Allee pits in metapopulations: critical dispersal rates for connectivity to be beneficial

Abstract Habitat fragmentation divides populations into smaller subpopulations. At the same time, the Allee effect reduces the growth and thereby the viability of small populations. Hence, habitat fragmentation and the Allee effect can synergistically amplify negative impacts on spatially distributed populations. To support endangered populations, management and conservation strategies aim to improve connectivity between subpopulations by creating corridors and stepping stones, for instance. This study investigates how enhanced connectivity (strength of connections between subpopulations in terms of dispersal rate) influences a fragmented population subject to the Allee effect. Using a generic two-patch discrete-time model with a positively density-dependent growth function, we study the impact of connectivity on the asymptotic total population size through simulations. Due to the Allee effect, low connectivity can lead to a decline in the asymptotic total population size, which we call the Allee pit. However, increased connectivity facilitates the rescue effect, wherein a persistent subpopulation in one patch can save an extinction-prone subpopulation in another patch. We find that for connectivity to benefit the asymptotic total population size, dispersal must be sufficiently large to push the smaller subpopulation above its Allee threshold. If dispersal is below this critical dispersal rate, there remains a detrimental effect on the asymptotic total population size. Therefore, this study implies that conservation strategies should not only aim to increase connectivity in fragmented populations subject to Allee effects but also ensure that the critical dispersal rate is surpassed.