The Effects of Livestock on California Ground Squirrels (Spermophilus beecheyii)

Abstract

Understanding the impacts of livestock grazing on wildlands is important for making appropriate ecosystem management decisions. Using livestock exclosures, we examined the effects of moderate cattle grazing on the abundance of California ground squirrels (Spermophilus beecheyii Richardson) and the spatial distribution of active burrows within their colonies in grassland and blue oak (Quercus douglasii Hook. & Arn.) savanna habitats in the coastal range of California over a 3-year period (1991)(1992)(1993)(1994). Overall, relative population densities of California ground squirrels declined significantly throughout the experiment, but did not differ between grazed and ungrazed colonies or between habitats. There was also no significant interaction between these 2 factors. The spatial distribution of burrows, as measured by the mean nearest neighbor distance of active entrances within a colony, did not differ significantly between grazed and ungrazed colonies or between habitats, nor was the interaction significant. Thus, low to moderate levels of cattle grazing did not appear to have a strong effect on the population dynamics of California ground squirrels, and grazing may be compatible with maintenance of ground squirrel populations. Based on multivariate analysis of variance of 1994 data, live plant cover, native plant cover, and standing biomass were lower where the number of burrows was higher on grazed colonies but were little affected on ungrazed colonies. Ground squirrels may increase the impact of livestock grazing and thus reduce the capacity of the land to support other activities. However, it is clear that the effects of livestock grazing are complex and that detailed studies of potential mechanisms by which grazing impacts California ground squirrel populations are necessary. ResumenEntender el impacto del apacentamiento del ganado en tierras silvestres es importante para tomar decisiones apropiadas sobre el manejo de los ecosistemas. Durante un periodo de tres añ os (1991-1994) y usando exclusiones de ganado, examinamos los efectos del apacentamiento moderado en la abundancia de ardillas terrestres de California (Spermophilus beecheyii) y la distribució n espacial de madrigueras activas dentro de sus colonias en há bitats de pastizal y ''Blue oak'' (Quercus douglasii) en la regió n costera de California, USA. En general, las densidades relativaas de la població n de ardillas declinó significativamente durante el experimento, pero no hubo diferencia entre colonias con apacentamiento pastoreo y sin apacentamiento, ni entre habitats. Tampoco hubo interacció n significativa entre estos dos factores. La distribució n espacial de madrigueras, medida por la distancia promedio del vecino más cercano de entradas activas dentro de una colonia, no difirio significativamente entre las colonias apacentadas y no apacentadas ni entre habitats, ni la interacció n fue significativa. Así, niveles de apacentamiento bajos a moderados no parecieron tener un efecto fuerte sobre las diná micas de la població n de a...