This preprint has been reviewed and recommended by Peer Community In Ecology (https://dx.doi.org/10.24072/pci.ecology.100004). Finding general patterns in the expansion of natural populations is a major challenge in ecology and invasion biology. Classical spatio-temporal models predict that the carrying capacity (K) of the environment should have no influence on the speed (v) of an expanding population. We tested the generality of this statement with reactiondiffusion equations, stochastic individual-based models, and microcosms experiments with Trichogramma chilonis wasps. We investigated the dependence between K and v under different assumptions: null model (Fisher-KPP-like assumptions), strong Allee effects, and positive densitydependent dispersal. These approaches led to similar and complementary results. Strong Allee effects, positive density-dependent dispersal and demographic stochasticity in small populations lead to a positive dependence between K and v. A positive correlation between carrying capacity and propagation speed might be more frequent than previously expected, and be the rule when individuals at the edge of a population range are not able to fully drive the expansion.

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 range expansions 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 e.g. genetic diversity and habitat quality sensitivity. However, empirical studies are lacking due to the challenge of generating reliably pushed versus pulled expansions in the laboratory, or discriminating them in the field. We here propose that manipulating the degree of connectivity among populations may prove a more generalizable way to create pushed expansions. We demonstrate this with individual-based simulations as well as replicated experimental range expansions (using the parasitoid wasp Trichogramma brassicae as model). By analyzing expansion velocities and neutral genetic diversity, we showed that reducing connectivity led to pushed dynamics. Low connectivity alone, i.e. without density-dependent dispersal, can only lead to 'weakly pushed' expansions, where invasion speed conforms to pushed expectations, but the decline in genetic diversity does not. In empirical expansions however, low connectivity may in some cases also lead to adjustments to the dispersal-density function, recreating 'classical' pushed expansions. In the current context of habitat loss and fragmentation, we need to better account for this relationship between connectivity and expansion regimes to successfully predict the ecological and evolutionary consequences of range expansions.

While species ranges have always moved, the ecological and evolutionary dynamics of range expansions have become especially relevant today, as human influence reshapes ecosystems worldwide. As a consequence, there have been many attempts to explain and predict evolutionary and demographic dynamics observed during range expansions. However, many of these predictions are based, explicitly or implicitly, on a subset of possible range expansion types, so-called “pulled” dynamics, in which the low-density front populations provide most of the “fuel” for the advance. Some expansions may exhibit very different dynamics, with high-density populations behind the front “pushing” the expansion forward. Studying the ecological and evolutionary consequences of pushed vs. pulled dynamics remains challenging, due to difficulties in reliably generating or identifying pushed and pulled waves in experimental or natural settings. Manipulations of the within-habitat quality to create Allee effects have successfully created pushed waves, but may only be applicable in some contexts. We here propose that manipulating, and specifically reducing the degree of structural connectivity among habitats may prove a more generalizable way to create pushed waves, through density-dependent dispersal. We demonstrate this using both individual-based simulations as well as replicated experimental range expansions (with the parasitoid wasp Trichogramma brassicae as model). Analysing expansion velocities and neutral genetic diversity, we showed that restricting connectivity did lead to pushed dynamics. Interestingly, our results suggest that reducing connectivity led to density-dependent spread (and thus pushed waves) through two different mechanisms in simulated and experimental expansions. In the current context of habitat loss and fragmentation, we need to better account for this relationship between connectedness and expansion regimes to be able to successfully predict the ecological and especially evolutionary consequences of range expansions.