The relationship between the richness of a community and its risk of being invaded (invasibility) goes back to Darwin. He thought that freshwater environments would be more invasible than terrestrial environments because the lower diversity of freshwater results in less severe competition. Nearly one hundred years later in The Ecology of Invasions by Animals and Plants, Charles Elton suggested that oceanic islands were more easily invaded because they had low diversity. Elton assumed that areas with greater diversity have fewer niches available for colonization. Anytime an idea can be traced back to Darwin, it’s going to become popular and by the late 1990s and early 2000s, everyone was on the diversity-invasibility bandwagon. One problem with these studies is the so-called invasion paradox, summarized nicely by Fridley et al. (2007). In this case, experiments suggest that greater diversity of an ecosystem should reduce the risk of invasion, whereas observational studies of natural systems tend to show that the most diverse ecosystems are also the most heavily invaded. Large-scale, long-term manipulative studies at Cedar Creek in MN, USA and the BIODEPTH experiment throughout Europe have mainly supported a negative diversity-invasibility relationship. What most of these studies did agree on was that abiotic factors such as resource availability or resource heterogeneity, and competition for these resources, were the drivers of any diversity-invasibility relationship. This is a pretty common paradigm, particularly in plant ecology.
Diversity-invasibility studies came quickly on the heels of numerous diversity-productivity studies. Interestingly, few considered the importance of top-down regulation by natural enemies in these studies. A pair of recent field studies suggests an alternative to this paradigm. Maron et al. (2010) and Schnitzer et al. (2011) found that the productivity of species-rich plots was high, not because of complementarity in resource use, but because density-dependent soil pathogens are not as abundant and subsequently exert smaller effects on residents. Conversely, monocultures have lower productivity because the species is strongly regulated in a density-dependent manner by its pathogens. Removing pathogens via biocide application increased the productivity of monocultures to similar levels as more diverse plots. This isn’t a new idea, though. It’s essentially an application of the Janzen-Connell hypothesis.
The idea that density-dependent pathogens more strongly regulate species poor communities has also been incorporated into diversity-invasibility studies. Turnbull et al. (2010) created a model to test this. Using negative plant-soil feedbacks, they demonstrated that monocultures are more easily invaded than diverse plots. Plants growing in their own soil experience more negative interactions with pathogens. Species in monocultures only experience home soil but individuals in diverse communities are more likely to germinate in ‘away’ soil. Thus, plants in less diverse communities are weaker competitors and less capable of excluding potential invaders. Compounding this disadvantage of being on home soil, exotic species that may lose enemies upon introduction are at even more of an advantage relative to native species colonizing these plots. This study is a welcome counter-balance to the pervasiveness of resource competition as the suspected mechanism of invasion resistance in diverse communities. It suggests that top-down regulation by pathogens can have a larger effect on community dynamics than many had previously acknowledged.
Turnbull et al. took an established hypothesis that had been studied to death, rethought it, and found another plausible mechanism for diversity-invasibility relationships. Perhaps other established invasion hypotheses could benefit from a similar overhaul.