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

Lead Research Organisation: University of Aberdeen
Department Name: Inst of Biological and Environmental Sci


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.


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Description Our aim was to quantify the contributions of variation in demographic parameters in predicting population dynamics in the face of changing environmental conditions. We used a unique data set on tawny owls to determine how life histories and dispersal behaviour are shaped by the environment and in turn, how life histories and dispersal behaviour influence population dynamics.

We elucidated processes that underpin variation in delayed reproduction and assessed the lifetime consequences of the age of first breeding in tawny owls subjected to fluctuating selection linked to cyclical variation in vole density (typically three-year cycles with low, increasing and decreasing vole densities in successive years). We found that owl cohorts suffered strikingly different juvenile survival prospects, with estimates ranging from 0.08 to 0.32 respectively for birds born in Decrease and Increase phases of the vole cycle. This resulted in a highly skewed population structure with >75% of local recruits being reared during Increase years. The probability of commencing reproduction was lower at age 1 than at older ages, and especially so for females. Variation in lifetime reproductive success was driven by the phase of the vole cycle in which female owls started their breeding career, more than by age at first breeding or by conditions experienced at birth. Females who postponed reproduction to breed for the first time at age 3 during an Increase phase, produced more recruits, even when accounting for birds that may have died before reproduction. These results were published in 2010 in Journal of Animal Ecology.

We also examined the interaction between natal conditions and senescence and found the former, measured in terms of vole density, explained 87% of the deviance in first-year apparent survival. We found evidence for senescence in survival for females as well as for males and an earlier decline in male than female reproductive performance. Long-lasting effects of natal environmental conditions were sex specific. Female reproductive performance was substantially related to natal conditions (difference of 0.24 fledgling per breeding event between females born in the first or third quartile of vole density) whereas male performance was not. We found no evidence for tawny owls born in years with low prey density having accelerated rates of senescence. Our results, combined with previous findings, suggest the way natal environmental conditions affect senescence varies not only across species but also within species according to gender and the demographic trait considered. These results were published in 2011 in Journal of Animal Ecology.

We investigated variation in tawny owl natal dispersal and recruitment. Tawny owls responded to the cyclic variation in prey abundance with a sit-and-wait strategy, implying a delayed reproduction. Variation in lifetime reproductive success matched the distribution of natal dispersal with higher fitness achieved for observed median dispersal distances. Males and females dispersed equally short distances, independently of prey abundance and age at recruitment. This resulted in a relatively high concentration of kin, with 48-55% of individuals having at least one close relative of the opposite sex alive in the breeding population. However, an emergent property of simple "rules" such as avoidance of recruitment in the natal territory and mate fidelity was to reduce the residual risk of inbreeding close to zero. These results will be considered for publication by Behavioural Ecology.

We developed an eco-evolutionary framework incorporating emigration, movement, settlement behaviour and the multiple costs involved in dispersal. Our results highlight that the joint evolution of dispersal characteristics can have major implications for spatial population dynamics and we argue that, in addition to increasing our fundamental biological understanding, a new generation of dispersal modelling, which exploits recent empirical advances, can substantially improve our ability to predict and manage the response of species to environmental change. This generic paper is published in Methods in Ecology and Evolution (2012).

The framework was elaborated upon in a study focussing explicitly on cyclic prey dynamics. We found that predators are likely to have evolved to emigrate more often and become more selective over their destination patch when their prey species exhibit spatio-temporally complex dynamics. We additionally demonstrate that the cost of dispersal can vary substantially across space and time. Perhaps as a consequence of current environmental change, many key prey species are currently exhibiting major shifts in their spatio-temporal dynamics. By exploring similar shifts in silico, we predict that predator populations will be most vulnerable when prey dynamics shift from stable to complex. The more sophisticated dispersal rules, and greater variance therein, that evolve under complex dynamics will enable persistence across a broader range of prey dynamics than the rules which evolve under relatively stable prey conditions. These results have been published in PlosONE.

We also analysed empirically how observed season-specific changes vole dynamics alters owl dynamics. Breeding probabilities were strongly dependent upon vole density in spring while vole density in autumn explained juvenile survival. Owl populations are expected to collapse under contemporary vole dynamics unless immigration is increased. These results will be considered for publication by Global Change biology. Overall, our work has substantially progressed understanding of the relationship between environmental variability, life history traits and population dynamics.
Exploitation Route The wider public has a huge interet for the natural world and we often draw upon those findings when given talks to the public The work inform conservation, especially by land manager who have biodiversity management as a remit. It is directly used by the Forestry commission

It is also relevant to politicians at it helps undertanding the magnitude of the impact of climate change will have on biodiversity
Sectors Environment

Description Local partners, e.g. Forest Enterprise and global partners worldwide an more aware of the potentially cascading impact of changes in seasonal resources availability caused by environmental change on predators.
First Year Of Impact 2013
Sector Environment
Impact Types Societal

Description Natural History Society of Northumbria Great North Museum: Hancock: Field voles and tawny owls in Kielder Forest: living in an unstable world. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Natural History Society of Northumbria Great North Museum: Hancock: Field voles and tawny owls in Kielder Forest: living in an unstable world.

Natural History Society of Northumbria Great North Museum: Hancock: Field voles and tawny owls in Kielder Forest: living in an unstable world. Oct 2013-10-07 attended by 70 + people
Year(s) Of Engagement Activity 2013
Description Press release highlighting Global Change biology paper 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact A Press release highlighting Global Change biology paper was issued and had substantial uptake by the media as well as by NERC Planet Earth news

Multiple interviews took place and several items in newspapers and online newsblogs were published
Year(s) Of Engagement Activity 2014