Shifts from pulled to pushed range expansions caused by reduction of landscape connectedness

Description

Range expansions are key processes shaping the distribution of species; their ecological and evolutionary dynamics have become especially relevant today, as human influence reshapes ecosystems worldwide. Many attempts to explain and predict these phenomena assume, explicitly or implicitly, so-called "pulled" expansion dynamics, in which the low-density edge populations provide most of the "fuel" for the species advance. Some expansions, however, exhibit very different dynamics, with high-density populations behind the front "pushing" the expansion forward. These two types of expansions are predicted to have different effects on factors such as genetic diversity and habitat quality sensitivity. However, studies are lacking due to the challenge of generating reliably pushed vs. pulled expansions in the laboratory, or discriminating them in the field. We here propose that manipulating the degree of structural connectivity (connectedness) among populations may prove a more generalizable way to create pushed expansions. We demonstrate this with both individual-based simulations as well as replicated experimental range expansions (using the parasitoid wasp Trichogramma brassicae as model). By analysing expansion velocities and neutral genetic diversity, we showed that reducing connectedness led to pushed dynamics. Our numerical and experimental results suggest that reducing connectedness can cause density-dependent spread (and thus pushed range expansions) both directly or by amplifying existing density-dependence. In the current context of habitat loss and fragmentation, we need to better account for this relationship between connectedness and expansion regimes to successfully predict the ecological and evolutionary consequences of range expansions.

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