Community Responses of Arthropods to a Range of Traditional and Manipulated Grazing in Shortgrass Steppe

COMMUNITY AND ECOSYSTEM ECOLOGY Community Responses of Arthropods to a Range of Traditional and Manipulated Grazing in Shortgrass Steppe T. A. SCOTT NEWBOLD,1,2,3 PAUL STAPP,1,4 KATHERINE E. LEVENSAILOR,4 5 1,6 JUSTIN D. DERNER, AND WILLIAM K. LAUENROTH Environ. Entomol. 43(3): 556Ð568 (2014); DOI: http://dx.doi.org/10.1603/EN12333 ABSTRACT Responses of plants to grazing are better understood, and more predictable, than those of consumers in North American grasslands. In 2003, we began a large-scale, replicated experiment that examined the effects of grazing on three important arthropod groupsÑbeetles, spiders, and grasshoppersÑin shortgrass steppe of north-central Colorado. We investigated whether modiÞcations of the intensity and seasonality of livestock grazing alter the structure and diversity of macroarthropod communities compared with traditional grazing practices. Treatments represented a gradient of grazing intensity by cattle and native herbivores: long-term grazing exclosures; moderate summer grazing (the traditional regime); intensive spring grazing; intensive summer grazing; and moderately summer-grazed pastures also inhabited by black-tailed prairie dogs (Cynomys ludovicianus Ord). Beetles and spiders were the most common groups captured, comprising 60% and 21%, respectively, of 4,378 total pitfall captures. Grasshopper counts were generally low, with 3,799 individuals observed and densities Ͻ4mϪ2. Two years after treatments were applied, vegetation structure differed among grazing treatments, responding not only to long-term grazing conditions, but also to the short-term, more-intensive grazing manipulations. In response, arthropods were, in general, relatively insensitive to these grazing-induced structural changes. However, species-level analyses of one group (Tenebri- onidae) revealed both positive and negative effects of grazing treatments on beetle richness and activity-density. Importantly, these responses to grazing were more pronounced in a year when springÐsummer rainfall was low, suggesting that both grazing and precipitationÑwhich together may create the greatest heterogeneity in vegetation structureÑare drivers of consumer responses in this system. KEY WORDS grasshopper, habitat structure, livestock grazing, spider, tenebrionid beetle Livestock grazing on grasslands and shrublands is a “healthy” conditions, whereas exclusion of grazers global phenomenon, with grazing impacts strongly leads to more degraded conditions, such as increased context dependent (Milchunas and Lauenroth 1993). susceptibility to invasion by exotic or weedy species For example, in systems where grazing is considered (Milchunas et al. 1992). The shortgrass steppe of an exogenous disturbance (Milchunas et al. 1998)Ñ North America is one system where plant communi- that is, where large herbivores are not an integral part ties are well adapted and resilient to grazing (Milc- of the systemÕs evolutionary historyÑgrazers and hunas et al. 1988, Milchunas and Lauenroth 1993). The their activities can exact a heavy ecological toll (Jones resilience of the plant community raises the question 2000). In contrast, in systems where plant communi- of whether consumer communities are similarly resil- ties coevolved with large herbivores, grazing is con- ient (Milchunas et al. 1998). Comparative studies of sidered an endogenous disturbance, and responses the effects of varying intensities of livestock grazing on can be quite different (Mack and Thompson 1982). shortgrass steppe animals suggest that some groups, Grazing in these latter systems is needed to maintain namely birds and macroarthropods, are more sensitive to grazing than others (reviewed in Milchunas et al. 1 Shortgrass Steppe LTER, Colorado State University, Fort Collins, 1998). It is increasingly recognized, however, that CO 80523. modiÞcations of traditional grazing practices, i.e., of 2 Department of Life Sciences, Sheridan College, Sheridan, WY the intensity, patchiness, and seasonality of grazing, 82801. can be used to increase spatial and temporal hetero- 3 Corresponding author, e-mail: [email protected]. 4 Department of Biological Science, California State University, geneity in vegetation at the landscape scale, and im- Fullerton, CA 92834. prove habitat quality for species of conservation con- 5 U.S. Department of Agriculture-Agricultural Research Service, cern in some grasslands (Fuhlendorf and Engle 2001, Rangeland Resources Research Unit, 8408 Hildreth Rd., Cheyenne, Fuhlendorf et al. 2006, Derner et al. 2009, Toombs et WY 82009. 6 Department of Botany, University of Wyoming, Laramie, WY al. 2010). To assess their utility for meeting conserva- 82071. tion goals, studies are needed that demonstrate the June 2014 NEWBOLD ET AL.: ARTHROPOD RESPONSES TO MANIPULATED GRAZING 557 effects of these nontraditional grazing practices on respond negatively, though responses to grazing can native consumer communities. be variable and species-speciÞc (Gibson et al. 1992). In shortgrass steppe, arthropods represent a critical Similarly, grasshoppers (Capinera and Sechrist 1982, component of the consumer community (Lauenroth Welch et al. 1991) and beetles (Lavigne et al. 1972) and Milchunas 1991). They comprise an important also appear to be sensitive to grazing-induced changes resource base for small vertebrates (Flake 1973, in habitat structure, and are predicted to decline with Lauenroth and Milchunas 1991, Stapp 1996), and play simpliÞed vegetation and increased bare ground (e.g., important roles in community-level dynamics and for tenebrionids, McIntyre 1997, Stapp 1997). In con- ecosystem functioning (reviewed in Crist 2008). Al- trast, we assumed that an increase in the availability of though effects of livestock grazing on arthropods tend cattle dung, an important food resource for dung bee- to be taxon-speciÞc (e.g., Di Giulio et al. 2001, Swengel tles (Scarabaeidae), would promote increased abun- 2001, Cagnolo et al. 2002, DeBano 2006, Batary et al. dance of scarab beetles on the most heavily grazed 2007, Joern and Laws 2013), in shortgrass steppe many treatments (e.g., Verdu et al. 2007). Those most heav- groups decline with increased grazing intensity (Lavi- ily grazed plots may also have positive effects on ar- gne et al. 1972, Capinera and Sechrist 1982, Lauenroth thropods, in general, owing to associations with the and Milchunas 1991, and references therein, Welch et unique microhabitats generated by prairie dog bur- al. 1991). While we might expect these consumer rows (Davidson and Lightfoot 2007); overall, how- groups to show similar sensitivity to novel, nontradi- ever, we anticipated arthropod declines with in- tional practices such as heavier grazing intensity than creased grazing intensity. Our study incorporates two previously experienced, the response of arthropods to levels of taxonomic resolution, family and species such conditions has not yet been evaluated. level, and includes an assessment of how plant com- Here, we used grazing treatments that expanded on munity composition may differ among grazing treat- the natural range of grazing conditions on shortgrass ments in the Þnal year of the study (2006), both of steppe to ask how vegetation structure and arthropods which may help further explain any observed re- respond to grazing. SpeciÞcally, we assessed how ma- sponses among these arthropod groups to grazing. jor macroarthropod groupsÑcarnivores (ground spiders), omnivores (beetles), and herbivores (grasshop- Materials and Methods pers)Ñrespond to changes in the seasonality (spring vs. summer) and intensity (i.e., twice the stocking Study Area and Sampling Design. The study was rate) of grazing relative to moderate summer grazing conducted at the United States Department of Agri- by cattle that represents the traditional grazing re- culture (USDA)-Agricultural Research Service Cen- gime in shortgrass steppe (Bement 1969, Hart and tral Plains Experimental Range (CPER) in north-cen- Ashby 1998). Because arthropod abundance and dis- tral Colorado, located Ϸ60 km northeast of Fort tribution are strongly inßuenced by changes in veg- Collins, CO. Vegetation is classiÞed as shortgrass etation structure (e.g., spiders, Hatley and MacMahon steppe, and is dominated by two perennial C4 grasses, 1980; ground-dwelling beetles, Mazia et al. 2006; insect Bouteloua gracilis ([Willd. ex Kunth] Lag. ex Grif- herbivores, Stinson and Brown 1983), we focused on Þths) and Bouteloua dactyloides (Nutt.) J. T. Colum- evaluating how grazing-induced simpliÞcation of veg- bus. Climate is semiarid, with long-term mean annual etation architecture affects abundance and richness of precipitation of 341 mm, with 70% received as rainfall these arthropod groups. The relatively simple struc- between April and September (Lauenroth and Milc- tural complexity and short stature of vegetation in hunas 1991). Annual precipitation during both sam- shortgrass steppe (Lauenroth and Milchunas 1991) pling years of the study was below the long-term mean suggests that grazing effects may be less dramatic than (2004: 293 mm; 2006: 301 mm). A drier spring and early in other more productive systems (e.g., tallgrass prai- summer (AprilÐJune) occurred in 2006 (54 mm) ver- rie, Sims and Singh 1978). Nevertheless, even small sus 2004 (114 mm). Sampling during the Þrst year of changes in important characteristics, such as vegeta- the study (2004) coincided with the end of a multiyear tion height (e.g., Noordijk et al. 2010), may affect the drought (2000Ð2004). arthropod communities. In 2003 and 2004,

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