Interacting effects of fire and animal disturbance on a prairie restoration
Bunchgrass prairies of western North America were historically maintained by natural disturbance regimes including fire and animals. These prairies are threatened by invasive annual grasses (IAGs) which create monocultures and degrade ecosystem function. Restoration efforts focus on controlling invasives and restoring the natural fire regime. However, interactions between invasive species and natural disturbance can complicate restoration.
Natural disturbance regimes of prairie remnants at Turnbull National Wildlife Refuge (TNWR) in Eastern Washington include burning and burrowing by northern pocket gophers (Thomomys talpoides). TNWR is invaded by cheatgrass (Bromus tectorum) and North Africa grass (Ventenata dubia). Restoration efforts have focused on increasing native species and removing IAGs. Although controlled burns appeared successful after one year, the interactions between fire, pocket gophers, and their longer-term effects on IAGs have not been studied.
We tested the hypothesis that in disturbed areas fire and pocket gophers would interactively increase IAGs and decrease native species. We sampled 80 1-m2 plots located in burned and unburned areas, half with and half without gopher disturbance. In plots we recorded plant species cover and subsampled IAG stem counts. We analyzed the effects of burning and gophers on IAGs and native species using mixed linear models.
Burning and pocket gopher disturbance interacted to increase cover of IAGs. Compared to undisturbed plots, total IAG cover was twice as great on plots with both forms of disturbance. Compared to undisturbed plots, V. dubia cover was two times greater in plots with only one form of disturbance and five times greater in plots with both. Total IAG stem counts were influenced by burning only; burned plots had nearly 50% more IAG stems than unburned plots. Unlike total IAG stem count, V. dubia stems were affected interactively. V. dubia stems were more numerous in plots with either disturbance, but they were three times higher in plots with both disturbances than in plots with neither. Gopher disturbance and burning did have an interactive effect on native species richness. Burning increased native species richness about 20%, but the presence of gopher activity reduced diversity.
These results support our hypothesis that burning and disturbance through gopher activity would interact to increase IAG abundance and decrease native species richness. Our results suggest that natural disturbances can interact to magnify each other’s effects on invasive species. Therefore we recommend careful planning in the use of controlled burns for restoration of prairies invaded by IAGs.