Grass invasion increases top-down pressure on an amphibian via structurally mediated effects on an intraguild predator.

Plants serve as both basal resources and ecosystem engineers, so plant invasion may exert trophic influences on consumers both via bottom-up processes and by altering the environmental context in which trophic interactions occur. To determine how these mechanisms affect a native predator we used a mark-recapture study in eight pairs of 58-m2 field enclosures to measure the influence of Japanese stilt grass invasion on 3200 recently metamorphosed American toads. Toad survivorship was lower in invaded habitats despite abiotic effects that favor amphibians. Prey densities were also lower in invaded habitats, but growth was unaffected. Frequent spider predation events in invaded habitats led us to use factorial field cage manipulations of stilt grass and lycosid spiders to determine if invasion increases predation rates. Spiders persisted at higher densities in the presence of stilt grass, and toad survival was lowest in cages with both grass and spiders. Invasion alone did not significantly reduce toad survival. Our results demonstrate that despite prey reductions and abiotic effects, it is increased spider persistence that reduces toad survival in invaded habitats. Invasion therefore affects resident forest floor consumers by modifying trophic interactions between native species, causing structurally mediated reductions in intraguild predation rates among spiders, with cascading implications for toad survival.