Using phenological drivers of demography in conservation

点击翻译按钮获取中文摘要

Understanding density-dependent processes and the conditions that influence their fitness consequences is a key component of population ecology, but the mechanisms determining density-dependent effects in wild populations are rarely understood. Most studies of density-dependent processes have focussed on identifying conditions influencing spatial variation in densities. However, phenological variation among individuals within breeding populations may also influence the strength of local density-dependence, if individual contributions to these processes vary with stage of the reproductive cycle. Synchronous breeding could thus be a major contributor to density-dependent processes within local populations. Identifying links between reproductive synchrony, density-dependence and demography is particularly important in species of conservation concern, for which management to facilitate reproductive synchrony and associated fitness benefits may be possible. In lowland areas of Europe, wading bird populations have undergone severe and widespread declines, and are increasingly restricted to small numbers of nature reserves. Funding mechanisms have been developed within the EU Common Agricultural Policy to attempt to halt and reverse these declines, through agri-environment schemes (AES), but these attempts have failed despite the very large sums of money involved. There is therefore growing recognition of the need to target actions through a much more profound understanding of the interactions between land management and species demography. AES that deliver suitable breeding habitat can successfully attract breeding waders, but high levels of nest and chick predation by generalist predators (primarily red foxes, Vulpes vulpes) consistently prevent population recovery. Ground-nesting waders are inherently vulnerable to predation, and many species have evolved strategies such as semi-colonial breeding and group mobbing of predators to counteract these impacts, but these behavioural adaptations have not previously been considered in the design of AES, and piecemeal deployment of AES that support very small populations may in fact be exacerbating the population declines. Predation rates on wader nests and chicks are generally lower when nesting densities are high, but there is often a great deal of variation in these relationships. The effectiveness of anti-predator behavioural adaptations may be enhanced when breeding is synchronous, as defence behaviour often intensifies through the breeding cycle, but the contribution of reproductive synchrony to these density-dependent processes is unknown. High levels of predation typically result in reproductive asynchrony, as predated nests are replaced throughout the breeding season. The actions of predators may therefore be reducing the effectiveness of anti-predator strategies; a cycle which can potentially be broken through management to facilitate reproductive synchrony. To test the role of reproductive synchrony in driving productivity and population growth, and the potential for management to facilitate reproductive synchrony in species of conservation concern, we propose to carry out a large-scale experimental manipulation of predator distribution and reproductive synchrony in breeding wading bird (northern lapwing, Vanellus vanellus, and common redshank, Tringa totanus) populations on lowland wet grasslands, and to quantify the mechanisms through which synchrony can enhance local productivity. This information will be used to model potential rates of population growth that could be achieved by land management strategies that integrate both environmental and behavioural drivers of population growth. Within-population phenological variation could influence density-dependent processes in a wide range of settings, and identifying these links may therefore substantially improve our understanding of key processes in population ecology and conservation management.