Mass Effects

In the last two decades, community ecology has matured to consider biotic communities as a product of both local and regional processes. Therefore, local communities are currently thought to be connected by the dispersal of organisms, thereby forming a metacommunity. A metacommunity is organized by multiple processes, including environmental filtering, biotic interactions, dispersal, and ecological drift. Thus, spatial variations in local diversity (i.e., alpha diversity) and community composition (i.e., beta diversity) result from the relative roles of these major processes. In turn, these processes are mediated by organisms’ characteristics, environmental heterogeneity, and the connectivity between localities in a metacommunity. For a given environmental gradient, the role of environmental filtering is likely to be dependent on the dispersal rates shown by organisms. Unsuitable habitat patches (i.e., sinks), in terms of biotic and abiotic characteristics, may be occupied by different species due to high dispersal rates from suitable habitat patches (i.e., sources). Thus, mass effects occur when species are established at localities where their populations cannot be self-maintained. Even though it may be difficult to prove the action of mass effects per se, given the complex interactions between different mechanisms shaping biotic communities, there is some empirical evidence supporting their importance in nature. In addition, high dispersal rates that lead to mass effects may have important implications for biomonitoring and biodiversity conservation. This is because species occurring at sites beyond their niche requirements may provide false information about a site’s ecological quality or result in misleading plans to conserve species at sites where they cannot persist in the absence of continuous influx of propagules.