Dispersal and depensation in low density culled mink populations

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Understanding the ecological dynamics of populations at range margins where populations may be patchy and at low density is fundamental for our understanding of how range expansion can ensue and how invasive species spread. Species of conservation concern are by definition found at lower than expected densities. Therefore understanding processes causing low density populations to perform poorly is fundamental to conservation and management. While there is a rich body of theory predicting its impact on species spread and persistence, empirical studies of depensation are scarce owing to the inherent difficulty in studying populations at very low density. Allee effects however have been detected in a range of invasive species. Strikingly however, even though depensatory processes define the early demographic spread of invasive species, it is only very recently that modelling studies have started considering depensation in the management of established invasive species. The aim of this application is to improve the link between theory and practice and, in doing so improve both ecological understanding of depensation and test its potential as an effective tool for invasive species management. Our program of research is integral to a cooperative conservation project that has already removed invasive American mink from 10,000km2. Given the conservation project area is surrounded by uncontrolled mink populations, it is exposed to re-invasion from its periphery in a process not dissimilar to the initial invasion and to the spread of species in a new range. This represents a unique opportunity to test predictions on patterns of immigration and demography over a large spatial scale in a natural setting normally inaccessible to investigation, making it possible, for the first time, to gain an empirical understanding of processes operating at low density. By combining genetic and age information from all mink caught as part of the conservation project, we can reconstruct genealogies, and subsequently measure the extent to which mink spread through the environment. We will do this for all mink caught since January 2010 and combine these data with information already obtained from 365 mink caught when the mink population was at saturation. Thus we will have information on dispersal patterns from area that were either saturated (pre-control), partially depleted (first year of raft deployment) or severely depleted of conspecifics (subsequently). We will establish when the spread of mink may be affected by the inability to locate mates at very low density by comparing patterns of dispersal and reproduction in low density mink populations immigrating from the periphery of the (expanding) project area with patterns from saturated populations. We will also consider whether the fact that un-mated females mink remain in heat and after some time may suffer from anemia contribute to heightened mortality of females. We will also determine whether the observation that dispersing mink tend to settle in productive areas could lead to emergent depensation described as "range pinning", whereby isolated colonies fail to spread because population loss due to emigration is not be compensated for by immigration. We will also use our data to test the effectiveness of management options that aim to maximise depensation. It has been suggested that a potential control strategy for some invasive species may be to suppress peaks in population abundance close to range borders. This could increase the likelihood of invasion pinning by reducing the flow of individuals from higher density populations behind the front. If effective, such management might be cost effective given the effort required to detect individuals at low density in vast controlled areas. Our project is unique in combining ecological theory with practical management of an invasive species in partnership with communities.