Ground crab spiders (Thomisidae: Xysticus) more abundant in grasslands invaded by cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae).
Effectively managing invaded grasslands requires a comprehensive understanding of effects of the invader on key species in those ecosystems. Annual grasses, such as cheatgrass (Bromus tectorum L.) and medusahead (Taeniatherum caput-medusae), are significant invaders of grasslands in the western United States and their presence has been found to influence vegetation and vertebrate wildlife. However, little is known about how annual grass invasions affect invertebrates, including spiders, which provide significant ecosystem services. This study examined the response of an important grassland predator, ground crab spiders (Thomisidae: Xysticus), to annual grass invasion in semi-arid grasslands of eastern Oregon. Crab spiders are ambush predators, and Xysticus is a ground-dwelling genus that forages primarily in litter. Spiders were sampled using pitfall traps and cover of invasive annual grasses and litter was surveyed for three years (2014-2016). Results revealed that abundance of ground crab spiders was positively associated with increased cover of invasive annual grasses and litter. A positive relationship between invasive annual grass and litter cover suggests that cheatgrass and medusahead contribute significantly to litter, which may result in the higher observed Xysticus abundance. Increased litter may provide these spiders with more prey, refugia, and foraging habitat or modulate extreme temperature and moisture conditions in this arid grassland.