Epigaeic spider response to sagebrush steppe restoration treatments

Abstract

Epigaeic (ground-active) spiders are dominant predators of arthropods and are important prey for vertebrates in sagebrush steppe systems. As part of the Sagebrush Steppe Treatment Evaluation Project (SageSTEP), the response of epigaeic spiders to sagebrush steppe restoration treatments was evaluated. Spiders were pitfall trapped pre-treatment and for up to 7 years post-treatment at 20 sites in the interior western United States. Spider species, functional groups, and communities were analyzed as regards site conditions and in response to treatments (prescribed fire, mechanical, and herbicide) designed to restore sagebrush steppe lands encroached by piñon–juniper woodlands (Pinus and Juniperus spp.) and invaded by cheatgrass (Bromus tectorum). A total of 10,149 epigaeic spiders were caught during the 8-year sampling period (2006–2014), comprising 18 families, 51 genera, and 109 species. Ten species are currently undescribed, including two species of jumping spiders (Salticidae), two species of funnel spiders (Agelenidae), and a species of trapdoor spider (Euctenizidae). Nearly 70% of adult catch comprised the 10 most commonly collected species; 50 species were caught fewer than five times. Spiders of the family Gnaphosidae (ground spiders) dominated the collection (65% of adults), followed by wolf spiders (Lycosidae; 12%), jumping spiders (8%), and crab spiders (Thomisidae; 4%). Fewer gnaphosids were caught in plots with higher tree cover, and this pattern was reversed by tree removal. There was no significant epigaeic spider response to the broadleaf herbicide tebuthiuron, nor to mowing or clear-cutting. Mastication caused a subtle increase in epigaeic spider richness at Utah juniper sites. Prescribed fire decreased abundance and richness of epigaeic spiders in the short term (1 year post-treatment), with catch at most sites converging to control levels by Year 2 post-treatment; application of the annual herbicide imazapic at treeless sites may have amplified this effect slightly in the short term. In general, gnaphosid catch and richness closely paralleled litter and bunchgrass cover. These patterns suggest that while gnaphosid spiders are not sagebrush-obligate species per se, these spiders prefer habitat conditions that provide more hiding cover, in the form of litter and vegetation, found more often in treeless sagebrush steppe plant communities and in woodland communities that have relatively lower tree cover.