Grasshopper species composition shifts following a severe rangeland grasshopper outbreak

Abstract

Little is known about how grasshopper species abundances shift during and following severe outbreaks, as sampling efforts usually end when outbreaks subside. Grasshopper densities, species composition and vegetation have infrequently been sampled during and after a severe outbreak in the western U.S., which is needed to better understand the cause of outbreaks and population declines. In this study, grasshopper densities, species composition and vegetation were monitored at a northern mixed rangeland site from 1999 to 2003 where densities reached 130 per m 2 during a severe outbreak. Phoetaliotes nebrascensis (Acrididae: Melanoplinae) comprised 79% of the outbreak in 2000, but declined to 3% by 2003. The dramatic shifts in proportional and actual abundance of P. nebrascensis over a 5 year period illustrate that species dominance can change rapidly, even for a highly dominant outbreak species. The difficulty of fully understanding factors causing shifts in grasshopper populations is illustrated by population declines in all species observed in 2002 and 2003. The data can help predict the intensity and decline of outbreaks and points to the critical importance of long term simultaneous monitoring of grasshopper densities, species composition and vegetation for outbreak prediction. Key wordsPhoetaliotes, Melanoplinae, rangeland, Acrididae, prairie , Jonas et al. 2015. Even less is known about shifts in the relative abundance of species during and following outbreak periods, as sampling efforts either fail to monitor species composition or end when outbreaks or chemical control efforts subside (Onsager 2000). Little, if any, data exist where grasshopper densities, species composition and vegetation were sampled during and after a severe outbreak, which is needed to better understand the cause of outbreaks and population declines in the western U.S. (Branson et al. 2006, Branson andHaferkamp 2014). Given species differences in food preference and phenology, grasshopper species should differentially respond to abiotic and biotic conditions (Joern 2000, Onsager 2000, Branson et al. 2006. In this study, grasshopper densities and species composition were sampled at a northern mixed prairie site during and after a severe outbreak. Materials and methodsGrasshopper sampling occurred in a large livestock exclosure at the USDA, Agricultural Research Service, Fort Keogh Livestock and Range Research Lab located near Miles City, Montana, U.S., from 1999 through 2003. Cattle were the only mammal excluded from the site and insects were not controlled. The site consisted of mixed grass prairie, with Western Wheatgrass (Pascopyrum smithii) initially comprising over 90% of vegetation (Branson 2008, Branson andHaferkamp 2014). In the area of the study site, greater than 90% of plant production typically occurs by July 1 (Heitschmidt and Vermeire 2005), with annual precipitation highly variable but averaging ~34 cm (Heitschmidt and Vermeire 2006).Grasshopper density was estimated by counting grasshoppers flushing from within e...