Comparing pocket gopher ( Thomomys bottae) density in alfalfa stands to assess management and conservation goals in northern California

Abstract

Pocket gophers ( Thomomys bottae) affect alfalfa ( Medicago sativa L.) production in Yolo County, California, as well as the distribution of special status, rare species that either prey on gophers or use their burrows as habitat. Farming practices, as well as attributes of the landscape and of alfalfa fields, were compared to 134 estimates of gopher density among 35 alfalfa stands scattered throughout the County during 1992–1994. Gophers in alfalfa fields averaged only one-fourth the average density among published reports, and the range from low to high density was much smaller in alfalfa fields. Gopher density was greater at the field edge, especially during the first 2 years of stand production. Preference for the edge decreased by the third year of alfalfa production as gophers used the available space in the field interior. A stepwise multiple regression model could explain 73% of the variation in the 134 estimates of gopher density. This variation was explained by years since sowing of the alfalfa (standardized slope coefficient, β≈0.52), annual frequency of flood irrigation ( β≈−0.43), habitat area as a percentage of the landscape within a 500 m buffer around the field ( β≈0.31), season of the year ( β≈0.25), field size ( β≈−0.20) and percentage of sand within the top soil layer ( β≈0.16). This model can be used to predict the distribution of special status species that depend on gophers, and can be used to guide conservation efforts by increasing the spatial extent of non-cultivated gopher habitat on suitable areas intervening alfalfa fields. Non-cultivated gopher habitat is currently rare in the valley portion of Yolo County. Gopher control failed to influence density to the magnitude sought by the alfalfa growers, and cessation of control would benefit both production and conservation goals in some alfalfa growing regions.