Reinvasion dynamics of northern pocket gopher ( Thomomys talpoides) populations in removal areas

Abstract

This study was designed to test the hypothesis that continuous removal of northern pocket gophers ( Thomomys talpoides) from natural habitats and tree fruit orchards would result in successful population reduction. A secondary objective was a detailed analysis of demographic responses (reinvasion dynamics) of gopher populations in control and removal sites. Pocket gopher populations were intensively live-trapped in replicate control and removal sites in natural and orchard habitats at Summerland and Vernon, British Columbia, Canada. New pocket gophers readily colonized removal sites in the natural habitats, particularly during spring, summer, and fall months. Fewer gophers colonized vacant sites during winter months (⩽1 per ha at Summerland and ⩽9 per ha at Vernon). Mean abundance of gophers ranged from 15 to 31 per ha on control sites. Population increases due to recruitment on control sites were reflected in periods of high reinvasion of removal sites. Spring reproduction and subsequent dispersal of juvenile gophers resulted in high colonization of vacant habitat. The recovery ratio (colonization rate) of gophers in natural habitats averaged 26.9% in summer and 0.9% in winter at Summerland, and averaged 44.2% in summer and 40.4% in winter at Vernon. Recovery ratios to previous densities on removal sites averaged >100% in summer at Summerland and in both seasons at Vernon. A relative recruitment index also followed this pattern at both study areas. New adult gophers that reinvaded removal sites produced young there despite continuous removal of animals. Mean body mass of gophers was significantly higher in control than removal populations at Summerland, but not Vernon. This study is the first intensive live-trapping evaluation of reinvasion dynamics of northern pocket gophers in removal sites. It seems likely that colonization of sites treated with toxicants or kill-trapping will be at least as high as the reinvasion rates recorded in our study. The timing of removals is crucial to achieving an overwinter reduction in gopher abundance.