Hierarchical density dependence in large animal ecology and evolution

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A principal driver of the life histories of long-lived species like large mammals, from bears to horses to people, has been competition which tends to intensify as population size (or density) increases. To me, little about the ecology and evolution of these species can really make sense except in the light of density dependence. I have come to know density dependence as a hierarchical process of ecology. One of my predictions is that density-dependent limiting factors to a population-like competition, predation, parasitism, or even mutualism-at a given space (leaf to landscape) and time (minutes to millenia) will match with proxies of fitness (energetics to extinction). The conclusion is that the very definition of carrying capacity (K) for members of the same species is scale dependent. I believe this to be of great significance as each organism is an `observer' of the environment and life-history adaptations can alter perceptual scales. It also means that there is a role for researchers in how we interpret the causes and consequences of density-related dynamics and apply our results to conservation. While formalizing theory on hierarchical density dependence is one of my long-term goals, the short-term goals of my program relate to density-dependent processes emerging across several scales and levels of ecological organization for the free-ranging horses at Sable Island National Park Reserve, Nova Scotia. Launched in 2007, my long-term, individual-based study of the horses has recently come of age: with 1385 life histories tracked it is now one of the world's largest studies of its kind. Building on recent advances in the literature to ask interesting hypotheses but also take advantage of current ecological conditions on Sable Island, I address scale- and density-dependent process regarding: (1) behaviour, including benefits of band cohesion at high density; what determines band size; and density- and sex-ratio impacts on the horse mating system; (2) life history traits like extension of the interbirth interval in response to parasitism; (3) landscape genetics and density-dependence of inbreeding depression; (4) how and why K for horses might vary in space and time from sea-to-land nutrient transfers and a marine trophic cascade that has been strong enough to bounce out of the ocean and onto Sable Island; and (5) island biogeography and how body size evolution of vertebrates on islands might be determined from the balance of density-dependent competition and extinction processes. My short-term goals all have exceptional potential to train high-calibre students of a diversity of backgrounds, leading to ground-breaking advances which is remarkable as each highlighted objective targets a different level of ecological organization. Potential impact is thus both broad-based and deep. This speaks to the power of applying a hierarchical perspective of density dependence to ecology and evolution.