Since its inception as a subfield, invasion ecology has sought to address two fundamental questions: 1) what makes a species invasive? and 2) what makes a community invasible? A few particularly dominant hypotheses have surfaced to answer these questions. One dominant hypothesis, the ‘enemy escape hypothesis,’ posits that many species are invasive because they are released from natural enemies. A second dominant hypothesis suggests that we should see a negative relationship between diversity and invasibility because introduced species are more likely to face biotic resistance to invasion in a more diverse community. Each of these hypotheses has been subject to decades of rigorous theoretical and empirical research, and there are certainly cases in which they may explain invasion success. However, empirical support is mixed. In many cases enemy escape cannot explain invasion, and diversity cannot predict invasibility. Placing invasion in a more complete food web context by considering more than one or two trophic levels at a time may help to explain why these hypotheses often fail.
Results/Conclusions
First, I present theoretical and empirical work that demonstrates how the invasive plant garlic mustard (Alliaria petiolata) can undermine its own invasion success by altering the native food web to reduce the role of enemy escape as an invasion driver. By promoting web-building spiders, garlic mustard indirectly improves conditions for some native plant species. Then, I present a global synthesis that examines how food web complexity relates to biotic resistance to invasion. By considering invasion in the context of broader food-web interactions, we can identify reasons that common hypotheses, such as enemy escape, might fail, while also gaining new power to explain global patterns of species invasions.