Deer in Britain: population spread and the implications for biodiversity

Abstract

Owing to their cascading effects on ecosystems, keystone species should be a focus of conservation and management efforts. However, in order to manage populations of keystone species – such as deer – effectively, there is a need to quantify how the distribution and abundance of populations may change in the future, and to understand the potential consequences of these changes for biodiversity.

By relating species’ abundance data within their historic ranges to habitat variables, I developed a novel methodology to assess habitat suitability for deer across Britain, none of which are currently at equilibrium with their environment. The resultant models performed well in describing the observed, contemporary distribution of roe, red and Chinese muntjac deer in Britain. In particular, the model for the non-native muntjac deer highlighted the potential for this species to expand its range significantly in the future.

I estimated, for the first time, the rate and pattern of the future spread of deer at a landscape scale across Britain, using a spatially explicit population model (SEPM). The SEPM accounts for range-limiting processes such as species-specific dispersal ability and environmental barriers to dispersal. The SEPM performed well in describing the observed spread of roe, red and muntjac deer in Britain between 1972 and 2007, and predicted the spread of, and overlap between, species to continue to increase in the future. From sensitivity analyses, I found that the spread of deer was strongly influenced by adult and juvenile survival.

I also investigated the impacts of deer on their environment using two approaches. First, using vegetation data collected in 35 woodland sites across Britain, I found negative relationships between the density of roe deer and the diversity and abundance of shrub-layer vegetation. Second, I investigated the potential cascading effect of this negative relationship between deer density and shrub layer vegetation on bird populations. I used a long-running dataset on bird abundances across Britain, to construct and contrast two multi-species composite population trends for birds: one for deer-sensitive woodland birds and the second for deer-tolerant woodland birds. The divergence in these trends showed a striking association to a composite population trend for deer (also calculated using data from long-running volunteer surveys). These results demonstrate the potential for rapid and profound impacts of keystone herbivores across multiple tropic levels, which are likely to increase as a consequence of the predicted future expansion of such species.