Evolutionary responses of communities to extinctions

Question: What are the evolutionary consequences of extinctions in ecological communities? Can evolution restore pre-extinction communities by replacing lost ecological strategies with similar ones, or will communities change in fundamental ways and never be the same again? Mathematical approach: We develop and explore a new framework based on evolutionary domains of attraction (EDAs), defined as sets of strategy combinations from which a particular ESS community can be attained through gradual evolution. The latter dynamics may include three types of evolutionary processes: continuous strategy adaptation in response to directional selection, evolutionary branching in response to disruptive selection, and evolutionarily driven extinction. Key assumptions: We consider gradual frequency-dependent evolution in ecological communities, with evolutionary dynamics being fully determined by the strategy composition of a community's resident species. Results: The EDA approach distinguishes ESS communities that gradual evolution can restore after extinctions from ESS communities for which this option does not exist or is constrained. The EDA approach also offers a natural definition of 'evolutionary keystone species' as species whose removal causes a community to shift from one EDA to another. Our study highlights that environmentally driven extinctions can readily cause such shifts. We explain why the evolutionary attainability of an ESS Community through gradual evolution from a single precursor species does not imply its evolutionary restorability after extinctions. This shows that evolution driven by frequency-dependent selection may lead to 'Humpty-Dumpty' effects and community closure on an evolutionary time scale. By establishing EDAs for several example food webs, we discover that evolutionarily driven extinctions may be crucially involved in the evolutionary restoration of ESS communities.