Isolation and secondary contact of lineages are fundamental to the origins, maintenance and consequences of biodiversity. Secondary contact is well studied at long-established stable or quasi-stable hybrid zones, but the dynamics at the initiation of secondary contact are poorly studied. The outcome of secondary contact depends upon ecological and evolutionary dynamics that occur at the very initiation of secondary contact. Here we take advantage of whole-lake restoration efforts to study the ecological and evolutionary dynamics during the first five years after secondary contact between landlocked and anadromous forms of alewife (Alosa pseudoharengus). Early colonial dams isolated alewife populations in numerous coastal lakes 250-350 years ago. The resulting landlocked populations have diverged from ancestral anadromous alewife in morphology, physiology, diet and habitat use. In the spring of 2015, a fishway was opened that allowed anadromous alewife access to Rogers Lake, Connecticut, for the first time in nearly 350 years. We conducted this restoration project as a whole-ecosystem experiment to study the ecological and evolutionary outcomes of secondary contact from its onset. We collected high temporal resolution data on anadromous spawning success and hybridization with resident landlocked alewife; morphology, habitat use, and diet of anadromous, landlocked, and hybrid individuals; and data on whole ecosystem responses to secondary contact.
Results/Conclusions
We found significant interannual variation in the magnitude and direction of gene flow, caused by differences in overlap in spawning between the two alewife forms. Overlap in spawning was largely explained by spring water temperatures, which regulate the timing of initiation of spawning by landlocked alewife. Years with substantial spawning overlap produced high rates of hybridization. Years with little spawning overlap produced low rates of hybridization but high rates of backcrossing with the landlocked form. Just five years after the initiation of secondary contact, we have found F1, F2, and multiple generations of landlocked-backcross individuals. Anadromous alewife spawned in Rogers Lake have returned to spawn after two years in the ocean and some hybrid individuals have remained in Rogers Lake to spawn in their second summer. Five years after the initiation of secondary contact, we found differences in alewife habitat use, diet, and morphology among anadromous, hybrid, and landlocked individuals, but no changes in the zooplankton community or ecosystem function of Rogers Lake. Our results illuminate the early ecological and evolutionary dynamics that are likely to shape the long-term outcome of secondary contact.