The restructuring of arthropod trophic relationships in response to plant invasion
Date
2019
Authors
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Publisher
University of Delaware
Abstract
Arthropod populations are declining across the globe, and biological invasions are one of the contributing factors to arthropod biodiversity loss. The effects of invasive plant species have been well-documented among plant-feeding arthropods, but many questions remain. How non-native plants affect trophic structure and how we can best predict shifts in trophic structure have not been well-documented. The purpose of my research is to investigate how arthropod trophic structure is altered through changes in plant origin, as well as to provide empirical evidence to support predictions of shifts in arthropod trophic structure based on an evolutionary experience concept. For my first chapter, I proposed that non-native plants shift the direction of arthropod trophic structure from an herbivore-driven “green” food web to a “brown” food web, i.e., driven by detritivores. I predicted this “green-to-brown” shift to be the common mode of restructuring arthropod communities following plant invasion. To test my hypothesis, I developed a field study comparing arthropod communities between native plant communities and plant assemblages dominated by a suite of non-native plants in the Mid-Atlantic region. I observed sweeping declines in herbivore species richness and density in 7 of 8 non-native plant assemblages, increased detritivore densities in 5 of 8 non-native plant assemblages, and declines in both species richness and density for predators associated with the green food web for all non-native plants. Furthermore, I found 2-to-24-fold decreases in the ratio of herbivores to detritivores following plant invasion, offering substantial support for a “green-to-brown” shift in arthropod trophic structure. Many arthropods collected with non-native plants were either non-native themselves or shared home distributions with non-native plants, suggesting that, in addition to non-native plants promoting dominance of detritivores in their introduced range, non-native plants may also facilitate further invasions and promote community homogeneity. In my second chapter, I developed an empirical study to test my prediction for the evolutionary-experience concept from a non-native plant, native arthropod perspective. The framework posits that an invader’s success can be predicted based on its evolutionary familiarity with organisms in its introduced range as well as how familiar native species are to the invader (“high” for familiar, “low” for naïve). In a “high-low” scenario, I predicted that herbivorous arthropods will decrease in abundance due to a lack of coevolution between herbivores and hosts; in contrast, I predicted that detritivores would increase following an abundant food source (i.e., plant litter). I compared non-native plants without congeners of functionally similar native species in the landscape. In a “low-high” scenario, I compared non-native crop or forage plants with native refugia and I predicted a decrease in detritivores and an increase in herbivores due to an abundant and susceptible food source. Finally, in a “high-high” scenario, I expected no difference in arthropod abundance between non-native plants and their native congeners. I provide evidence to support the “high-low” scenario, as herbivores decreased while detritivores increased following plant invasion, consistent with my findings for a “green-to-brown” shift in arthropod trophic structure. However, I was unable to support my hypotheses for the “low-high” or “high-high” scenario with all non-native plants selected; instead, non-native plants either reduced or maintained the richness or abundance of arthropod functional groups where dominant. Arthropods provide vital ecosystem services, and the trophic restructuring of native arthropod communities following plant invasion can have negative consequences to energy flow, nutrient cycling, and the quality of habitat for organisms at higher trophic levels. I provide evidence for “green-to-brown” shifts in arthropod trophic structure as a common mode of restructuring in native arthropod communities across landscapes in the Mid-Atlantic and suggest that this trend may occur with plant invasions among other landscapes beyond the scope of my study. I recommend management strategies that limit successful invasions for non-native plants that demonstrate traits of a “green-to-brown” shift in arthropod trophic structure.