Sex and social hierarchies affect populations across landscapes

Ecologists have long been interested in the factors that drive the species composition of ecological communities. I propose that variation within species, particularly in dispersal behavior, plays an underappreciated role, as it affects population distributions across landscapes. To study this in the field, I censused dragonflies in a north Texas metacommunity, differentiating between adult males, adult females, and larvae. I found that while adult males were not well-explained by any environmental or spatial variables, adult females and larvae were significantly explained by their environment. Therefore, considering variation within species (in this case, sex and life stage) can clarify our understanding of species distributions. This is one example of sex-biased dispersal, which is universal amongst animals. However, research has mostly been focused on its potential benefits, while its costs remain largely unexplored. I developed an individual-based simulation model, to see how populations with varying degrees of sex-biased dispersal were able to persist in the face of frequent disturbances. I found that increasing sex-biased dispersal made it difficult for individuals to find mating partners in suitable habitat, and so these populations did not persist very long, in comparison to populations with equal dispersal. These heavy costs may be alleviated by habitat selection or alternative mating systems, however these costs should remain in the discussion of understanding sex-biased dispersal. Even within males, there may be significant variation in social status and physical condition, which can then affect dispersal strategies; these differences could then produce unique signatures in the distributions of populations. I developed another individual-based simulation model, in which individuals were sensitive to crowds, to environment, to both, or to neither. In comparison to passive dispersal, I found that crowd-sensitive populations were disproportionately abundant in medium-quality patches. In contrast, environment- sensitive populations were scarce in these patches. This approach requires fine-scale environmental data, but may be easier to acquire than the fine-scale demographic data that would otherwise be required. Overall, considering variation between individuals, particularly in their dispersal behavior, can improve our understanding of species distributions.