Modelling the spatial dynamics of a recovering species: the Grey Heron Ardea cinerea in France

In many countries, the implementation of measures to protect declining species has led to the recolonization of some areas by bird species within their historic range. There is, however, a lack of quantitative tools for exploring and projecting such large-scale dynamics. Here, we present an approach that makes use of both census and habitat suitability data and integrates them into a spatially explicit model of colonization dynamics (cellular automaton). The Grey Heron Ardea cinerea, which has recently recolonized much of France following legal protection measures, was used as a focal species. Data analysis allowed us to quantify the positive effects of habitat quality on the probability of colonization, and to uncover both positive and negative density-dependent colony settlement processes. Projective simulations then allowed us to determine an inflection point in global colonization dynamics and predict a significant reduction in the colonization rate during the next 50 years. This makes it possible to project the spatial distribution of the species at different timescales, and to estimate the carrying capacity of France for the species (around 1500 colonies).