Partial migration to seasonally-unstable habitat facilitates biological invasions in a predator-dominated system

Although partial migration, a phenomenon in which some individuals in a population conduct seasonal migrations while others remain resident, is common among animals, its importance in facilitating biological invasions has not been demonstrated. To illustrate how partial migration might facilitate invasions in spatially complex habitats, we developed an individual-based model of common carp Cyprinus carpio in systems of lakes and winterkill-prone marshes in the Upper Mississippi River Basin (UMRB). Our model predicted that common carp are unable to become invasive in lakes of the UMRB unless they conduct partial migrations into winterkill-prone marshes in which recruitment rates are high in the absence of native predators that forage on carp eggs and larvae. Despite low dispersal rates of juveniles and higher mortality rates of migrants, partial migration was adaptive across a wide range of migration rates and winterkill frequencies. Partial migration rates as low as 10% and winterkill occurrence as infrequent as once in 20 years were sufficient to cause invasiveness because of carp's reproductive potential and longevity. Consistent with the results of our model, empirical data showed that lake connectivity to winterkill-prone marshes was an important driver of carp abundance within the study region. Our results demonstrate that biological invasions may be driven by a small, migratory contingent of a population that exploits more beneficial reproductive habitats.