Responses of salamanders to earthworms influenced by recent and historical co-occurrence

Background/Question/Methods   Although invasive species often negatively impact native species, invasives also can have positive effects on some native organisms through facilitation. Facilitation is most likely to occur when an invasive species provides habitat for a native species or becomes an ecological substitute for a similar native species. Invasive earthworms can positively impact Plethodon cinereus, a woodland salamander native to eastern North America, through both habitat provisioning and as prey. At one time, earthworms occurred throughout North America, but were extirpated from northern North America during glaciations ending 10-14,000 BP. Plethodon cinereus quickly re-colonized these areas while earthworms did not. Recently, earthworms of European origin have invaded North America. The goal of this study was to determine if the behaviors of salamanders to an invasive earthworm are influenced by historical co-occurrence with native earthworms. I compared responses of P. cinereus from several Michigan and Virginian populations to either a native (Eisenoides carolinensis) or invasive (Lumbricus terrestris) earthworm in burrow use and feeding experiments. Salamanders from Michigan were separated into two groups: having no exposure to earthworms, or having recent (last 10-20 years) but no evolutionary history with earthworms. Salamanders from Virginia have both a recent and evolutionary history with earthworms.

Results/Conclusions   For both burrow use and feeding trials, I found no differences in response to earthworm species per se. However, I found that salamanders from populations that had co-occurred with earthworms historically (e.g., Virginia salamanders) began to use earthworm burrows more quickly and spent more time in earthworm burrows than salamanders from Michigan populations where earthworms had invaded within the last 10-20 years. In turn, salamanders from these Michigan populations with recent exposure to earthworms began to use earthworm burrows more quickly and spent more time in earthworm burrows than salamanders from populations that had never been exposed to earthworms. In contrast, differences in feeding behaviors (e.g., handling times and latencies to attack) depended only on historical co-occurrence, with Virginia salamander populations exhibiting shorter handling times and latencies to attack compared with Michigan populations with a recent history with earthworms. Different selective pressures on these behavioral traits may explain these differences and have implications for behavioral responses in other organisms to invasive species.