More harm than good: when invader vulnerability to predators enhances impact on native species

Invasion biologists typically regard susceptibility of an invasive species to native predators as a fortuitous condition that increases biotic resistance to the invasion. The line of reasoning is that predation weakens the net impact of the invader and reduces its ability to displace native competitors. However, predation on invasives is a coupled interaction; every invader consumed also enhances the predator population. If these predators also consume native species, then the invader's indirect effect via predators (i.e., apparent competition) could be more harmful to natives than the effect of resource competition from the exotic. We apply general community ecology theory to determine the conditions under which the net effect of predation on the exotic species is to extirpate the native competitor. An approximation to these conditions provides a simple metric to estimate the threat of native species extinction due to apparent competition posed by an invader. Key words: apparent competition; biocontrol agent; biological control; biological invasions; biological resistance; competitive exclusion; exotic species; exploitative competition; nonindigenous species.