An exercise in complexity

Abstract

Plants play a major role in the construction and maintenance of ecosystems, supporting green food webs through herbivory and brown food webs through detrital inputs, affecting abiotic habitat variables, and influencing energy flow through these systems by changing the structural context in which trophic transfers take place. Alterations in these communities, such as those following plant invasion, may therefore affect consumers within impacted habitats. However, the multifaceted roles of plants makes the nature and strength of these effects difficult to predict. This dissertation explores detritally and structurally-mediated effects through which invasion by the Asian grass Microstegium vimineum influences forest floor consumers through investigating belowground C dynamics, abiotic parameters, energy flow, and the growth and survival of 3200 American toads, 800 southern leopard frogs, and 200 eastern newts across eight independent invasion fronts. The mechanisms behind observed effects were further explored through manipulations of the presence of lycosid spider predators and short-term foraging and survival trials within paired invaded and uninvaded enclosures. Belowground, this plant was found to influence abiotic habitat parameters and carbon dynamics, potentially contributing to observed effects on the production of microarthropods. However, it also serves as an important basal resource, making it likely that bottom-up influences of this plant are a consequence of changes in detrital communites in invaded habitats rather than resource sequestration of nonindigenous plant tissues. Investigation of the mechanisms through which stilt grass influences the American toad demonstrated that structural changes initiated a contextually-mediated dampening of intraguild predation rates among lycosid spiders, increasing their survival, and thereby augmenting predation pressure on metamorphic toads. Interspecific comparisons of the effects of invasion on three amphibians utilizing disparate metamorphic strategies revealed that the nature of the influence of invasion may be predicted by the metamorphic strategy of the species in question, as small, actively foraging toads are susceptible to top-down pressures while larger, sit-and-wait leopard frogs are sensitive to bottom-up effects and cryptic, toxic eastern newts did not respond to trophic effects. This research demonstrates that invasion indirectly affects consumers through alterations in detrital inputs and habitat structure, although the nature and strength of these effects varies interspecifically.