Designing Invasion-Resistant Plant Communities: The Role of Plant Functional Traits

Abstract

Establishing and maintaining weed-resistant plant communities is a central goal of sustainable invasive plant management.1 Based on this need, much research has been directed toward understanding properties that confer plant community invasion resistance. Early work identified a positive relationship between resource availability and an invader’s ability to establish and spread in native plant communities.2 This work led researchers and managers to question how plant community composition influences resource availability and invasion resistance. Initial efforts focused on the impact of the number of species present (i.e., species richness) and/or the number of species present as well as their relative abundances (i.e., species diversity). Although some research found that increasing species richness or diversity decreased resource availability and increased invasion resistance, most evidence suggested species richness and diversity were relatively poor predictors of invasion resistance.3 One reason was the high degree of functional overlap between species. For example, researchers found that different perennial bunchgrass species tended to have similar patterns of resource capture, suggesting there was some functional redundancy in how these species influenced resource availability.4 Similar relationships were found between different shrub and perennial forbs species. As a result, researchers and managers began focusing on coarse functional groups (e.g., perennial grasses, forbs, shrubs) rather than on individual species. With this approach, the array of species in a plant community was condensed into only a handful of functional groups. Unfortunately, more than two decades of research indicates that neither species diversity nor functional group diversity adequately predict invasion resistance. In most cases it appears that a single species or functional group is the major driver of invasion resistance, with the species or functional group varying across different invasion scenarios.5,6 At the outset, this observation can be frustrating to managers, causing them to ask the reasonable question: “Which species or functional group should my management target?” To answer this question, we need to focus less on the identity of the species, initially, and more on the suite of functional traits shared by species that are able to successfully resist invasion. The burning question is, “Which functional traits are important?” Fortunately, there are some very timely answers to this question.