The downward spiral: eco-evolutionary feedback loops lead to the emergence of ‘elastic’ ranges

Abstract

In times of severe environmental changes and resulting shifts in the geographical distribution of animal and plant species it is crucial to unravel the mechanisms responsible for the dynamics of species ranges. Without such a mechanistic understanding reliable projections of future species distributions are difficult to derive. Species ranges may be highly dynamic and subject to elastic behavior, i.e. a range contraction following a period of range expansion as a consequence of eco-evolutionary feedbacks due to (rapid) dispersal evolution. It has been proposed that this phenomenon occurs in habitat gradients, which are characterized by a negative cline in selection for dispersal from the range core towards the margin, as one may find with increasing patch isolation, for example. Using individual-based simulations we show in this study that the presence of Allee effects is a necessary condition for ranges to exhibit elastic behavior. A pronounced source/sink-structure at the range margin caused by Allee effects, leads to selection for decreased dispersal and subsequently to lowered colonization rates and increased local extinction risk. In addition, the nature of the gradient is crucial, as gradients which do not select for lower dispersal at the margin than in the core (patch size, growth rate, demographic stochasticity, extinction rate) did not lead to elastic range behavior.\n\nThus, we argue that range contractions are likely to occur after periods of expansion for species living in gradients of increasing patch isolation, which suffer from Allee effects.