MODELING THE DISTRIBUTION OF THE RED AND GREY SQUIRREL AT THE LANDSCAPE SCALE - A COMBINED GIS AND POPULATION-DYNAMICS APPROACH

1. An integrated Geographical Information System (GIS)-Spatially Expli cit Population Dynamics Model (SEPM) for investigating the dynamics of red squirrel Sciurus vulgar is and grey squirrel S. carolinensis popu lations in different landscapes is described. 2. Using the model, we s imulated processes of reproduction, mortality and dispersal in individ ual populations of squirrels in habitat blocks identified within a GIS . We modelled dispersal as a process whereby individual animals moved between blocks of habitat separated by areas of unsuitable habitat. An interference model simulating the effects of competition between the grey and red squirrels was incorporated where both species occupied th e same habitat blocks. 3. The model was used to investigate the spread of grey squirrels and its impact on the distribution of red squirrels in Norfolk, UK, where historical information on the decline in the re d squirrel and the expansion of the grey squirrel were available. 4. W e examined the effect of a range of life history scenarios differing i n terms of adult mortality, juvenile mortality, fecundity, habitat car rying capacity and maximum dispersal distances. Model predictions were closest to the observed expansion of the grey squirrel and decline in the red squirrel when mortality was lower and fecundity higher than t he averages recorded for the grey squirrel in the United Kingdom.