Cattle grazing decreases abundance but not foraging success in Great Basin desert steppe

Ines Anuncibay-Childs1, Keitty Calderon-Chalco2, Sanam Noorani3, and Brandon Mukogawa4

1University of California, Davis; 2University of California, Los Angeles; 3University of California, Santa Cruz; 4University of California, San Diego

ABSTRACT

Grazing has been documented to have widespread effects on flora and fauna in a variety of ecosystems. However, research has yielded conflicting results on determining the impact of grazing on ; some have found that grazing increases ant abundance and alters composition while others have found the opposite pattern. It is critical to accurately survey the consequences of grazing in order to understand the implications of this widespread land management practices on ecosystems especially on the Great Basin sagebrush steppe, which is vulnerable to degradation from grazing, urbanization, and other anthropogenic pressures. We sought to determine the effects of grazing on ant species richness and abundance using pitfall traps and transect surveys. We found no effect of grazing on ant species richness, however, grazing decreased ant abundance and altered ant species composition. Additionally, we studied foraging success of the harvester ant salinus and found that grazing had no effect on foraging success. The implications of cattle grazing on the ecosystem’s organisms, such as ants should be considered before implementing grazing as a land management practice.

Keywords: ants, cattle grazing, foraging, sagebrush steppe, Pogonomyrmex salinus

INTRODUCTION trophic levels. For instance, species that feed on grazed vegetation may be Cattle grazing is one of the most negatively impacted while other organisms widespread—but also controversial—land that utilize less dominant vegetation may management practices (Eldridge et. al benefit. However, grazing can compact soil, 2016). Grazing can have neutral or positive increase erosion, decrease water impacts on plant species richness, permeability, and reduce vegetative cover increasing the diversity of flora and fauna that is used for food and/or shelter (Donkor by removing dominant vegetation. This in 2001; Eldridge et. al 2016). With less turn allows less competitive plants to foliage, there are often less available niches become more successful by making for organisms to utilize—limiting resources available and increasing available biodiversity (Newbold and MacMahon sunlight. These effects can have varying 2008). Limiting biodiversity and various impacts on different functional groups and other effects on the ecosystem are

CEC Research | https://doi.org/10.21973/N36W9J Summer 2019 1/8

important to consider before implementing decline following the reduction of cattle grazing. vegetative understory in sagebrush steppe. Ants, specifically, have been historically Thus, it is essential to understand the used as bioindicators—organisms that have effects of human disturbances such as been monitored to assess the health of cattle grazing to preserve this critical ecosystems because they are easy to survey habitat. and have well-studied community Our study tracked changes in the ant responses to environmental stressors and community structure of grazed and disturbances that are representative of ungrazed areas within the Eastern Sierra other organisms’ responses in the same sagebrush steppe at the Sierra Nevada habitat (Hoffmann 2010). They are Aquatic Research Laboratory in Mammoth especially indicative of the vegetative Lakes, CA. This research is particularly community due to their utilization of a important as many studies have focused on variety of plants for both food and shelter the impact of grazing on grasslands as (Andersen 1990). Additionally, ant opposed to the sensitive sagebrush steppe, communities are often determined by the which has been largely reduced by the type of soil and vegetation in a habitat— invasion of exotic plants such as cheatgrass which is largely influenced by management (Bromus tectorum) as a result of grazing practices like grazing (Hoffmann 2010). In (Hemstrom et. al 2002). fact, some forest management agencies Because there is a lot of conflicting have used ant surveys to monitor areas literature on whether livestock grazing has with fire, industrial, and grazing practices positive or negative consequences on ant (Andersen 1997). Thus, using ants to assess communities, we wanted to explore the health of areas where cattle grazing possible mechanisms that drive this occurs is an effective measure to determine potential difference in ant distribution whether such practices should be between grazed and ungrazed areas. One implemented within certain habitats. ultimate cause in this different ant One particularly important ecosystem to distribution may be a difference in foraging study the consequences of cattle grazing is ability as grazing has been known to the desert sagebrush steppe. This habitat, decrease vegetative cover and increase the characterized by sagebrush Artemesia spp., amount of bare ground (Usnick and Hart is found throughout the Great Basin of the 2002). This may allow ants to forage more United States, but has been reduced by at easily in grazed areas—with less debris and least eighty percent of its natural occurring plants to navigate through, they may both range due to a variety of anthropogenic find food and bring it back to the nest alterations to the landscape such as quicker. Therefore, we developed an agriculture, urbanization, extensive grazing, experiment specifically studying one of the and climate change (Hemstrom et. al 2002). most common ant species in the area, This specific ecosystem is essential for Pogonomyrmex salinus (Nonacs 2002), to species such as the greater sage grouse test whether ant foraging efficiency differed (Centrocercus urophasianus), which has the between the grazed and ungrazed plots. potential of being listed in the Endangered Species Act because of its precipitous

CEC Research | https://doi.org/10.21973/N36W9J Summer 2019 2/8

We hypothesize that: grasses dispersed amongst the landscape (1) Grazed plots will have less vegetation include Great Basin wild rye (Elymus than ungrazed plots, specifically less grasses cinereus) and cheatgrass (Bromus and forbs. tectorum). (2) Grazed plots will have less ant colonies In order to examine the effects of grazing than ungrazed plots as a result of soil on ant communities in this region, we compaction and trampling of ant mounds. conducted a five-day survey between July (3) Grazed plots will have a lowered ant 31 and August 4, 2019, examining ant species richness than ungrazed plots communities in grazed and ungrazed because of a reduction in grasses and forbs. sagebrush steppe as well as an experiment (4) Species compositions will differ looking at differences in foraging efficiency between grazed and ungrazed plots between the two sites. Foraging efficiency because the shift in vegetation will create was calculated using the formula below: different niches for ant species to exploit. (5) P. salinus will have a higher foraging 2.2 Ant Community Surveys efficiency in grazed areas than ungrazed areas because grazing will create more In order to conduct an ant community ground cover and reduce vegetation that survey of the ungrazed and grazed plots, we impairs their movement to and from their walked 30 meters away from any road or nest. fencing separating the ungrazed and grazed plots to avoid potential differences in METHODS vegetation. We surveyed eighteen 40 x 4 m plots with 22.5 m between each plot: nine 2.1 Study system in the grazed and nine in the ungrazed. Before conducting our experiment, we used We sampled two sites, one within the a pilot study to determine what time Sierra Nevada Aquatic Research Laboratory, periods to use during our survey of ant an area protected from cattle grazing, and nests. During which, we determined that a an adjacent area with free-range cattle 10 minute period was sufficient in surveying grazing. Both were located within Mono all nests in the grazed area, but this time County on the Eastern slope of the Sierra period was insufficient in the ungrazed plot Nevada (37°36'51" N latitude, 118°49'47" W because of the more dense vegetation. longitude). The sites were at an elevation of Instead, we found a 12 minute survey around 7,000 ft. and receive about 25 cm to period in the ungrazed plot to be equally 38 cm of precipitation per year, mostly in sufficient to the 10 minute period in grazed the form of snow melt. The mean plots. temperature typically fluctuates between - Additionally, we calculated visual percent 23°C and 11°C in the winter and from 0°C to ground cover for shrubs, grasses (including 29°C in the summer. The reserve is a forbs), and dead vegetation using a 0.25 m2 semiarid Great Basin sagebrush steppe quadrat four times per transect at 10 m dominated by sagebrush (Artemesia spp.), intervals. We conducted a t-test bitterbrush (Purshia tridentata), and determining the effects of treatment on the rabbitbrush (Ericamerianauseosa). Common different types of ground cover. A t-test was

CEC Research | https://doi.org/10.21973/N36W9J Summer 2019 3/8

also conducted to determine the effect of treatment on the total number of ant nests per m2. Furthermore, we set up a total of 80 pitfall traps: four pitfall traps across 20 transects (10 in the grazed and 10 in the ungrazed areas) positioned in the center of the 4 m width of the plots in 10 m intervals. The pitfall traps were left for 48 hours and collected in the evening. Ant specimens were collected and placed in jars with ethanol alcohol before being identified to their species under a microscope. Figure 1. Foraging experiment setup. Each nest had three plates with 10 red colored seeds placed 75 cm 2.3 Foraging Experiment away from the nest entrance in the north, southeast and southwest direction.

In addition to the ant survey, foraging RESULTS efficiency trials were conducted with P. salinus. We identified 49 nests total (24 in Overall we sampled 18 40 m x 4 m plots in the ungrazed and 25 in the grazed). Three grazed and ungrazed areas. Ungrazed plots paper plates with an area of 143.14 contained a larger abundance of total ant 2 cm ±3.14 cm2 were placed 75 cm away from colonies per m2 when compared to grazed the nest entrance in the north, southeast plots (N=9, P=0.0362, F=5.23, Figure 2). The and southwest directions per nest. To keep vegetative composition of the ungrazed the bait plate level with the ground, we plots contained a higher average percent used local soil to cover the edges of the grass cover than the grazed plots (N=9, plate. On each plate we placed 10 oats dyed P=0.0037, F=11.5, Figure 3). However, the with red food coloring that were cut in ¼ average percent shrub cover (N=9, P=0.678, pieces for a total of 30 oat pieces per nest F= 0.179, Figure 3) and the average percent (Figure 1). Upon delivery of the first oat to dead cover (N=9, P=0.266, F=1.33, Figure 3) the colony, ants were given 15 minutes to were not different between the grazed and collect as many oats as possible. After the ungrazed plots. 15-minute period, the total number of oats There was no difference in mean species remaining on the plate was recorded as well richness per pitfall trap in grazed areas as the number of oats successfully delivered when compared to ungrazed areas (N=10, to the colony. We also noted the number of P=0.295, F=1.12, Figure 4). Ant species oats taken by other species. composition was different between the two We conducted t-tests to determine treatments, with P. salinus, Formica whether treatment affected foraging planipilis, Conomyrma insana, efficiency (FE), which was calculated as: Brachymyrmex depilis all being found in both the grazed and ungrazed pitfall traps. ������ �� ���� ��������� �� = (����� ������ �� ���� − ������ �� ���� ������) Formica sibylla was unique to the ungrazed area pitfall traps and Lasius alienus and

CEC Research | https://doi.org/10.21973/N36W9J Summer 2019 4/8

Formica neogagates were unique to the grazed ones (Figure 5). In total we sampled, 49 P. salinus colonies in grazed and ungrazed areas, and there was no difference in their foraging efficiency across grazed areas and ungrazed areas (NGrazed=25, NUngrazed=24, P=0.211, F =1.61, Figure 6).

Figure 2. Total number of ant nests per m2 per treatment. There were more ant nests per m2 found in the ungrazed areas than in the surveyed grazed areas. (N=9, P=0.036, F=5.22). Vertical axis represents the total number of ant colonies per meter squared, using a 40m2 plot. Horizontal axis represents the grazing treatment as either ungrazed or grazed. Bars represent + 1 S.E.

Figure 4. Effect of treatment on ant species richness per pitfall trap. There was no difference in species richness per pitfall trap in the grazed and ungrazed areas. (N=10, P=0.295, F=1.12). X-axis represents average ant species richness, as obtained from pitfall-traps along a transect. Y-axis represents the grazing treatment as either ungrazed or grazed. Bars represent + 1 S.E.

Figure 3. Effect of treatment on mean percent cover. There was no difference in the percent of dead vegetation cover (N=9, P=0.266, F=1.33) and the percent shrub cover (N=9, P=0.678, F=0.179)in the grazed and ungrazed areas. Percent grass cover was higher in the ungrazed areas than in the grazed areas(N=9, P=0.0037, F=11.5). X-axis represents the average percent cover as estimated by visual cover estimates. Y-axis represents the grazing treatment as Figure 5. Effect of treatment on ant species either ungrazed or grazed. The bars, in order of composition found in pitfall traps. X-axis represents appearance left-to-right, represent percent dead the grazing treatment as either ungrazed or grazed. vegetation, percent grasses, percent ground, and Numbers represent the total occurrences of species percent shrub. Bars represent + 1 S.E. in pitfalls from the two treatment sites (NGrazed=10,NUngrazed= 10).

CEC Research | https://doi.org/10.21973/N36W9J Summer 2019 5/8

soil (Donkor et. al 2001). Alternatively, grazing may also limit food availability by decreasing the grass cover that provides seeds granivore species feed on. Therefore, more research can be done to understand the relationship between a gradient of grazing and mound trampling, soil compaction, and reduction of grass. These results could potentially help determine what is the ultimate cause(s) that explain how grazing reduces ant abundance in the Great Basin steppe. Despite observing that grazing reduces ant abundance, grazing did not appear to Figure 6. Effect of treatment on foraging efficiency of P. Salinus. There was no difference in the foraging affect ant species richness. Though efficiency between P. salinus in the grazed and contradictory to our initial hypothesis, this ungrazed areas (NGrazed=25, NUngrazed=24, P=0.211, finding has been supported by other F=1.61). X-axis represents foraging efficiency, research in a semi-arid shrubland calculated by the total number of oats delivered to ecosystem (Read and Andersen 2000). the colony by foragers, divided by the number of oats initially minus the number of oats stolen by However, another study was also other species or colonies. Y-axis represents the conducted in the sagebrush steppe, but grazing treatment as either ungrazed or grazed. Bars found that grazing decreased ant species represent + 1 S.E. richness (Nash et. al 2001). Other research has stated that grazing has positive effects DISCUSSION on ants in highly productive ecosystems, Overall, our results indicated that grazing but negative effects in areas of low reduces ant abundance, contradicting productivity (Schmidt et al. 2012). However, literature that asserts grazing increases ant our study and those performed by Read and abundance (Read and Andersen 2000, Wells Andersen (2000) as well as Nash et al. et al. 1976) or has no effect on it (Tadey and (2001) were all conducted at low primary Farji-Brener 2007, Heske and Campbell productivity areas. Therefore, the 1991). However, previous studies have difference in our results may be explained found that ants are most negatively by a difference in grazing intensity as impacted in heavily grazed areas (Rogers et moderately grazed areas have been known al. 1972), which suggests that our site was to promote richness while heavily grazed potentially overgrazed. Grazing may reduce areas can have decreased richness ant abundance because of the trampling of (Abensperg-Traun et al. 1996). Therefore, it ant mounts—requiring the colony to is possible that the sagebrush steppe expend extra energy to maintain their studied by Nash et al. (2001) was more mounds (Usnick and Hart 2002)—as well as heavily grazed than the area we examined. soil compaction which may hinder new Although we found no effect of treatment colonies from building their mounds in the on ant species richness, species composition changed with grazing: two ant

CEC Research | https://doi.org/10.21973/N36W9J Summer 2019 6/8

species, L. alienus and F. neogagates, were a lack of effect of grazing on the foraging only found in the grazed area. The presence efficiency of P. salinus was that there was of two unique species in the grazed area no difference in the ground, shrub, or dead could prompt further research to determine vegetation cover between the grazed and if there is a potential shift towards more ungrazed areas. Thus, our assumption that grazing-resistant species (Usnick and Hart ungrazed areas would have more vegetative 2002). In addition to the presence of unique debris and therefore impeded foraging may species, there were differences in the have been incorrect. But it is possible that relative abundance of ant species found in the difference observed in grass cover still both treatments. For instance, F. planipilis may affect foraging. In order to better was over three times more abundant in understand how foraging efficiency is ungrazed plots than in grazed ones. This affected by cattle grazing, further research effect of treatment on species composition, could be conducted on different species of may perhaps be due to different extents of ant such as F. planipilis because it was three available resources as F. planipilis, which times more abundant in the ungrazed plot constructs mounds out of plant matter than the grazed area. (Nonacs 2002), may have less resources in In conclusion, we found that grazing grazed areas because of the reduced grass decreased ant abundance, which may imply cover. This pattern in grass cover across the that the Sierra Nevada Aquatic Research treatments may explain why some species’ Laboratory may have overgrazed sagebrush relative abundance was largely affected by steppe, as ants are widely used as a treatment, while others’ abundance were bioindicator species (Hoffmann 2010). This not affected. Thus, it may be possible that pattern of ant abundance is especially grazing effects cannot be generalized to all important in this study area—Great Basin species, but are instead species-specific as desert steppe—which is sensitive to each species occupies its own niche. overgrazing and other practices (Hemstrom We were unable to support the et. al 2002), and may inform agencies like hypothesis that grazing would increase the the U.S. Forest Service and U.S. Bureau of foraging efficiency of P. salinus, as we found Land Management in determining how to no effect of grazing on foraging efficiency. A best manage the land beyond the Great study on the close relative of P. salinus, the Basin. Furthermore, our methods could be western harvester ant, Pogonomyrmex applied more generally as a metric for occidentalis, found that P. occidentalis was overgrazing of communities at large in able to successfully tolerate stress from order to potentially determine patterns of cattle grazing (Usnick and Hart 2002). It may overgrazing globally. be possible that much like P. occidentalis, P. salinus can also successfully tolerate ACKNOWLEDGMENTS grazing. This potential toleration may explain the ability of P. salinus to efficiently We would like to thank the University of forage in both grazed and ungrazed areas, California Natural Reserve System and its as the species might easily acclimate to the Sierra Nevada Aquatic Research Laboratory, effects of grazing on the landscape. doi:10.21973/N3966F, for allowing us to However, perhaps a better explanation for conduct research within the reserve. We

CEC Research | https://doi.org/10.21973/N36W9J Summer 2019 7/8

would also like to give thanks to Dr. Joe Nash, M.S., W.G. Whitford, D.F. Bradford, S.E. Sapp for helping us to identify our Franson, A.C. Neale, and D.T. Heggem. 2001. Ant communities and livestock grazing in the Great samples, our faculty advisors Basin, USA. Journal of Arid Environments 49:695– Dr. Timothy Miller and Dr. Krikor Andonian, 710. as well as Ph.D. student Prahlada Papper for their endless support and guidance. Newbold, T.A.S. and J.A. MacMahon. 2008. Consequences of cattle introduction in a REFERENCES shrubsteppe ecosystem: indirect effects on desert horned lizards (Phrynosoma platyrhinos). Western Abensperg-Traun, M., G.T. Smith, G.W. Arnold, D.E. North American Naturalist 68:291–302. Steven. 1996. The effects of habitat fragmentation Nonacs, P. 2002. Patterns of energy allocation within and livestock-grazing on communities in remnants of gimlet Eucalyptus salubris woodland foragers of Formica planipilis and Pogonomyrmex in the Western Australian wheatbelt . salinus. Western North American Naturalist Journal of Applied Ecology 33:1281–1301. 62:188–196.

Donkor, N.T., J.V. Gedir, R.J. Hudson, E.W. Bork, D.S. Schmidt, A.C., L.H Frasers, C.N Carlyle, and E.R.L. Basset. 2012. Does cattle grazing affect ant Chanasyk, and M.A. Naeth. 2001. Impacts of abundance and diversity in temperate grasslands? grazing systems on soil compaction and pasture Rangeland Ecology & Management 65: 292-298. production in Alberta. Canadian Journal of Soil Science 82:1–8. Tadey, M. and A.G. Farji-Brener. 2007. Indirect Eldridge, D., A. Poore, M. Ruiz-Colmenero, M. Letnic, effects of exotic grazers: livestock decreases the and S. Soliveres. 2016. Ecosystem structure, nutrient content of refuse dumps of leaf-cutting function, and composition in rangelands are ants through vegetation impoverishment, Journal negatively affected by livestock grazing. Ecological of Applied Ecology 44:1209–1218. Applications 26: 1273–1283. Read, J.L., and A.N. Andersen. 2000. The value of ants as early warning bioindicators: responses to Hemstrom, M.A., M.J. Wisdom, W.J. Hann, M.M. Rowland, B.C. Wales, and R.A. Gravenmier. 2002. pulsed cattle grazing at an Australian arid zone Sagebrush-steppe vegetation dynamics and locality, Journal of Arid Environments 45:231–251. restoration potential in interior Columbia Basin, Rogers, L.E., R.J. Lavigne, and J.L. Miller. 1972. USA. Society for Conservation Biology 16:1243– Bioenergetics of the western harvest ant in the 1255. shortgrass plains ecosystems, Environmental Entomology 3:994–997. Heske, E.J., and M. Campbell. 1991. Effects of an 11- year livestock exclosure on rodent and ant Usnick, S.J and R.H Hart. 2002. Western harvester numbers in the Chihuahuan Desert, southeastern ants’ foraging success and nest densities in Arizona. Southwestern Naturalist 36:89–93. relation to grazing intensity, Great Plains Research Hobbs, R.J. 1985. Harvester ant foraging and plant 12:261–273. species distribution in annual grassland. Oecologia Wells, T.C.E., J. Sheail, D.F. Ball, and L.K. Ward. 1976. 67:519–523. Ecological studies on the Porton Ranges: relationships between vegetation, soils and land Hoffman, B.D. 2010. Using ants for rangeland use history. Journal of Ecology 64:589–626. monitoring: Global patterns in the responses of ant communities to grazing. Ecological Indicators 10:105–111.

CEC Research | https://doi.org/10.21973/N36W9J Summer 2019 8/8