Disentangling the roles of mobility and propagule pressure in species responses to landscape structure

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Species extinctions are driven mainly by loss of natural habitats, especially in regions with high native biodiversity such as southern Canada. Connectivity conservation has been suggested as a means of slowing species losses in such regions. In a landscape with high connectivity the natural habitat is arranged such that organisms can safely move between remnant habitat patches. Connectivity conservation preserves elements of natural habitat (e.g. hedgerows) that link habitat patches. This emphasis on connectivity conservation arises largely from research suggesting that species with higher dispersal abilities are more likely to persist in human-modified landscapes. However, the link between connectivity conservation and dispersal ability is not straightforward, because dispersal ability is conceptualized in two ways: (i) mobility - the ability of individuals to move to distant sites -, and (ii) colonizing ability - the ability of a species to colonize distant sites. Colonizing ability depends on both mobility and the number of dispersing individuals, called propagule pressure. Propagule pressure, in turn, depends on population size and fecundity. Over 90% of studies of the role of dispersal ability in species responses to landscape structure (forest amount, road density etc.) measure dispersal ability in ways that combine mobility and propagule pressure. This is problematic because, if dispersal ability is more strongly driven by propagule pressure than by mobility then, rather than prioritizing connectivity conservation, we should focus efforts on increasing propagule pressure by increasing habitat amount and quality. This will increase population sizes and fecundity and therefore increase the number of dispersing individuals (propagule pressure). I propose to determine the role of mobility itself in species responses to landscape structure. We will conduct spatial simulations to predict situations when mobility or propagule pressure should drive species responses. We will conduct meta-analyses to determine the current state of knowledge, and we will conduct empirical tests of the model predictions, using our own and others' existing data on a wide range of taxa, and collecting additional data where needed. Overall, I predict that mobility plays a minor role in species responses to landscape structure, though I am prepared to be surprised by the results. But if I am correct, then conservation efforts should aim to maximize the total area and quality of habitat, irrespective of its presumed role in facilitating movement. In other words, governments and NGO's should take and encourage every opportunity to preserve, restore, and improve natural habitat, because even small or isolated parcels can increase population size and therefore increase propagule pressure.