Assembly of a community typically results from dispersal from the regional species pool, filtered by interactions among species and the environment, as well as chance fluctuations in demography, environment, and order of arrival. Historical contingencies, such as priority effects, result from events that leave lasting effects on the local community (e.g. changes in colonization order that lead to differences in community composition at equilibrium). These differences have been defined as alternative stable states, and they can lead to differential resource-use and changes in interspecific interactions. The amphibian microbiome-chytridiomycosis (Bd) relationship presents an ideal study system for testing how priority effects may lead to changes in community function. Past studies have shown that protection from Bd is directly related to antifungal bacterial composition on host skin. The traits of these bacteria are well characterized, but it is unknown how changes in assembly may alter disease states. We tested two strongly and weakly antifungal bacterial species against Bd under simultaneous and ordered introductions. qPCR and spectrophotometric assays were used to track abundances over time and quantify Bd-inhibition. We hypothesized that high resource-use overlap between bacteria and Bd and the release of anti-Bd metabolites by bacteria would produce strong priority effects.
Results/Conclusions
We observed substantial priority effects irrespective of high antifungal ability. While strongly antifungal bacteria are favored for their Bd-inhibitory ability, they may be preempted by weakly antifungal species in multispecies communities. For example, in microcosms where Bacillus cereus (weakly antifungal) was introduced before Janthinobacterium lividum (strongly antifungal), B. cereus was 1.4-1.7 times as abundant at 144 h compared to simultaneous cultures, while J. lividum showed no clear change in abundance compared to introduction at 36 h. We infer this may be partially due to B. cereus’ rapid growth rate. We also observed an increase in Bd-inhibition when bacteria were introduced before vs. after Bd. However, microcosms containing two strongly antifungal bacterial species did not always produce significantly higher levels of Bd inhibition, suggesting that interactions between bacterial taxa are not always synergistic and may be more complex than previously thought. Our results provide the first empirical evidence that priority effects and composition influence bacterial assembly on amphibian skin. Changes in assembly may ultimately lead to altered states of disease protection, influencing how we conserve amphibians currently at risk of extinction due to chytridiomycosis.