Analysis of Animal Excursions and Dispersal from Radio-Tracking
Robert E. Kenward, Ralph T. Clarke, Sean S. Walls, and Katharine H. Hodder, Institute of Terrestrial Ecology, Furzebrook Research Station, Wareham BH20 5AS United Kingdom
There are many different methods available for estimating the size, shape and internal structure of home ranges, but all are based either on densities or on distances. Density-based methods either involve creation of ellipsoids about one or more focal points, or of contours across a spatial matrix. Distance-based methods either involve varying degrees of edge restriction, which provide a continuum from convex polygons to grid cells if there is a resolution-strip round the fixes, or depend on internal distances to fixes, in peeled and cluster polygons (or tesselations). We have combined distance-based methods with parametric techniques to provide objective estimates of excursive activity and of dispersal from a home-range.
Estimation of range shape and internal structure is important for analyses of sociality, of links between movements and food supply, and of relationships between movements and animal performance (expressed for example as survival or breeding success). In many species, relationships are best defined if a range core can be separated from an area used for infrequent excursions. We use the Gaussian kernel function to define a frequency distribution of nearest neighbor distances between fixes, and then separate outliers from core fixes on the assumption that core activities involve different traveling behavior than excursive activities. An outlier exclusion distance (OED) can be used to form tight multinuclear cores, either with cluster analysis or edge-restriction polygons. These objective cores approximate to those estimated subjectively by inspection of utilization distributions. A similar process can be used to define dispersal, as the time when the vectors of three consecutive locations exceed the expected distribution of usual movements from a range center.
We use Ranges V software to compare the effectiveness of objective range cores with other techniques for separating appropriate from unsuitable habitat; data are from squirrels on a small island where woodland is interspersed with oil-production facilities. We demonstrate the dispersal detector with data from juvenile raptors.
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