Plant invasion can alter important ecosystem properties such as habitat complexity, resource availability, and abiotic conditions, which can drive cascading effects on resident species. However, the multifaceted roles of plants can make the nature of these effects on consumers living within affected habitats difficult to predict, especially given the variation in life history strategies that exists even among related consumer species. In order to explore the interplay between life history strategy and susceptibility to invasion-driven trophic effects we monitored the performance of 2,600 recently metamorphosed individuals from three amphibian species in 58m2 pens spanning eight Japanese stilt-grass (Microstegium vimineum) invasion fronts. The selected amphibian species varied in both initial size and behavior as a consequence of their highly disparate metamorphic strategies, which are designed to optimize the transition between the aquatic and terrestrial environment. Certain trade-offs are inherent in these strategies; for example, species that metamorphose early and small have frequently minimized mortality in the first environment, but face increased risk in the second. To ensure that they reach reproductive size, their post-metamorphic behavior is often compensatory to the timing of this transition, potentially further affecting their susceptibility to invasion-driven changes in prey availability and predator densities.
Results/Conclusions
We found that the initial 6-week survival of American toads was lower in invaded habitats due to increased predation pressure from lycosid spiders (which had benefited from the structurally complex habitat provided by stilt-grass invasion), but that this effect diminished as they grew. Southern leopard frog survival was not affected by invasion status; however, growth rates were lower in invaded habitats, likely due to reduced prey availability post-invasion. Neither the high survival nor slow growth of eastern newt efts were influenced by invasion status. These results show species-specific responses to invasion. We hypothesize that species that metamorphose early and forage actively to support rapid post-metamorphic growth are more susceptible to increased predation pressure resulting from M. vimineum invasion, whereas species that metamorphose at a larger size and are cryptic, ambush foragers are more affected by altered prey availability. In contrast, species with flexible time horizons to reproduction may prioritize survival over growth by aestivating during adverse conditions, minimizing trophic effects. These results demonstrate that metamorphic strategy can influence the nature and strength of the responses of related species to invasion, and support theoretical models that predict the prioritization of either growth or survival as a function of the timing of metamorphosis.