Young species are highly prone to extinction via increased gene flow after human-caused environmental changes. This mechanism of biodiversity loss, often termed reverse speciation or introgressive extinction, is of exceptional interest because the parent species are typically highly differentiated ecologically. Reverse speciation events are potentially powerful case studies for the role of evolution in driving ecological changes, as the phenotypic shifts associated with introgressive extinction can be large and they occur over particularly short timescales. Furthermore, reverse speciation can lead to novel phenotypes, which may in turn produce novel ecological effects. Here we investigate the ecological shift associated with reverse speciation in threespine stickleback fish using a field study and a replicated experiment. We find that an instance of introgressive extinction had cascading ecological consequences that altered the abundance of both aquatic prey and the pupating aquatic insects that emerged into the terrestrial ecosystem. The community and ecosystem impacts of reverse speciation were novel, and yet they were also predictable based on ecological and morphological considerations. The study suggests that knowledge about the community ecology and changes in functional morphology of a dominant species may lead to some predictive power for the ecological effects of evolutionary change. Moreover, the rapid nature and resultant ecological impacts associated with reverse speciation demonstrates the interplay between biodiversity, evolutionary change, and ecosystem function.
A tremendous body of research has demonstrated that when species are lost the function of ecosystems is altered. All of these studies model the process of extinction by demographic decline, where species decline until they are extirpated. The ecological effects of extinction by reverse speciation, where previously separate species interbreed leading to a single hybrid population, were largely unknown. Using a case of reverse speciation in threespine stickleback in a replicated experiment and field study we demonstrated that this type of extinction can have profound effects on aquatic ecosystems-altering the benthic invertebrate community, the breakdown of organic matter, and the emergence of insects from the aquatic environment into the terrestrial. These ecological effects were largely predictable based on the morphological evolution that had resulted from reverse speciation. As such, our study provides a guideline for predicting how reverse speciation may effect ecology in many ecosystems.