The impact of interacting processes on population dynamics.

For a wide range of applied (pest control, disease outbreak, harvesting strategies) and fundamental reasons (climate change) ecologists need to understand not only what determines the abundance of a species but also how abundance varies over time and why these patterns differ from one location to another. Considerable effort has been devoted to exploring this issue, especially in species with unstable dynamics but in only a few instances do we now have the insight into the major mechanisms. These studies have tended to focus attention on the role of single biotic factors, such as predation or parasitism, and some studies have also considered the interaction between a biotic process and climate. Yet we know that populations are governed by a variety of biotic processes and that these commonly interact. So, a major challenge, and the overall aim of this proposal, is to quantify the impact of interactions between biotic factors in relation to their climatic context on population dynamics. We will work on the red grouse, a species in the British uplands whose populations commonly show cyclic dynamics. There is considerable variation in grouse population dynamics between areas, with some populations not cyclic, and the cycle period of the strongly cyclic populations varying from 4 to 12 years. This study system and the variation within it provides us with an excellent opportunity to quantify the impact of interactions between biotic processes. Grouse have been well studied, so we can identify and manipulate the two dominant biotic processes: parasitism and territorial behaviour. Moreover, we also know that territorial behaviour and parasite intensity affect each other. However, we do not know how abiotic factors affect these interactions and how the interactions in turn affect individual fitness and emergent dynamics. Our specific aim is to test the hypothesis that the spatial variation we observe in the cyclic dynamics of grouse is the result of parasites and territorial behaviour interacting within a gradient of rainfall. To achieve this aim we have built on a successful team of empiricists by collaborating with two theoreticians, skilled in dynamic game theory and population modelling. With this team we will test our hypothesis through experimentation, modelling and testing predictions on long-term time series. We will conduct two experiments that will tell us a) how parasite intensity varies with territorial behaviour and vice-versa, b) how these interactions influence breeding success and survival across an environmental gradient; and c) what the transmission rate is between male and female grouse within pairs. A quantification of transmission between sexes is necessary to combine previous models and data. Given that more aggressive grouse pickup more parasites, we will use game theory to explore the implications of this interaction for how much individual males should invest in territorial behaviour. With input from the experiments and game theory, the population dynamic consequences of the strategy decisions will be investigated through population models. For grouse populations in locations with given values of rainfall the population models will predict what cyclic patterns would be expected. We will the test these predictions against the long-term time series of harvest records available from managed grouse moorlands across the country. Our findings will highlight the role played by interactions between biotic factors on population dynamics. This is an important issue, as it will help us understand how climate change and management will interact to influence abundance and dynamics. In Britain, these studies are both timely with respect to our current knowledge and the dramatic ecological changes being observed in parts of the uplands.