Spatiotemporal Impacts of Invasive Species Removal and Reinvasion

Abstract

Invasive species are a major driver of native species declines and reduce ecosystem function. Though eradication of invasive species is often beneficial, it can create other ecological issues and if not implemented correctly, can result in reinvasion. A model invasive species to test the effects of removal is the red-imported fire ant (Solenopsis invicta, hereafter RIFA), which depredates and competes with native species, though these impacts are debated. Eradication attempts of RIFA often result in reinvasion, though patterns of reinvasion, which give insight into the efficacy of removal, are not well understood. I hypothesized that RIFA would reinvade treated areas with higher densities due to elimination of competition from native species. I also hypothesized that RIFA removal would positively influence altricial Peromyscus species, due to increased survival of young in the nest, but would not impact semi-precocial cotton rats (Sigmodon hispidus), which are mobile more quickly after birth and thus at less risk of depredation by RIFA. To test our hypotheses, I applied a granular insecticide (Extinguish® Plus) on two 400-500 ha study sites, and left two 400-800 ha areas untreated. On all sites, I trapped ant species and small mammals. Extinguish® Plus effectively removed RIFA, but they reinvaded about 14 months after treatment with higher densities than on untreated areas, which indicates that treatment could have negative outcomes for native ant species. Results for small mammals were dependent upon species, with some results contrary to what I had expected, indicating that both the magnitude and mechanism of effects on small mammals require further investigation. In managing for invasive species, impacts on all species should be considered. Removal of long-established invasives involves many trade-offs, and if not eradicated, reinvasion of the species can result in worse ecological outcomes for native species.