CONSEQUENCES OF CATTLE INTRODUCTION IN A SHRUBSTEPPE ECOSYSTEM: INDIRECT EFFECTS ON DESERT HORNED LIZARDS (PHRYNOSOMA PLATYRHINOS)

摘要

Livestock grazing is one of the most common forms of land use in the western United States, yet scientists struggle to accurately predict grazing impacts. This study examined the initial response of desert horned lizards (Phrynosoma platyrhinos) to cattle introduction at a site in northwestern Utah. We sampled 21 grazed and 7 ungrazed study plots before grazing (2001), after grazing (2002), and 1 year after the cessation of grazing (2003). We were specifically interested in whether grazing influenced lizards through biotic or abiotic pathways. Lizard response (based on scat counts) to the initial impact of grazing (<6 months after cattle introduction) suggests that lizards abandoned areas protected from grazing (i.e., ungrazed exclosures) presumably in favor of grazed areas. Avoidance of ungrazed plots by lizards coincided with a decline in shrub and grass cover on grazed plots and with no significant change in relative abundance or richness of prey (ants) on grazed plots. In 2003, one year after cattle had been removed from the site, prey species richness declined on ungrazed plots, and percent bare ground increased on previously grazed plots; however; we detected no response of lizards to these changes. Overall, these results add to growing consensus that responses by small vertebrates to grazing are largely the result of changes to habitat structure(i.e., vegetation cover), rather than to changes in prey availability. Importantly, results from our relatively unique evaluation of initial grazing impacts corroborate results from more classic grazing studies conducted sometime after grazing has commenced and suggest that responses of small vertebrates to changes in habitat structure may be generalizable.