Environmental variability counteracts priority effects to facilitate species coexistence: evidence from nectar microbes

Background/Question/Methods

The order of species arrival during community assembly can affect species coexistence, but the strength of these effects, known as priority effects, is variable among ecosystems, organisms, and experiments. The causes of this variation remain unclear, but make it difficult to predict the end state of a community when use arrival order tends to be stochastic. Environmental variability also tends to be present in many systems, and we suggest that interactions between environmental variability and arrival order is one potential cause of variability between effects. Using a system of nectar-inhabiting microorganisms for which priority effects have known importance, we manipulated spatial and temporal variability in temperature, and examined the consequences for community assembly. We allowed communities in artificial flowers to assemble from a set of four microbes (2 bacteria, 2 yeasts), fully crossing arrival order treatments with temperature variability treatments. We then plated, identified, and ennumerated the abundance of each species in each community over a 32 day period.

Results/Conclusions

When species arrive simultaneously to the artificial flowers, temperature variability had no effect on the final diversity and identity of the community. In contrast, multiple species coexisted when temperature was variable, but not when it was constant, if species arrived sequentially. Temperature variability prevented extinction of late-arriving species that would have been excluded due to priority effects if temperature had been constant. We suggest that the effects of temperature variability on growth rates, in combination with differential tolerances for temperature variability among species, could alter the effectiveness of well-known priority effect mechanisms such as habitat modification and resource preemption. Our results suggest that understanding variable effects of arrival order on species coexistence requires consideration of how environmental variability alters the strength of priority effects. Further, they suggest that interactions between different mechanisms can make the outcome of experiments that focus on a single mechanism less general.