Although conventional wisdom states that non-native invasive species are detrimental to ecosystems, recent studies have shown that their impact is in fact extremely complex and includes both direct and indirect effects with either positive or negative implications for other species in the community. Invasive plants have direct impacts on the composition of insect communities through provision of resources, and on plant communities through competition. They also have indirect impacts on other plants, mediated by pollinators and consumers. Indirect effects mediated by pollinators are especially important because they directly relate to the seed set of the plant, a measure of fitness. However, competing hypotheses surround the magnitude of this indirect impact: an invasive species might be a “magnet” species and facilitate pollinator attraction to the patch, or it may compete for pollinators, drawing them away from other plants in the patch. The same conflicting hypotheses can be applied to the attraction of herbivorous insects. We conduct an experiment to quantify the positive and negative impacts of Carduus acanthoides, a non-native invasive thistle species, on the existing community. We describe how the insect communities differ between plots with and without thistles, using network theory. The comparison between these networks will provide support for either the magnet or competition hypothesis.
Preliminary results show that insects, mostly pollinators, from more than 16 families representing 4 orders are associated with the flowers of C. acanthoides. Other observed insects include herbivores, parasitoids, and predators of herbivores, indicating that a complex community of insects with multiple levels of trophic interactions associates with C. acanthoides. Thus, the thistle has the potential to dramatically affect the structure of the community of plants and insects in areas where it has invaded.