As both basal resources and autogenic ecosystem engineers, plants play an important role in structuring consumer communities. Plant invasion can alter important ecosystem properties such as habitat complexity, nutrient availability, and abiotic properties, and the hyper-productive, high-nutrient nature of many invaders may make the influence of these plants especially pronounced. While direct influences of these processes on competitors and predators may be relatively foreseeable, the consequences of indirect effects on consumers living within these invaded habitats are more difficult to predict. In order to determine the response of various forest floor consumers to invasion by the exotic grass Microstegium vimineum we stocked large enclosures spanning eight independent invasion fronts with metamorphic individuals of three amphibian species; Anaxyrus [Bufo] americanus (American toads), Lithobates [Rana] sphenocephalus (southern leopard frogs), and Notophthalmus viridescens (eastern newts). Using robust capture periods we tracked the survival and growth of these individuals. We also investigated the mechanisms behind these responses by tracking invertebrate abundance and abiotic factors on either side of each of these invasion fronts and performing additional manipulations, such as predator manipulations and analyses of foraging success, designed to investigate the effects of both bottom-up and top-down influences on these consumers.
Results/Conclusions
We found that invasion by M. vimineum could have significant effects on consumer performance (for example, the survival of metamorphic A. americanus was significantly lower in invaded areas) but that the nature and strength of these influences varied between species as well as ontogenetically, as L. sphenocephalus was not similarly affected and the influences on A. anaxyrus dissipated over time. It appears that the negative influences of invasion on metamorphic toad survival were largely driven by top-down processes associated with the accumulation of invertebrate predators in invaded areas, while abiotic and bottom-up influences were more important factors for individuals large or fast enough to escape capture by invertebrate predators. We conclude that the influences of plant invasion on consumer communities can be varied, complicated, and far-reaching.
