds and leaves of Jatropha dioica and Larrea tridentata, whichare not found or are scarce in Mapimi grasslands, but that are, conversely, commonin shrubs. Honeydew feeder ants are currently found in environments where plantspecies that offer nectar and/or homopterans are common, none of which arefound in desert grasslands.

(3) The granivorous guild was represented by species of the genera Pheidole (4 species)and Pogonomyrmex (5 species). The average number of granivorous species by sitewas 6 (with a range of 4 to 9).

(4) In shrub groups (rich and poor), all 9 granivorous species were found; in grasslands,on the other hand, only 5 granivorous species were found. In two grassland sites(HG, SG), however, this guild was relatively important because ca. half of specieswere granivorous accounting for more than 75% of the total biomass (Fig. 3(a,c)).In grasslands there was a dominance of genus Pheidole (four to five species) overgenus Pogonomyrmex (one species: P. rugosus).

(5) In terms of abundance, there was not a clear dominance of granivores in grasslandsites (except for SG; Fig. 3(b)).

Figure 4. Species distribution by size in Mapimi ant communities. (a) All species, (b) omnivor-ous species, and (c) granivorous species. Large, medium, small, very small.


(6) Omnivores were relatively more abundant in 7 sites (four rich shrubs, two poorshrubs and one grassland; Fig. 3(b)); in terms of biomass, this guild dominated onlyin three sites (two rich and one poor shrub; Fig. 3(c)).

Size patterns

Mapimi ant species were classified by size into 8 very small, 12 small, 7 medium, and5 large species. These four size classes were represented in all sites, with slight inter-sitedifferences and with a preponderance of the small group of species (Fig. 4(a)).When species size was analysed for the two most important trophic guilds (omnivorous;Fig. 4(b), and granivorous, Fig. 4(c)), it was observed that small species dominated theomnivorous guild in all sites (xN"5)3 species per site, S.D."2)01), a pattern signifi-cantly different from the granivorous guild (xN "1)45 species per site, S.D."0)68).In this last guild, conversely, very small species dominated (xN "3)27 species per site,S.D."0)64), accounting for more than 60% of the total abundance in 7 of 11 sites.Very small granivorous species were important in grasslands, where they represented onaverage 88% and 71% of abundance and species richness, respectively. This patternwas significantly different from that found in rich shrubs (52% and 39% of averageabundance and species richness, respectively).

Discussion

In North American deserts, the structure of ant communities is determined by severalfactors, including predation (Rissing, 1981), competition (Brown & Davidson, 1977;Davidson, 1977a; Chew & De Vita, 1980), parasitism (Feener, 1981), and disturbance(Wisdom & Whitford, 1978). Ant species richness, in particular, has been correlatedwith primary productivity (measured as annual rainfall) and competition (Davidson,1977a), but few studies have assesed the effect of environmental heterogeneity(Nash et al., 1998). The number of ant species in Mapimi (32 species) is similar to themaximal value hitherto published for North American arid shrubs (33 species; Gaspar& Werner, 1976 quoted by Morton, 1993), but lower than the value mentioned byMacKay (1991) (50 species) for a 3 km transect in the New Mexican ChihuahuanDesert. The number of species in Mapimi could increase if sampling is extended toother months (especially after the wet season).

At the local (vegetation type) scale, the average number of species in Mapimi hardlyaccounts for more than 50% of the regional pool. In addition, the richest site (CH"26species) comprises 2)4 times more species than the poorest one (SG"11 species),indicating that environmental heterogeneity has promoted an increase in the speciesregional pool.

Montan8 a (1990) mentioned that, in Mapimi, composition and species richness ofplant communities varied in sites distributed along a water disponibility gradient,ultimately determined by relief. This author found a bell-shaped response of speciesrichness, with the highest values occurring in well drained sites (‘bajadas’), and thelowest values found in excessively and poorly drained sites (‘sierras’ and ‘playas’,respectively). Ant species richness (Table 2) partially displayed a similar trend, withhighest values ocurring in four shrubs sites (CH, LS, MS, MA, which correspond to‘bajadas’), and lowest values occurring in the three grasslands sites (MG, HG, SG,which correspond to ‘playas’). Following Montan8 a (1990), we suggest that our resultsindirectly reflect the influence of water availability ("plant productivity) on ant speciesrichness. In our single ‘sierra’ site (SIH), however, ant species richness was high,indicating that ant and plant species richness do not always respond to the samevariables (i.e. relief and water disponibility).


Total abundance and biomass estimates should be used with caution as no nest countswere made. It is widely known that pitfall traps are a good estimator of ant speciesrichness (Samways, 1983; Donnelly & Giliomee, 1985; Romero & Jaffe, 1989), butthe same is not true for abundance. However, in spite of their limitations (Topping& Sunderland, 1992), pitfall traps can be used to estimate abundance, if it is assumedthat the number of ants trapped with this method is directly related to the number ofactive nests. More than the total number of nests, active nests have been suggested asa more important variable when impacts on ecosystems are to be evaluated (Heatwole& Muir, 1991).

Average pitfall abundances were similar in rich shrubs (‘bajadas’) and in grasslands(‘playas’) (Table 2), a different pattern to that found by Whitford (1978a), whofound that all species of ants were more abundant in the single ‘playa’ site. Samplingmethods (nest counting vs. pitfall traps) and differences in species richness andspecies composition could explain this difference.

As has been found in other desert studies (Heatwole & Muir, 1991), ant bio-masses in Mapimi were determined more by the size of the ants than by the number ofindividuals. For example, the most abundant species, Forelius maccooki, only accountedfor 1)7% of the total biomass, whereas the species with the highest biomass (P. rugosus)contributed 8)1% to the total abundance.

Discrimination of Mapimi ant species in different guilds revealed 9 species ofgranivores, accounting for 32% of the total abundance, which means that 68% offoraging ants trapped by pitfall traps belong to other guilds. In terms of biomass,granivorous ants are more important (63% of total biomass), but this is mainly due tothe large species P. rugosus, which accounted for almost half of the total ant biomass(Table 1). If this species is excluded, the percentage of biomass accounted by granivoresis reduced to 27)4%.

Although Whitford (1978a) states that in ‘bajadas’ omnivorous ants are more impor-tant than granivorous ants and granivores constitute the largest guild in ‘playas’, their

Table 3. Abundance and biomass of omnivorous and granivorous ants of Mapimi.Sites are grouped by landforms; (pft"pitfall traps)

Omnivores GranivoresSite Abundance

(ind per 25 pft)Biomass

(mg per 25 pft)Abundance

(ind per 25 pft)Biomass(mg per25 pft)

‘Bajada’ sitesCH 617 718)5 97 96)9LS 240 136)5 242 230)2MS 261 102)1 72 141)2MA 171 139)4 61 60)8MO 79 124)6 552 2054)5PE 134 30)8 53 12)8xN 250 208)65 179)5 432)7S.D. 191)7 252)9 195)4 797)9

‘Playa’ sitesMG 375 24)9 71 15)9HG 264 14)7 109 114)5SG 63 3)1 702 212xN 234 14)2 294 114)1S.D. 158 10)9 353)8 98


data is qualitative and not conclusive (Whitford, 1978: Table 1, p. 84). Our results,however, partially support his conclusion because in Mapimi granivorous biomassdominate in ‘playas’ but omnivores and granivores are equally important in ‘bajadas’(Table 3). In some bajada sites, however, dominance of omnivores is more marked (e.g.in CH, abundance and biomass are three to six times greater than other bajada sites).Wesuspect that discrepancies with Whitford (1978a) are related to the number of sitessampled in each landform.

As a whole, the ant community of Mapimi is dominated by small and very small ants.Only in the guild of granivores were all ant sizes present, indicating a resource (seeds)partitioned according to ant body size. A correlation between seed sizes and granivorousant body size has been shown in several studies (Davidson, 1977a; Chew & De Vita,1980), which has been invoked to explain the coexistence of ant species consuming thesame resource.

An interesting pattern found in grasslands was the coexistence of four very small andonly one large granivorous species (P. rugosus). The lack of intermediate sizes in thesesites suggests a resource (one or few plant species) that is partitioned into two contrast-ing sizes: florets (and/or bare seeds) vs. spikelets, as has been recorded by Chew & DeVita (1980) in Arizona. As far as small ants have small foraging areas, the dominance ofvery small species over other sizes in grasslands could indicate high densities of smallseeds (Bernstein, 1975). On the other hand, coexistence of four species of similar sizecould be explained by differences in foraging strategies (Davidson, 1977a; Hansen,1978; Whitford, 1978b; Brown et al., 1979). Conversely, a wider spectrum of granivor-ous ant sizes in shrubs could indicate a larger diversity of seed sizes.

In the guild of omnivores, small ants dominated in all vegetation types. With sucha diverse food spectrum (corpses, faeces, plant and insect exudates, etc.), it is diffi-cult to determine to which degree resources influence ant body size.

Some of the results presented in this study indicate that comprehensive analysis ofNorth American desert ant communities can be implemented if two actions are under-taken: (1) analysis of the whole species pool, and (2) sampling of environmentalheterogeneity. This approach will reveal patterns that are not always detected with othermethods.

This work was supported by the National Council of Science and Technology of Mexico(CONACyT) project P220CCOR880873. We thank Eduardo Rivera, Gema Quintero, GabrielaChaH vez and the people of ‘Ejido La Flor’ for their help with the field work. Araceli Cartas isacknowledged for helping in obtaining biomass data. We also acknowledge the suggestions of twoanonymous reviewers that greatly improved the manuscript.

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