COMMUNITY AND ECOSYSTEM ECOLOGY (: Formicidae) Responses to Environmental Stressors

in the Northern Chihuahuan Desert Downloaded from https://academic.oup.com/ee/article-abstract/29/2/200/342528 by New Mexico State University Library user on 08 November 2019

1 1,2 2 2 MALIHA S. NASH , WALTER G. WHITFORD , JUSTIN VAN ZEE, AND KRIS M. HAVSTAD

Environ. Entomol. 29(2): 200Ð206 (2000) ABSTRACT We studied responses of ant communities to shrub removal and intense pulse seasonal grazing by domestic livestock for four consecutive years. Weighted relative abundance and percent of traps in which an ant occurred were analyzed using randomized complete block design, split in time analysis of variance to test for signiÞcant differences between means of ant groups. The ant community in the Chihuahuan Desert grassland is dominated by small, liquid-feeding , Conomyrma insana (Buckley), and large seed harvesting ants, desertorum Wheeler. The weighted relative abundance of C. insana was signiÞcantly reduced on the plots without shrubs. The relative abundance of P. desertorum was signiÞcantly lower on grazed plots without shrubs than on the ungrazed plots without shrubs. There were no detectable effects of shrub removal or intense, pulse grazing on the less abundant ant species. These results suggest that the recent encroachment of shrubs into Chihuahuan Desert grasslands has increased the relative abundance of the dominant ant species in these communities. Intensive grazing by livestock has had an adverse effect on the most abundant seed-harvester, P. desertorum.

KEY WORDS ants, Conomyrma insana, Pogonomyrmex desertorum, Jornada Desert, grazing, mes- quites

THE ARID RANGELANDS of the southwestern United Although livestock production is the primary land States have experienced extensive and dramatic use in the region, there are few studies designed to changes in species composition and physiognomy of speciÞcally to determine how the grassland fauna is the vegetation since the establishment of the livestock affected by livestock grazing (Bock et al. 1984, Heske industry in the late 19th century (BufÞngton and Her- and Campbell 1991). Grazing by cattle not only re- bel 1965, Gross and Dick-Peddie 1979,Grover and Mu- moves a portion of the above ground biomass of pal- sick 1990, Bahre 1995). The desert basins of the north- atable grasses and forbs, but trampling also effects soil ern Chihuahuan Desert were originally dominated by structure and vegetation composition (Taboda and C4 perennial grasses, particularly black-gramma (Bou- Lavado 1993, Cole 1995). Trampling of soil and teloua eriopoda (Torrey) Torrey (Brown 1994). The changes in vegetation composition and biomass are northern Chihauhuan Desert grasslands are now a predicted to have dramatic effects on soil dwelling mosaic of shrublands and grasslands with patches of organisms. shrubs. These changes in vegetation are thought to be Despite documentation of major shifts in the veg- the results of overgrazing by domestic livestock com- etation of the Chihuahuan Desert grasslands, we have bined with naturally occurring drought (Grover and no mechanistic understanding of the processes that Musick 1990, Schlesinger et al. 1990, Bahre 1995). produced those changes. Therefore, we conducted an Currently, there are few shrub-free patches of grass- experiment to examine changes in plant and land in the region, and there are no large areas in the intermountain basins that are not grazed or have not communities and soil properties and processes in re- been grazed at some time in the past. Although the sponse to intensive seasonal grazing. Because the historical changes in vegetation composition and black-grama grasslands of the Jornada Experimental physiognomy are well documented, data of faunal Range were shrub-free in the 1860s (BufÞngton and community structure in these grasslands are nonex- Herbel 1965) we eliminated mesquite shrubs from half istent. Published faunal studies primarily involve of the plots. The shrub-free plots are hypothesized to woody shrub such as mesquite (Prosopis glandulosa) revert to the plant and animal community structure or creosotebush (Larrea tridentata (DC.) Cov. The that was present before the introduction of domestic presence of these shrubs inßuences the fauna, espe- livestock. cially the communities (Whitford et al. 1995). Ants are among the most ubiquitous in ter- restrial ecosystems. Because desert ants nest in the soil and feed on seeds, honey-dew, liquids derived from plants, and insects, grazing by domestic livestock 1 US EPA, NERL, ESD, P.O. Box 93478, Las Vegas, NV 89193Ð3478. 2 Jornada Experimental Range, USDAÐARS, MSC 3JER, New Mex- should have multiple effects on them. Ants have been ico State University, Las Cruces, NM. 88003. used as bioindicators of disturbance in Australia (Ma- April 2000 NASH ET AL.: ANT RESPONSES TO STRESS 201

Ant communities were sampled in September of each year beginning in 1993. Ants were sampled using pit fall traps arranged in a 7 ϫ 7 trap array with 9.14 m spacing between traps on each of the 0.5-ha plots (see

plot 2, Fig. 1). Pit-fall traps (38 mm diameter by 70-mm Downloaded from https://academic.oup.com/ee/article-abstract/29/2/200/342528 by New Mexico State University Library user on 08 November 2019 tall plastic vials) were Þlled to a depth of 30 mm with a mixture of 70% ethanol and 30% glycerol (Green- slade and Greenslade 1971). Traps were buried in the soil ßush with the surface and left in place for 24 h. Traps were retrieved, labeled with grid location in- Fig. 1. Multiple stressor exclosure plot layout. Number in formation, and stored before processing. lower left corner for each plot denotes plot number. A pit-fall All ants in a vial were identiÞed to species or to ant sampling grid is shown in plot 2. plus an operational taxonomic unit (e.g., Phei- dole spp. with no major workers) and counted. Rel- ative abundance of species was assigned as 0Ð5 ants ϭ jer 1983) and were hypothesized to be sensitive indi- 1, 6Ð10 ants ϭ 2, 11Ð20 ants ϭ 3, 21Ð30 ants ϭ 4, 31Ð40 cators of disturbance in Chihauhuan Desert grass- ants ϭ 5, 41Ð50 ants ϭ 6, and Ͼ50 ants ϭ 7. The pit-fall lands. The establishment of C3 shrubs in the desert trap data for each species in each plot were expressed grassland provided reliable sources of liquid food for in two ways: (1) the percent of traps in which a species ants. Both P. glandulosa and L. tridentata are pheno- was found and (2) a weighted relative abundance logically predictable, producing new leaves and ßow- determined by multiplying the percent of traps in ers in the spring and early summer months even in which a species occurred by the sum of the abundance years when there is insufÞcient soil moisture for any rankings for this species in the plot. The weighted grass growth. Here we examine changes in ant com- relative abundance was designed to eliminate bias munities in response to shrub removal and cattle graz- resulting from species that forage over long distances ing. We hypothesized that ant genera that feed di- and from species that forage intensively near the nest rectly or indirectly on plant liquids (Conomyrma spp. and which occur at high nest densities (i.e., Cono- and Forelius spp.) would be affected by the removal of myrma spp.) The weighted relative abundance calcu- shrubs. Because cattle grazing removes ßowering lations applied to the pit fall trap data are similar to tillers on grasses and trampling affects herbaceous those calculated for ant community studies by Lobry annuals, we hypothesized that seed harvesting ants de Bruyn (1993) and Whitford et al. (1999). To use (Pogonomyrmex spp.) would be affected by cattle abundance of individuals of a species in the traps in grazing. pit-fall trap data, it is necessary to transform the data to account for differences in the “trapability” of spe- cies with different foraging strategies. Column forag- Materials and Methods ers are less likely to encounter traps than random The experiment was conducted on the Jornada Ex- foragers and wave foragers (such as C. insana). For perimental Range Ϸ40 km N-NE of Las Cruces, NM. example, C. insana may have 150 individuals from a Eighteen 0.5-ha plots were established in a 1,284-ha single colony in a trap, whereas a column forager like grassland pasture that had been grazed during winter P. desertorum will have only 2Ð3 individuals from a and spring at an average stocking rate of 259 animal single colony in a trap. Relative abundance scales unit months per year since 1957 (an animal unit numbers of individuals in the traps to correct for the months is one adult cow for 30 d). The plots were behavioral differences in trapability. arranged in two rows of nine plots blocked in three We examined the responses of the ants to treat- blocks of six plots per block along the long axis (Fig. ments and time by analysis of variance (ANOVA). Six 1). The treatments were a combination of two factors: treatments of three levels of grazing (winter-grazed, (1) removal of the mesquite shrubs (P. glandulosa), summer-grazed, and not grazed) and two levels of and (2) intensive seasonal grazing (winter or sum- habitat (shrubs-removed and shrubs-intact) were mer). These treatments were applied in a randomized used in a randomized complete block design (see Fig. complete block design (Fig. 1). The plots were fenced 1). Measurements were repeated over time. Split plot in JulyÐAugust 1993. Mesquite shrubs were com- in time ANOVA was used to Þnd the differences in ant pletely removed from nine plots in January and Feb- responses between treatments over time. ruary 1994. Because of the extreme drought, plots were not grazed in the summer of 1994. Plots were winter-grazed in February 1995 and 1996 and summer Results grazed in August 1995 and 1996. Stocking rate was adjusted for the estimated forage available in the plots. Except for the most abundance species, the abun- Plots were stocked with between 20 and 40 yearlings dance of all ants of a genus was used in the analysis. per plot for 24Ð36 h to remove 65Ð80% of the esti- The ant community in the Chihuahuan Desert grass- mated available forage. This stocking rate produced a land was dominated by the small, liquid feeding ant grazing intensity Ϸ30 times greater than the grazing species, Conomyrma insana Buckley (Table 1). The intensity imposed on the pasture in the previous 40 yr. other abundant liquid feeding species was Forelius 202 ENVIRONMENTAL ENTOMOLOGY Vol. 29, no. 2

Table 1. Weighted relative abundance of ant species captured Table 3. Ant responses as affected by different treatments in pit fall traps on the grazing, shrub removal experimental plots using the weighted relative abundance (WRA) and percent traps

Species 1993 1994 1995 1996 WRA Species Source of variation F-value Pr Ͼ F Aphaenogaster cockerelli 27 64 35 27 Downloaded from https://academic.oup.com/ee/article-abstract/29/2/200/342528 by New Mexico State University Library user on 08 November 2019 Camponotus spp. 0100Conomyrma insana Year 11.0 0.0001 Conomyrma insana 2,691 3,073 2,343 1,848 Shrub*Year 3.5 0.025 Forelius foetidus 0 8 34 43 Grazed*Shrub 9.5 0.005 Forelius pruinosus 326 286 625 676 Solenopsis spp. Year 13.6 0.0001 Monomorium minimum 01143Forelius pruinosus Year 32.8 0.0001 depilis 92 229 104 231 Grazed 6.1 0.018 Myrmecocystus mexicanus 21 230 88 197 Grazed*Shrub*Year 3.7 0.006 Myrmecocystus mimicus 2 310 163 295 Myrmecocystus spp. Year 41.05 0.0001 Myrmecocystus placodops 05220Pheidole spp. Year 13.3 0.0001 Myrmecocystus romainei 3 8 16 32 Pogonomyrmex Year 20.6 0.0001 Pheidole crassicornis 18 18 37 10 desertorum Grazed 6.4 0.0165 Pheidole hyatti 14 51 89 72 Grazed*Shrub 28.6 0.0001 Pheidole militicida 49 59 67 97 Grazed*Shrub*Year 3.3 0.011 Pheidole rugulosa 11 92 11 13 Pheidole S4 5229847 SigniÞcant F-values are given for each group of species (signiÞcant Pheidole S8 20 82 6 52 level Ͻ 0.05) Pheidole sitarches soritis 4 60 4 207 Pheidole tucsonica 00125 Pheidole xerophila 69 132 212 221 Pogonomyrmex californicus 5 117 29 74 Pogonomyrmex desertorum 219 532 363 470 among years. A similar response was recorded in Pogonomyrmex rugosus 4226Pogonomyrmex spp. (Fig. 4) in which the greatest Solenopsis aurea 15 25 103 77 within and among plot variance was recorded during Solenopsis krockowi 13 65 66 119 the extreme drought year (1994). Solenopsis xyloni 126 321 315 524 Trachymyrmex smithii 0100The overall ANOVA showed a signiÞcant shrub- grazing interaction for C. insana (F ϭ 9.53; df ϭ 2, 36; P Ͻ 0.01; Table 3). Multiple comparison of means pruinosus Roger. The large seed harvesting ants were indicated that the weighted relative abundance of C. mostly P. desertorum with very few P. rugosus and P. insana was signiÞcantly higher in the pretreatment californicus. The native Þre ant Solenopsis xyloni Mc- shrubs-removed plots than in the shrubs-intact plots ϭ ϭ Ͻ Cook was more abundant than the other two Solenop- (t 2.55, df 36, P 0.02; Fig. 2). Five months after sis spp. which were in fairly equal in abundance. There the completion of the shrub removal treatment, the were six species of small seed harvesting ants, Pheidole signiÞcant differences in Conomyrma spp. weighted ϭ ϭ ϭ spp. that occurred in similar abundance plus three relative abundance disappeared (t 2.07, df 36, P additional species that were relatively rare (Table 1). 0.05; Fig. 2) in the shrubs-, shrubs-intact comparison. The remaining species of ants were relatively rare and The abundance of C. insana in the summer-grazed unevenly distributed among plots. with shrub removed were consistently higher than The pretreatment sampling of the ant community that of summer-grazed with shrubs-intact over the was conducted during a year with close to average years (Fig. 2). The differences in abundance were, rainfall (Table 2). Drought characterized the subse- however, not signiÞcant over the years (t Ͻ 1.09, df ϭ quent two years. There was so little rainfall in the 36, P Ͼ 0.28). In contrast, the weighted relative abun- summer of 1994 that none of the grasses produced dance of C. insana was signiÞcantly higher on the green leaves during the growing season. Rainfall close winter-grazed plots with shrubs than that on the win- to the long-term average characterized the Þnal year of the study (Table 2). Ants exhibited signiÞcant an- nual variation in weighted relative abundance (Table 3; Figs. 2Ð7). The most dramatic temporal differences were recorded for Myrmecocystus spp., which in- creased in relative abundance and in the within plot variance during the extreme drought year (1994) (Fig. 6). In all of the other years, the relative abundance of Myrmecocystus spp. was the same among plots and

Table 2. Annual and growing season rainfall (mm) at the ex- perimental plots during the study period Ⅺ Year Annual JuneÐSeptember Fig. 2. Mean ( ) and one standard deviation (-) of Conomyrma spp. relative abundance as affected by grazing 1993 279.9 184.4 and shrub removal treatments and time. WI, winter-grazed, 1994 139.9 34.5 shrub intact; WR, winter-grazed, shrub removed; SR, sum- 1995 167.5 158.3 mer-grazed, shrub removed; SI, summer-grazed, shrub intact; 1996 221.4 183.1 NI, nongrazed, shrub intact; NR, nongrazed, shrub removed. April 2000 NASH ET AL.: ANT RESPONSES TO STRESS 203 Downloaded from https://academic.oup.com/ee/article-abstract/29/2/200/342528 by New Mexico State University Library user on 08 November 2019

Fig. 4. Mean (Ⅺ) and one standard deviation (-) of Fig. 3. Mean (Ⅺ) and one standard deviation (-) of Pogonomyrmex spp. relative abundance as affected by grazing Forelius spp. relative abundance as affected by grazing and and shrub removal treatments and time. WI, winter-grazed, shrub removal treatments and time. WI, winter-grazed, shrub shrub intact; WR, winter-grazed, shrub removed; SR, sum- intact; WR, winter-grazed, shrub removed; SR, summer- mer-grazed, shrub removed; SI, summer-grazed, shrub intact; grazed, shrub removed; SI, summer-grazed, shrub intact; NI, NI, nongrazed, shrub intact; NR, nongrazed, shrub removed. nongrazed, shrub intact; NR, nongrazed, shrub removed.

There were no signiÞcant treatment effects on the ter-grazed, shrubs-removed plots (t Ͼ 3.00, df ϭ 36, weighted relative abundance of Solenopsis spp., P Ͻ 0.01). Changes over the duration of the study Myrmecocystus spp., and Pheidole spp. (Figs. 5Ð7). The showed that the removal of mesquite shrubs resulted weighted relative abundance of Myrmecocystus spp. in signiÞcant reductions in the weighted relative abun- was highest during the drought year (1994). dance of C. insana in winter grazing treatments. There were signiÞcant differences in the overall Discussion ANOVA of weighted relative abundance of Forelius spp. resulting from grazing, and shrubs-removedÐyear Annual differences in relative abundance of ants interactions (Table 3). There was no signiÞcant dif- were directly related to the climatic variation during ferences in Forelius weighted relative abundance in this study. The growing season rainfall (June through the shrubs-removed and shrubs-intact plots in 1994 September) was 47% of the long-term average in 1994 (t ϭϪ0.321, df ϭ 36, P ϭ 0.750; Fig. 3). Highest and 88% in 1995. The drought caused the death of a response in abundance was found in 1995 (Fig. 3) in number of colonies of short-lived species such as all treatments. After the imposition of grazing, the Conomyrma spp. Drought not only affects the demo- abundance of Forelius spp. in summer-grazed shrub graphics of ant colonies, but it affects their foraging removed (1995) plots was signiÞcantly higher than behavior. Temperature and humidity are important other treatments (t Ͼ 2.286, df ϭ 36, P Ͻ 0.03; except variables affecting the foraging intensity of some spe- for ungrazed shrubs intact; 1995). Abundance in win- cies of ants (Schumacher and Whitford 1974, Whit- ter-grazed shrubs removed (1995) was signiÞcantly ford and Ettershank 1975, Whitford et al. 1980, higher than most of other treatments (t Ͼ 2.176; df ϭ Whitford et al. 1981, Ho¨lldobler and Wilson 1994). 36, P Ͻ 0.036; Fig. 3). However, food supply has also been shown to affect There was a signiÞcant combined effect of grazing the intensity of foraging and the spatial extent of and shrub removal on P. desertorum (Table 3). In foraging (Whitford 1975, Whitford and Ettershank general, the relative abundance of P. desertorum was 1975). Climate affects the results of pit-fall trap studies lower in the winter-grazed plots without shrub than of ants by its inßuence on foraging intensity. Increase others and it was signiÞcantly different except for the summer grazed plots with shrub (t Ͼ 2.423, df ϭ 10, P Ͻ 0.04). P. desertorum was the highest in ungrazed shrubs removed and it was signiÞcantly different than the others (t Ͼ 2.405, df ϭ 10, P Ͻ 0.04). Multiple comparison of means (Fig. 4) indicated that in 1996, the weighted relative abundance of P. desertorum was lower in winter-grazed and summer-grazed without shrub than that on the ungrazed shrubs-removed plots. But, the difference was signiÞcant only for the winter-grazed shrub removed plots (t ϭ 2.429, df ϭ 36, P ϭ 0.02; Fig. 4; 1996). The weighted relative abun- dance of P. desertorum was highest during the drought Fig. 5. Mean (Ⅺ) and one standard deviation (-) of year (1994) with large differences in the ungrazed and Solenopsis spp. relative abundance as affected by grazing and summer-grazed plots without shrubs (t Ͼ 2.5, df ϭ 36, Ͻ shrub removal treatments and time. WI, winter-grazed, shrub P 0.02; Fig. 4, 1994). In winter-grazed plots, abun- intact; WR, winter-grazed, shrub removed; SR, summer- dance were signiÞcantly higher in plots with shrubs grazed, shrub removed; SI, summer-grazed, shrub intact; NI, (t ϭ 4.312, df ϭ 36, P ϭ 0.0001; 1994; Fig. 4, 1994). nongrazed, shrub intact; NR, nongrazed, shrub removed. 204 ENVIRONMENTAL ENTOMOLOGY Vol. 29, no. 2

The most signiÞcant result from this study was the effect of removing the invasive mesquite (Prosopis glandulosa) from the black grama grassland on the relative abundance of the liquid-feeding ants Cono-

myrma spp. P. glandulosa spread into the black grama Downloaded from https://academic.oup.com/ee/article-abstract/29/2/200/342528 by New Mexico State University Library user on 08 November 2019 grasslands on the Jornada Experimental Range in the 1930s and 1940s (BufÞngton and Herbel 1965). The establishment of this shrub in the grassland provided a phenologically predictable food supply for ßuid- feeding ants. Mesquite shrubs initiate leaf production in late April and early May and produce ßowers and Fig. 6. Mean (Ⅺ) and one standard deviation (-) of fruits in June and July. Although new leader produc- Myrmecocystus spp. relative abundance as affected by grazing tion and the abundance of ßowers and fruits of mes- and shrub removal treatments and time. WI, winter-grazed, quite vary with rainfall, the green foliage produced in shrub intact; WR, winter-grazed, shrub removed; SR, sum- early summer remains green and hydrated even dur- mer-grazed, shrub removed; SI, summer-grazed, shrub intact; ing the most severe drought. This foliage supports NI, nongrazed, shrub intact; NR, nongrazed, shrub removed. ßuid-sucking insects, many of which are tended by ants (Conomyrma spp.). During severe growing sea- son drought such as that of 1994, the perennial grasses in foraging intensity appears to be a response of ants fail to produce new tillers and leaves. The reduction that forage as individuals in a somewhat random for- in quality of grasses for C. insana during the drought aging mode. Column foragers such as Pheidole spp. was reßected in the reduced abundance on the (Whitford et al. 1981) exhibited an increase in within grasses. During a severe growing season drought, mes- plot variance and among year variance in relative quite shrubs provide the only source of liquid food for abundance in comparison to the Þrst year of the study ßuid-feeding ants. The reduction in relative abun- but no differences between the severe drought year dance of C. insana on the shrubs-removed plots dem- and a year with average rainfall following the drought. onstrated that shrub removal had a greater effect on This was very different from the pattern recorded these ants than drought or grazing by domestic live- stock. for Myrmecocystus spp. and Pogonomyrmex spp. The Honey-pot ants, Myrmecocystus spp., are also ßuid- ßuid feeding ants recorded in this study did not feeding specialists. However, these ants did not exhibit exhibit column foraging behavior and P. desertorum any response to the removal of mesquite shrubs. forages as individuals in a random pattern (Whitford Myrmecocystus spp. store food reserves in the honey 1975). Some species increase their foraging inten- pots (Ho¨lldobler and Wilson 1994), which allows sity when food is scarce (e.g., Pogonomyrmex deser- them to survive droughts even in the absence of a torum and Myrmecocystus spp. in this study), which phenologically predictable supply of liquid food. The accounts for the increase in relative abundance and dramatic increase in variance and weighted relative within plot variance in relative abundance during abundance of Myrmecocystus spp. during the severe the drought year (Whitford and Ettershank 1975). drought year (1994) probably resulted from the hon- Pogonomyrmex spp. may also increase the size of the ey-pot ants foraging over a wider area and more in- foraging range in response to drought (Rissing tensely than in other years (i.e., they were more likely 1988a). to be coming in pitfall traps because of their greater activity level). The large within plot variance and high weighted relative abundance of Pogonomyrmex spp. in 1994 is attributed to the same type of behavioral re- sponse to reduced food supply during the growing season. The food storage behavior of both of these species contributed to the survival of the colonies and return to predrought weighted relative abundance the following year. Mesquite shrubs not only provide a reliable food source for liquid feeding ants, the shade also modiÞes the soil microclimate (Chew 1995). The nests of C. insana are concentrated around the periphery of mes- quite shrubs on our shrubs-intact study plots (unpub- lished data). The reduction in relative abundance of C. insana in the shrubs-removed plots were attributable Fig. 7. Mean (Ⅺ) and one standard deviation (-) of Pheidole spp. relative abundance as affected by grazing and to both food resource limitations and loss of soil mi- shrub removal treatments and time. WI, winter-grazed, shrub croclimate heterogeneity by shade from shrub cano- intact; WR, winter-grazed, shrub removed; SR, summer- pies. Other species such as Veromessor pergandei Mayr grazed, shrub removed; SI, summer-grazed, shrub intact; NI, and Pogonomyrmex badius Latreille are reported to nongrazed, shrub intact; NR, nongrazed, shrub removed. change foraging behavior or to move nests from April 2000 NASH ET AL.: ANT RESPONSES TO STRESS 205 shaded areas (Carlson and Gentry 1973, Smith et al. stock exclosure in a semidesert grassland site. J. Range 1987). Desert seed harvesting ants appear to be less Manage. 37: 239Ð242. affected by shade. Mackay et al. (1986) reported that Brown, D. E. 1994. Biotic communities of the southwestern none of the Þve most common diurnal seed harvesting United State and northwestern Mexico. Ecol. Monogr 35: ants moved their nests from artiÞcially shaded plots. 139Ð164. Downloaded from https://academic.oup.com/ee/article-abstract/29/2/200/342528 by New Mexico State University Library user on 08 November 2019 Colonies of other species such as Myrmecocystus mexi- Buffington, L. C., and C. H. Herbel. 1965. Vegetation cana Wesmael either die or move their nest entrances changes on semidesert range from 1858 to 1963. Ecol. Monogr 35: 139Ð164. away from shade (Chew 1995). Because Myrmecocys- Carlson, D. M., and J. B. Gentry. 1973. Effects of shading on tus spp. were sparse on the plots at the beginning of the the migratory behavior of the Florida harvester ant. experiment and at least one of these species avoids Pogonomyrmex badius. Ecology 54: 452Ð453. shaded nest sites, there were no changes in Myrme- Chew, R. M. 1995. Aspects of the ecology of three species of cocystus spp. relative abundance as a result of shrub ants (Myrmecocystus spp. and Aphaenogaster sp.) in de- removal. sertiÞed grassland in southeastern Arizona, 1958-1993. Rissing (1988b) found that P. rugosus nests were Am. Midl. Nat 134: 75Ð83. preferentially located near shrubs in some habitats Cole, D. N. 1995. Experimental trampling of vegetation. II and preferentially located away from shrubs in other Predictors of resistance and resilience. J. Appl. Ecol 32: habitats. The shrub removalÐgrazing effects on P. de- 215Ð224. sertorum may have resulted from the changes in pre- Greenslade, P.J.M. and P. Greenslade. 1971. The use of baits ferred nest sites for P. desertorum in the shrubs-re- and preservatives in pitfall traps. J. Aust. Entomol. Soc. 10:253Ð260. moved plots. The shrubÐgrazing interactions Gross, F. A., and W. A. Dick-Peddie. 1979. A map of prime- recorded for P. desertorum in this study are the result val vegetation in New Mexico. Southwest. Nat 24: 115Ð of multiple factors (i.e., changes in soil microclimate 122. resulting from shrub removal and reduction of grass Grover, H. D., and H. B. Musick. 1990. Shrubland encroach- canopy by grazing). Reduction of grass canopy affects ment in southern New Mexico, U.S.A: an analysis of de- soil microclimate and also modiÞes food supplies for sertiÞcation processes in the American southwest. Cli- seed harvesters. Grazing also caused reduction in matic Changes 17: 305Ð330. ßowering tillers and loss of mesquite seed inputs in the Heske, E. J., and M. Campbell. 1991. Effects of an 11-year shrubs-removed plots. The time lag in responses of P. livestock exclosure on rodent and ant numbers in the desertorum to the treatments is attributed to the seed Chihuahuan Desert in southeastern Arizona. Southwest. storage in the nests. Stored food ameliorates the im- Nat 36: 89Ð93. mediate effects of food resource ßuctuations. Ho¨lldobler, B., and E. O. Wilson. 1994. The ants. Belknap Press, Cambridge, MA. Forelius spp. exhibited signiÞcant responses to both Lobry de Bruyn, L. A. 1993. Ant composition and activity in grazing and shrub removal. However, the responses annually-vegetated and farmland environments on con- were inconsistent among years. Forelius spp. are more trasting soil at Kellerberin, Western Australia. Soil Biol. generalist feeders than specialized liquid-feeders Biochem. 25: 1043Ð1056. (Ho¨lldobler and Wilson 1994). This feeding ßexibility Majer, J. D. 1983. Ants: bio-indicators of mine site rehabil- may have contributed to the lack of response to shrub itation, land use and land conservation. Environ. Manage. removal by Forelius spp. 7: 375Ð383. SigniÞcant quantitative responses to treatments Mackay, W., S. Silva, D. C. Lightfoot, M. I. Pagani, and W. G. were limited to the dominant species in the ant com- Whitford. 1986. Effects of increased soil moisture and munity. The recorded responses were generally what reduced soil temperature on a desert soil com- was hypothesized based on the natural histories of the munity. Am. Midl. Nat 116: 45Ð56. dominant species. SigniÞcant responses of other ant Rissing, S. W. 1988a. Dietary similarity and foraging range of two seed-harvester ants during resource ßuctuations. species to the treatments may occur if the experiment Oecologia (Berl.) 75: 362Ð366. is continued as planned for a decade or more. Rissing, S. W. 1988b. Seed-harvester ant association with We thank Walter Smith, Joseph Daraio, and Gregory shrubs: competition for water in the Mojave Desert? Forbes for processing ant samples. This research was sup- Ecology 69: 8Ð9-813. ported in part by funds from the U. S. Environmental Pro- Schlesinger, W. H., J. H. Reynolds, G. L. Cunningham, L. F. tection Agency (EPA) through its OfÞce of Research and Huenneke, W. M. Jarrell, R. A. Virginia, and W. G. Development. It has been subjected to the AgencyÕs peer and Whitford. 1990. Biological feedbacks in global deserti- administrative review and approved as an EPA publication. Þcation. Science (Wash. D.C.) 247: 1043Ð1048. Schumacher, A. H., and W. G. 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Whitford, W. G. 1975. Foraging behavior of Chihuahuan Whitford, W. G., G. S. Forbes, and G.I.H. Kerley. 1995. Desert harvester ants. Am. Midl. Nat 95: 455Ð458. Diversity, spatial variability, and functional roles of Whitford, W. G., and G. Ettershank. 1975. Factors affecting invertebrates in desert grassland ecosystems, pp 152Ð foraging activity in Chihuahuan Desert harvester ants. 195. In M. P. Mcclaran and T. R. Van Devender [eds.], Environ. Entomol 4: 689Ð696. The Desert Grassland. University Arizona Press, Tuc- Downloaded from https://academic.oup.com/ee/article-abstract/29/2/200/342528 by New Mexico State University Library user on 08 November 2019 Whitford, W. G., E. Depree, P. Hamilton, and P. Johnson. son. 1980. Foraging ecology of two Chihuahuan Desert ant Whitford, W. G., J. Van Zee, M. S. Nash, W. E. Smith, and J. E. species: Novomessor cockerelli and Novomessor albiseto- Herrick. 1999. Ants as indicators of exposure to envi- sus. Insects Soc. 27: 148Ð156. ronmental stressors. Environ. Monitor. Assess 54: 143Ð Whitford, W. G., D. J. Depree, P. Hamilton, and G. Etters- 171. hank. 1981. Foraging ecology of seed-harvesting ants. Pheidole spp. in a Chihuahuan Desert ecosystem. Am. Received for publication 11 May 1999; accepted 2 December Midl. Nat. 105: 159Ð167. 1999.