Population dynamics of small mammals in relation to vegetation management in orchard agroecosystems: compensatory responses in abundance and biomass

Abstract

This study was designed to test the hypothesis that habitat alteration with intensive herbicide (glyphosate) treatment would adversely affect the small mammal community in apple orchards. Multiple applications of herbicide were applied to the total orchard floor in two experimental orchards, and this same herbicide regime was applied in a 2-m-wide strip within tree rows only in a third orchard. Intensive sampling of montane vole ( Microtus montanus), deer mouse ( Peromyscus maniculatus), and northwestern chipmunk ( Eutamias amoenus) populations was conducted in replicate control and treatment blocks of apple orchards at Summerland, British Columbia, Canada from 1983 to 1986. Average abundance of voles declined by 53% to 73% on treatment compared to control blocks after the first herbicide application. Vole populations were consistently reduced in response to the herbicide treatment, with average abundance ranging from 2.8 to 28.0 times higher on control than treatment blocks. Voles declined to, or near, extirpation in all orchards during the winter of 1985–1986. Either deer mouse and northwestern chipmunk populations were significantly higher on treatment than control blocks or there was no difference in abundance after the start of the herbicide applications. The average abundance of deer mice ranged from 1.3 to 11.1 times higher, and that of chipmunks ranged from 1.8 to 13.3 times higher, on treatment than control blocks. The high numbers of deer mice and chipmunks on treatment blocks were composed mainly of resident animals. There were no significant differences in biomass of small mammals between control and treatment populations in summer and winter periods in two out of three orchards. There appears to be a ‘compensatory response’ in this small mammal community whereby deer mice and northwestern chipmunks have essentially ‘replaced’ the montane vole on treatment blocks in an orchard agroecosystem. The quality of the altered habitats seemed sufficiently high to support these populations at comparable or higher levels than on controls. The species diversity of plants and animals in orchard agroecosystems should be investigated to determine their role as part of a mosaic of natural and agrarian habitats.