Beyond Competition: Incorporating Positive Interactions between Species to Predict Ecosystem Invasibility

One of the many unintended consequences of global commerce has been the translocation of countless plants and animals to new regions, continents, and oceans [1,2]. Such “exotic” species have colonized nearly every habitat on Earth, and modern ecosystems are now made up largely of species originating from geographically distinct regions [3–5]. Most exotic species have negligible or no negative effects, but a small handful have had substantial impacts on native species and ecosystem processes [3,6]. For example, the introduction of the Nile perch (Lates niloticus) into Lake Victoria has not only caused the extinction of two-thirds of the endemic fish fauna, but has changed the entire food web of the lake by reducing the grazing by phytoplanktivores [7,8]. Given the sizable ecological and economic costs of species invasions [9], understanding the environmental factors that regulate them has become a major goal for basic and applied ecologists. One major research theme is the investigation of the relationship between native species richness (the number of local native species) and the ability of exotic species to colonize and thrive in new habitats (termed community “invasibility”) [10,11]. A longstanding concept in ecology is that habitats with high levels of diversity are difficult to invade (the biotic resistance hypothesis—see Glossary) [11–15]. This is because, in theory, a more diverse assemblage of plants or animals can utilize resources more fully than a less diverse community, thus increasing the intensity of competition and making it harder for new species to become established. Predictions from this model are, however, based on the assumption that natural communities are largely structured by competitive interactions and that the effects of native species on invaders are predominantly negative. There is, however, growing evidence that facilitation (positive species interactions—see Glossary) plays an equally important role in shaping communities and ecosystems [16–20]. One species can facilitate another by ameliorating stressful abiotic conditions or by providing refuges from natural enemies such as predators. Nonetheless, positive species interactions are rarely incorporated into conceptual ecological theories that describe the complex dynamics of species invasions [19,21]. Facilitation has been included in invasion scenarios to describe the case of extant exotic species enhancing the colonization of new exotics (e.g., invasional meltdown [22]). Yet a large body of evidence from terrestrial and marine habitats indicates that native species also commonly facilitate exotic colonizers through a variety of mechanisms. For example, shading by the native shrub, Atriplex vesicaria, fosters the establishment of the exotic succulent, Orbea variegata, in South Australia [23], while native sessile invertebrates protect the introduced oyster, Crassostrea gigas, from predation on the rocky shores of Western Canada [24].