Understanding ecological complexity in a chemical stress context - a reflection on recolonization, recovery and adaptation of aquatic populations and communities.

Recovery, recolonization and adaptation in a chemical stress context are processes regenerating local populations and communities as well as the functions these communities perform. Recolonization, either by species previously present or by new species able to occupy the niches left empty, refers to a metacommunity process with stressed ecosystems benefitting from the dispersal of organisms from other areas. A potential consequence of recolonization is a limited capacity of local populations to adapt to potentially repeating events of chemical stress exposure when their niches have been effectively occupied by the new colonizers or by new genetic lineages of the taxa previously present. Recovery, instead, is an internal process occurring within stressed ecosystems. More specifically, the impact of a stressor on a community benefits less sensitive individuals of a local population as well as less sensitive taxa within a community. Finally, adaptation refers to phenotypic and, sometimes, also genetic changes at individual and population levels, allowing the permanence of individuals of previously existing taxa without necessarily changing the community taxonomic composition (i.e., not replacing sensitive species). As these processes are usually operating in parallel in nature, though at different degrees, it seems relevant to try to understand their relative importance for the regeneration of community structure and ecosystem functioning after chemical exposure. In the present critical perspective, we employed case studies supporting our understanding of the underlying processes with the hope to provide a theoretical framework to disentangle the relevance of the three processes for the regeneration of the biological community after chemical exposure. Finally, we provided some recommendations to experimentally compare their relative importance, so that the net effects of these processes can be used to parameterize risk assessment models and inform ecosystem management.