Species diversity, invasibility, and alternative community states in sequentially assembled communities

Background/Question/Methods

The relationship between resident species diversity and community invasibility is generally negative in experimental studies, but takes various forms in observational studies of natural communities. We hypothesized that stochastic species colonization, which applies to natural communities but not experimental communities generally assembled through simultaneous species introduction, may lead to non-negative diversity-invasibility relationships via incurring priority effects. To test this hypothesis, we manipulated both resident species diversity and colonization history in sequentially assembled communities of bacterivorous protist species.

Results/Conclusions

Despite a significant effect of assembly history on invasibility, invasibility declined with diversity, largely driven by positive selection effects associated with the dominant influence of an invasion-resistant species. This result reflected the generally weak priority effects observed, including those involving the invasion-resistant species. Increasing species diversity, however, greatly strengthened priority effects, providing the first experimental support for the idea that larger species pools promote alternative community states. We conclude that elucidating mechanisms regulating the strength of priority effects may help understand variation in diversity-invasibility relationships among natural communities.