Home Range Size and Population Social Structure

When faced with shifts in resource levels or intruder pressure, individuals that are occupying a home range can respond in ways other than adjusting their home range size. One alternative is to accept one or more additional individuals into their home range, and then cooperatively defend it against competing individuals. This kind of response is seen in the pied wagtails, which, as noted earlier, defend territories along river banks in England during the wintertime. At moderate resource levels, individuals occupy and defend individual home ranges. However, as resource levels increase, rather than decreasing their range size as might be expected from a simple cost-benefit model of territory size such as the one depicted in Figure 5, owners begin to share the territory with an additional individual who shares the home range but forages independently of the owner.

A simple model for evaluating the economic profitability of sharing a home range considers the costs individuals incur in the form of reduced food availability versus the benefits that arise from reduced per capita defense expenditures when sharing a home range with one or more additional individuals. An important factor influencing the impacts of sharing on the food intake of individuals is the dynamics of resource renewal. A resource that is being shared between individuals will be exploited at a higher rate of one exploited by a single individual. If the resource renews slowly, then the increase in the exploitation rate will have a significant impact on the foraging efficiency of the individuals; in contrast, if the resource renews quickly, then the increased exploitation rate may have a relatively small effect on the foraging efficiency and resulting per capita food intake of individuals.

A second key factor affecting the costs of sharing a home range is the spatial distribution of resources. The 'resource dispersion hypothesis' proposes that in situations where a species is foraging on a patchily distributed, ephemeral resource in which each patch, when available, can supply enough resources to meet the needs of more than one individual, an economically defendable home range that is large enough to sustain an individual or a mated pair of individuals is likely to support the foraging needs of one or more additional individuals. Two animals whose home range patterns appear to fit this reasoning are red foxes and badgers in England. Due to the patchy, ephemeral nature of earthworms (a key prey resource for both species), mated pairs of individuals defending a home range large enough to satisfy their own energetic requirements incur relatively little cost from resource consumption of additional individuals and benefit from the cooperative defense of the shared home range. Similar arguments have been made for other carnivores, and for species in other animal groups, including primates, ungulates, and birds.

A distinguishing feature of the resource dispersion hypothesis is that it argues that it is the spatial and temporal dynamics of the underlying resources, rather than the benefits of cooperative hunting or cooperative defense against predators, that favors the occupation and defense of shared home ranges by groups of individuals. Additional benefits of group living, such as improved hunting success, improved predator detection and defense, and the inclusive fitness benefits when sharing a home range with relatives, will, however, increase the benefits and reduce the costs of group living. Predictions that distinguish the hypothesis are: (1) group size should more strongly correlate with patch richness and heterogeneity rather than territory size; and (2) territory size is primarily determined by the patterns of resource dispersion. These predictions have been borne out in several studies of fox and badger populations, as well in studies of other species, including spotted hyenas, mara, and magpie jays. Thus, the spatiotemporal distribution of resources, in conjunction with other benefits of group living, such as shared defense costs, improved foraging success, or predator defense, can favor the occupation and defense of shared home ranges by groups of individuals. An important implication of this is that the size, shape, and degree of exclusivity of home ranges is a key determinant of the different social organization and mating systems found within species.

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