The resilience of alternative community states driven by priority effects: a microcosm investigation

Abstract

Within an ecosystem, there are a variety of interactions between species which affect the overall community. One of the strongest influences of community structure within a habitat is the order in which species arrive and establish; resulting in populations either coexisting or excluding one another to extinction. This can either be invading species excluding residents (competitive exclusion) or residents excluding invaders (priority effects), often due to freely depleting any shared resources before invaders arrive. Priority effects are predicted to be weaker when the invasion occurs simultaneously with warming towards and above the thermal tolerance of one species as the pressure put on the species can be too much to allow a population to grow or establish to survive. This experiment investigated whether an 8°C temperature range altered protist ability to invade or be invaded in simple aquatic microcosms, where the order of invasion of Colpidium and Tetrahymena was varied. I measured the changes to population density of both species over time, to identify changes in maximum population density and time to extinction. Results showed very strong priority effects between the two species, but this was never affected by temperature. In all treatments, resident Tetrahymena could never be invaded by Colpidium. However, Tetrahymena can invade resident Colpidium and populations can coexist for weeks, but Colpidium always eventually exclude Tetrahymena to extinction. The only factors temperature affected were maximum population density and time to extinction in single species microcosms, with earlier extinction and lower maximum population densities at warmer temperatures. This study suggests that arrival of species into an environment is vital in determining the final habitat composition. Although temperature does not affect priority effects, it does alter the duration species may be able to survive and coexist, which could be fundamental in conservation work in a world with changing habitats and climates.