Ant (Hymenoptera: Formicidae) Responses to Environmental Stressors
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COMMUNITY AND ECOSYSTEM ECOLOGY Ant (Hymenoptera: 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 species 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 ants, Conomyrma insana (Buckley), and large seed harvesting ants, Pogonomyrmex 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 animal 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 insect communities (Whitford et al. 1995). Ants are among the most ubiquitous insects 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. genus 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.