Shifts from pulled to pushed range expansions caused by reduction of landscape 1 connectedness 2

11 Range expansions are key processes shaping the distribution of species; their ecological and 12 evolutionary dynamics have become especially relevant today, as human influence reshapes 13 ecosystems worldwide. Many attempts to explain and predict these phenomena assume, explicitly or 14 implicitly, so-called “pulled” expansion dynamics, in which the low-density edge populations provide 15 most of the “fuel” for the species advance. Some expansions, however, exhibit very different 16 dynamics, with high-density populations behind the front “pushing” the expansion forward. These 17 two types of expansions are predicted to have different effects on factors such as genetic diversity 18 and habitat quality sensitivity. However, studies are lacking due to the challenge of generating 19 reliably pushed vs. pulled expansions in the laboratory, or discriminating them in the field. We here 20 propose that manipulating the degree of structural connectivity (connectedness) among populations 21 may prove a more generalizable way to create pushed expansions. We demonstrate this with both 22 individual-based simulations as well as replicated experimental range expansions (using the 23 parasitoid wasp Trichogramma brassicae as model). By analysing expansion velocities and neutral 24 genetic diversity, we showed that reducing connectedness led to pushed dynamics. Our numerical 25 and experimental results suggest that reducing connectedness can cause density-dependent spread 26 (and thus pushed range expansions) both directly or by amplifying existing density-dependence. In 27 the current context of habitat loss and fragmentation, we need to better account for this relationship 28 between connectedness and expansion regimes to successfully predict the ecological and 29 evolutionary consequences of range expansions. 30