Population dynamics of resource limited predators: Individuals differences and condition dependent dispersal

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Local populations are composed of individuals that have been subject to varying environmental conditions influencing their life history decisions, subsequent dispersal patterns and reproductive performance. These decisions generate heterogeneity in individual quality within and across annual cohorts as well as intergenerational effects, such as maternal effects. To date, consideration of these sources of heterogeneity in predictive models of population dynamics have been restricted to reproductive traits, and been based mostly on populations inhabiting enclosed systems, mainly islands, with no or limited dispersal. Yet, not only is dispersal an influential demographic trait but it is also increasingly recognised as key to understanding how populations respond to environmental change. In this project, we will investigate variation in individual quality and its role in population dynamics by simultaneously considering its influence on reproductive and dispersal traits. We will achieve this using a unique, large, long-term study of ringed Tawny owls specifically designed to detect dispersal at multiple scales and characterised by marked changes in the spatio-temporal dynamics of cyclic field vole populations, the main prey of tawny owls, and hence in the potential contribution of life history traits including dispersal to the overall dynamics of asynchronous predator populations. Of all the important demographic traits, estimating dispersal poses perhaps the biggest difficulties: obtaining unbiased estimates of its frequency at demographically relevant spatial scales is challenging. This project is made possible thanks to a uniquely extensive and very detailed data set of tawny owl Strix aluco, abundance and life history traits, together with detailed measures of prey abundance collected continuously by Petty and collaborators since 1980 over 600 km2 in Kielder Forest, in northern England. Reproduction of tawny owls has been measured annually since 1979 in a large area (175 km2) where most owls breed in nest-boxes. Regular nest-box checks provide data on territory occupancy, clutch size and egg volume, hatching order, fledgling success, as well as biometrics allowing the calculation of condition index. In addition, adult breeders were caught with an average 84% of the breeding females over the whole study period and 78% of breeding males over an 11-yr period. Overall, Capture-Mark-Recapture (CMR) data consist in more than 2000 individuals banded as chicks and more than 170 banded as adults, cumulating ca. 1000 recapture events between 1981 and 2005 in Kielder Forest alone, and more than 156 dead recoveries in the study area and beyond. Uniquely for the purpose of this study, the intensive Kielder Forest study area is supplemented by 3 satellite areas (Wark, Kershope and Redesdale forests) where a large fraction of chicks are also ringed. These data are highly suited to recently developed capture-recapture modelling techniques allowing simultaneous modelling of information from a variety of animal encounters so as to yield maximum statistical power. In particular, combining recapture data with dead recoveries is a powerful tool for the estimation of dispersal pattern Simultaneously, and guided by empirical data, we will clarify the relationship between dispersal behaviour and the resulting density dependence in dispersal, a key determinant of population synchrony. The system under study has experienced marked, season-specific changes in prey dynamics, namely loss in spatial and temporal autocorrelations, coincident with climate change. As such, it serves as a rare model system for investigating the response of a predator to observed and expected environmental change.