In an ecological community, a priority effect results from a single event that leaves a lasting effect on the local community. Through the combined influence of interspecific interactions and arrival order, priority effects lead to changes in community composition, structure, and function. The amphibian microbiome-chytridiomycosis relationship presents an ideal study system for testing the theory of priority effects and how they lead to changes in community function. Scientists have found that amphibian protection from the pathogenic chytrid fungus, Bd, is directly related to native bacterial composition on host skin. High resource use overlap between bacteria and Bd and the release of anti-Bd metabolites into the environment by bacteria may produce strong priority effects. In this study, we employed a two-way factorial design involving three orders of bacterial species introductions: simultaneous vs. ordered (A before B or B before A), and two orders of Bd introduction: before vs. after bacteria. Two strongly and weakly antifungal bacterial species were used with E. coli DH5α as a control. All bacterial introductions were made in liquid culture and then inoculated onto agar plates either before or after Bd introduction. A photometric method was use to quantify Bd-inhibition and track colony abundances over time.
Our preliminary results suggest: 1) order of arrival is the main driver of community assembly, since, under a constant environment, ordered microbial additions produced communities wherein the first colonizer had abundances two to four times as high as in simultaneous addition communities and some secondary species never established, 2) changes in the assembly process affect fungal inhibition, since ordered microbial additions wherein strongly antifungal species established first produced higher levels of Bd-inhibition, and 3) there is a priority effect of Bd presence, as evidenced by inhibition halos produced by the bacterial community that were on average 1.7 times as wide when Bd arrived after the bacteria compared to before. Our results provide the first empirical evidence that both priority effects and composition influence the assembly of bacterial communities found on amphibian skin. These changes in community assembly lead to altered states of Bd protection. This research will have major impacts on how we understand and treat amphibians currently at risk of extinction due to chytridiomycosis and it provides further evidence for the influence of priority effects in community assembly theory.