Identifying Patterns and Intensity of Home Range Use

Assessing patterns of concentrated and diffuse use of an animal's home range is an important component of understanding ecological and behavioral processes. We present the Area Independent Method (AIM), a graphical and quantitative technique for identifying the pattern of home range use (i.e., clumped or even) and for distinguishing between heavily and lightly used parts of an individual's home range when clumping is present. This technique does not require that the data have any specified statistical distribution. The results of the AIM are compared with those from Samuel et al. (1985) when both methods are applied to radio-telemetry locations of 18 black bears (Ursus americanus). Total home range size has a lesser effect on the size of the core area identified by the AIM than by Samuel et al.'s (1985) method. Both methods result in core area usage that is little affected by total home range size. This technique provides an objective method for comparing areas of concentrated and peripheral use among individuals. Int. Conf. Bear Res. and Manage. 8:243-249 Home ranges reveal the ecological requirements of animals since they include all of the resources that are used by a resident. Home ranges and the movements within them are the behavioral manifestations of basic biological requirements such as finding sufficient food, shelter, and mates. Knowledge about home ranges contributes to both management and ecological theory. Direct management applications include assessing how much space and what kind of habitat are necessary (e.g., Poelker and Hartwell 1973). Our theoretical understanding may benefit from learning the effect of various factors (e.g., food supply and breeding requirements) on spacing behavior and movements (Simon 1975, Carpenter and MacMillan 1976, Gass et al. 1976). Management efforts may suffer if their data lack detail; simple outlines of home ranges may include unused habitat and resources, and thus give distorted pictures of what animals use (Dixon and Chapman 1980). Of course, concentration of use by an animal does not imply that heavily used areas or resources are more important than others. The importance of different areas must be decided by studies of the activities associated with the areas of different levels of use (e.g., Belovsky 1978). From a theoretical standpoint, an ecological factor that changes the pattern of concentrated use (but does not change the perimeter of the range) may have a real and important effect. Realizing this can help us progress to a more thorough understanding of behavioral ecology. To perform detailed studies of home range we need a technique for identifying the core of a home range. The core is the area of most concentrated use, and the technique for identifying the core must reflect our understanding of the biological meaning of a core. Two factors are of primary importance in this concept. First, the core area is used much more heavily than other areas of the home range (this will be defined explicitly in the methods to follow). This is important because an area that is used only slightly more than average could result from random use. The requirement that the core be used much more than other areas may be considered vaguely analogous to the statistical requirement for a number to be far above (or below) the mean before we say that it is significantly different from the mean. Second, the core size must not be strictly determined by total home range size. The result of this condition is that animals with equal sized home ranges but different use patterns (e.g., central place foragers, strongly territorial animals, and extensive wanderers) can have different core sizes. Several methods exist for identifying core areas (e.g., Ford and Krumme 1979, Dixon and Chapman 1980, Samuel et al. 1985). Dixon and Chapman (1980) outline a subjective procedure in which the core is delineated by eye, which cannot form a legitimate basis for studies that involve statistical testing of hypotheses concerning core size, so we will only discuss objective procedures. Ford and Krumme (1979) developed a method called MAP (minimum area vs. probability) that designated any arbitrary constant amount of use as the core. In other words, the researcher is free to decide that the highest x percent of use represents the core of the home range for all individuals in the study. A crucial drawback to such a method is that the core is not strictly tied to intensity of use, and will almost certainly include areas of low use or exclude areas of high use. Samuel et al. (1985) presented a method for objectively identifying the maximum possible core area. This method defined the core as being all areas used more heavily than would result from the even distribution of use by the resident. The core area defined in this way is responsive to the patterns of use of different individuals, and cannot include areas used with frequency lower than the mean. This paper will present the Area Independent Method (AIM) for identifying areas of concentrated use, compare 244 BEARS-THEIR BIOLOGY AND MANAGEMENT it with the method of Samuel et al. (1985), and show the results of applying the 2 techniques to radio-location data from 18 black bears. We would like to thank M.D. Samuel, J.M. Reed and 3 anonymous reviewers for many helpful suggestions on the manuscript, and C. Brownie for statistical assistance. M.A. Fritz, M.A. Homer, T.H. May, C.B. Cooper, and numerous Earthwatch volunteers helped collect the data. The research was made possible by funding and cooperation from R. Bacon, Citibank Corp., Earthwatch/The Center for Field Research, G. and R. King, National Geographic Society, the U.S. National Park Service, North Carolina Agricultural Research Service, North Carolina Wildlife Resources Commission, and the Port Clyde and Stinson Canning Companies (who donated hundreds of cases of canned sardines for bait).