Plant-soil feedback may interact with drivers of global change, including species invasions. Plant invaders often participate in positive or neutral net-pairwise feedback, and evidence suggests that plant-soil feedback may commonly drive invader dominance. However, plant-soil interactions can be highly context dependent in response to both abiotic and biotic variables. Here, I discuss the influence of aboveground food-web interactions amongst herbivorous insects and spiders on the outcome of plant-soil feedback.
Some invasive plants have structural traits that alter aboveground food web interactions, such as physical features that provide habitat for predators. Beyond changing the aboveground community, alterations to the food web can influence the strength and direction of plant-soil feedback. Such changes can occur through altered plant chemistry in response to changing herbivore density or behavior, or through altered flow of nutrients through the system. Here, I present spatially-explicit models that examine the long-term implications of altered food web structure for net pairwise plant-soil feedback between native plants and an invasive plant that supports high densities of predators.
Using the invasive plant garlic mustard (Alliaria petiolata) as a case study, I present the results of spatially explicit models that show how the presence of plant-associated predators can alter the outcome of plant-soil feedback. Garlic mustard forms a fruit structure that is frequently colonized by web-building spiders, resulting in a strong spatial association between the invasive plant and these predators. This interaction was explored in a series of cellular automaton models that included garlic mustard, native plant species, herbivorous insects, and predatory spiders. Model results suggest that by contributing nutrient subsidies (in the form of prey carcasses) to the microsite occupied by an invasive plant, a predator can generate positive plant-soil feedback that may lead to invader dominance. However, in cases where native plants gain a greater benefit from nutrient subsidies than invasive plants do, predators may ultimately drive a negative feedback loop that promotes diversity. The results of these models emphasize the importance of considering the biotic context of plant-soil feedback. In particular, results show that aboveground food-web interactions can alter the outcome of plant-soil feedback in a way that typical greenhouse experiments cannot detect.