Experimental evidence that the importance of stochasticity and an environmental filter varies among three sites/latitudes across North America
The variation in community composition in space and time can result from a variety of mechanisms, including both determinisitic (e.g. environmental filtering and species interactions) and stochastic processes (e.g. colonization and extinction). In natural communities these processes occur simultaneously, making it difficult to identify the relative importance of any single process on its own. Using experimental mesocosms, we ask whether the relative importance of stochastic colonization and extinction versus environmental filtering varies across three sites/latitudes in North America (Florida, Missouri, and Alberta, CA). To isolate these processes, we experimentally manipulated the degree to which colonization (and subsequent extinction) was stochastic by initially introducing either a small or large number of aquatic organisms to the mesocosms at all sites. To assess the importance of an environmental filter (drought) at each site, we slowly drained the mesocosms after a year of assembly (post initial introductions) and then refilled them a month later. All mesocosms were sampled each year after initial assembly for two years. Using these data, we simulated communities assuming purely stochastic assembly and calculated the expected dissimilarity values within and between treatments. These expected values were then compared to our observed values and used to calculate an effect size for each treatment at each site. The effect sizes were then compared across all sites to assess 1) if the effect of drought and stochasticity varied among sites, and 2) if the relative importance of those processes varied across the three sites/latitudes.
For both the stochastic and drought treatments, all sites showed significant treatment effects indicating that the mesocosms within each treatment were more similar in composition than expected assuming random assembly. Additionally, the most northern site had a significantly greater effect of the stochastic treatment than the other two; the most southern site significantly differed for the drought treatment. These results are consistent with the ideas that environmental filtering and that priority effects are strongest at higher latitudes, and that the relative importance of environmental filtering increases with latitude.