An individual-based quantitative approach for delineating core areas of animal space use

Abstract

Core areas are often employed in space-use studies to emphasize sections of a home range where an animal supposedly concentrates its time spent (i.e., areas of intense use). The area designated as the core is often subjectively selected based on aggregations of animal locations, thus not quantitatively repeatable, or selected based on arbitrary areas from a home range estimator, thus not biologically significant. We present an explicit ecological model of space use for objectively delineating areas of intense use, or core areas, based on further refining the definition of core area as the area within which an animal spends a maximum amount of time. Core areas were delineated using a time-maximizing function derived from kernel analyses. Essentially, we plotted utilization distribution area against volume and determined the point at which the slope of the line fitted to the data is equal to 1; this point represents a threshold where the proportional home range area begins to increase at a greater rate than the probability of use and the corresponding isopleth value defines the boundary of the core area; an animal's time spent within this area is maximized relative to the periphery. We used summer locations from 60 adult female moose ( Alces alces) to demonstrate our method. Moose core areas were bounded by the x ¯ = 58 % (SD = 0.60) isopleth and comprised x ¯ = 77 % (SD = 6.09) of animal relocations. Core areas represented x ¯ = 27 % (SD = 1.99) of the home range ( x ¯ = 3837 ha , SD = 4336.15; 95% utilization distribution) and were used an average of 2.16 (SD = 0.20) times more intensely than home range peripheries. Proportions of intensively used areas, as defined by isopleth values, were not related to home range size. Our technique reflects a biological understanding of a core area and provides a repeatable, quantitative method that enables statistical testing of hypotheses related to the effects of environmental factors on home range and core area size and usage. Adopting our approach will greatly improve our capacity to understand and ability to compare spatial dynamics of resource selection within home ranges of wildlife.