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

Pocket gophers (Thomomys bottae) affect alfalfa (Medicago sativa L.) produc tion in Yolo County, California, as well as the distribution of special sta tus, rare species that either prey on gophers cruse their burrows as habita t. 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 report s, and the range from low to high density was much smaller in alfalfa field s. Gopher density was greater at the field edge, especially during the firs t 2 years of stand production. Preference for the edge decreased by the thi rd year of alfalfa production as gophers used the available space in the fi eld interior. A stepwise multiple regression model could explain 73% of the variation in the 134 estimates of gopher density. This variation was expla ined by years since sowing of the alfalfa (standardized slope coefficient, beta approximate to 0.52), annual frequency of flood irrigation (beta appro ximate to 0.43), habitat area as a percentage of the landscape within a 500 m buffer around the field (beta approximate to 0.31), season of the year ( beta approximate to 0.25), field size (beta approximate to 0.20) and percen tage of sand within the top soil layer (beta approximate to 0.16). This mod el can be used to predict the distribution of special status species that d epend on gophers, and can be used to guide conservation efforts by increasi ng the spatial extent of non-cultivated gopher habitat on suitable areas in tervening alfalfa fields. Non-cultivated gopher habitat is currently rare i n the valley portion of Yolo County. Gopher control failed to influence den sity to the magnitude sought by the alfalfa growers, and cessation of contr ol would benefit both production and conservation goals in some alfalfa gro wing regions. (C) 2001 Elsevier Science B.V. All rights reserved.