Plants serve as autogenic ecosystem engineers that give structure to their communities and the basal resources upon which foods web depend. Given this, the influences of changes in plant communities are undoubtedly varied and far-reaching. Invasion by exotic plants provides an opportunity to study the influence of the addition of a single species to a community; since exotic invasives are typically high in resources and hyper–productive they have the potential to dramatically alter the food web dynamics and physical structure of their environments. Japanese stilt-grass (Microstegium vimineum) is one such plant; a widespread invader of forest floors, it forms dense monocultures that differ in structure and quality from natives, making invasion likely to initiate ecosystem-level effects. In order to investigate the influence of stilt-grass invasion on forest floor consumers we confined metamorphic American toad (Bufo americanus) populations on either side of eight independent invasion fronts for six weeks in 2007 and 2008. We also measured habitat parameters such as belowground soil C mineralization rates, C pool sizes, and hunting spider abundances, determined the foraging success and growth rates of toads in invaded and uninvaded habitats, and isotopically measured the contribution of stilt-grass to toad tissues.

Results/Conclusions

We found that toad survivorship was significantly lower in invaded habitats. Growth of surviving toads was not related to invasion status, and stable isotope analysis indicated stilt-grass derived C was represented in toad tissues in proportion to its environmental presence. Investigations into the mechanism behind the reductions in survivorship have revealed that invasion by this plant may influence these consumers through both bottom-up and top-down processes; microbial activity is greater and C pool size smaller in invaded habitats, which may be linked to reduced soil invertebrate abundance [toad prey], and increased structure may be increasing predatory spider abundances in invaded habitats, resulting in rapid mortality of recently metamorphosed toads.