Life history strategies predict responses of lacustrine fish communities to eutrophication.

The demographic traits of an organism are key components of its fitness and life history theory aims at identifying the environmental drivers underlying the evolution of life history strategies. For fishes, the equilibrium species, those investing into larval survival (large eggs, parental care) rather than into absolute fecundity, are hypothesized to have evolved in stable and predictable environments with high biotic pressure. Human induced nutrient enrichment in many lakes around the world makes them increasingly subjected to perturbations such as anoxia and toxic algal blooms. We hypothesized that eutrophication results in lakes becoming more unstable, unpredictable and less resource-limited, in turn less favorable to equilibrium species. Another hypothesis states that lacustrine environment stability increases with ecosystem size. This study presents the first attempt to compare the two hypotheses in a group of 26 lakes. We found that the population abundance of equilibrium species was negatively related to increasing eutrophication. Long-lived and highly fecund periodic species responded more positively to eutrophication than short lived opportunistic species, with no parental care. This result could be demonstrated by seasonality in primary productivity which favors periodic species, disconnection from the river which prevents good colonist (i.e., opportunistic) species to (re)-establish after perturbation events, and predation by periodic species on opportunistic species. In contrast, we found no support for the ecosystem size hypothesis. Overall, we showed that human driven eutrophication affected species according to their life history strategies, reinforcing the usefulness of life history theory as a framework for assessing fish community response to a large array of human perturbations. More generally, our study emphasizes the importance of species traits to assess, explain, and predict community responses to human and natural perturbations.