Loss of trophic regulation by rodents, and disturbance shift a native perennial plant community to an invaded annual plant community
A major goal in ecology is to identify mechanisms of invasion and to forecast vulnerabilities to invaders based on ecological conditions. Biotic resistance and disturbance are fundamental processes regulating plant invasion, but the role of consumers in early invasion dynamics and how disturbance modifies resistance mechanisms is unclear. Using experimental exclusion of small mammal consumers and burn treatments in a factorial design in five replicated blocks, we document synergistic effects of disturbance and reduction in small mammals in shifting a native, perennial plant community to an invaded, annual plant community in three years.
Small mammal consumers exerted strong resistance to the establishment and proliferation of two aggressive plant invaders: cheatgrass and halogeton. At the end of three years the combination of fire and small mammal reduction had increased cheatgrass density 3.7 and 6.5 fold relative to burned, small mammal access plots, and unburned plots. Fire and small mammal exclusion increased halogeton densities 26 and 25 fold relative to control plots in year 2. With the release of cheatgrass in small mammal exclusion plots in year 3, there was evidence of competition between cheatgrass and halogeton that reversed the invasion momentum of halogeton. Results provide evidence that fire may facilitate plant invasions indirectly through interference of consumer-mediated biotic resistance. While small mammals drastically reduced invasion in our study system, fluctuations in rodent populations due to reproduction cycles, disturbance and extreme climatic events that are intensifying with global change, may provide windows of opportunity for exotic plant species to escape biotic resistance and initiate invasions.