ECOLOGICAL AND EVOLUTIONARY ASYNCHRONY IN CARIBBEAN EXTINCTION

Evolution occurs within an ecological context. Ecological dynamics are profoundly nonlinear and exhibit threshold effects, especially under extreme conditions such as those caused by human exploitation and climate change. Thus time lags occur between environmental change and biotic responses in ecological time, and the question arises how such time lags may scale up into macroevolutionary time. The isolation of the Caribbean from the Pacific by the rise of the Panamanian Isthmus resulted in profound oceanographic changes, including a dramatic decline in productivity, that are believed to have precipitated widespread extinction. However, extinction in most groups lagged 1-2 Myr behind environmental change. We explored the reasons for this time lag in terms of the mechanistic basis for changes in life histories of cupuladriid bryozoans that are abundantly well preserved throughout the Caribbean Neogene. Through surveys and experiments, we established that clonal reproduction is more lucrative than sexual reproduction in eutrophic conditions whereas sexual reproduction is favored under oligotrophic conditions. Thus, sexual reproduction should have become more prevalent than cloning when Caribbean productivity declined, as was indeed the case. Species that switched from primarily clonal to sexual reproduction survived to the Recent, and all species that originated after the rise of the Isthmus were predominantly sexual. In contrast, species that for some reason failed to reduce their rate of clonal reproduction became extinct. As for other taxa, species extinction lagged 1-2 Myr behind environmental change, but species destined to become extinct became progressively rarer and were found in fewer collections than predominantly sexual species. This pattern of gradual, drawn out extinction is consistent with predictions of metapopulation theory and the concept of extinction debt when recolonization of local populations fails to compensate for local extinctions.