Keeping an Eye on the Neighbors

摘要

For decades ecologists have sought to understand the principles underlying how mammals optimize their space requirements. It is intuitive that mammals need home ranges: areas they routinely traverse that are large enough to meet their energy needs, but small enough to be protected from intrusions by same-species neighbors that occupy adjacent home ranges. Early attempts to understand the relation between body mass and home-range area suggested that home-range area increases at the same rate as metabolism (7). As metabolic rate is proportional to body mass raised to the 3/4 power, then home-range size should also have the same proportion to body mass (2). However, abundant data on the home ranges of mammals, primarily derived from wildlife telemetrystudies, suggest that this is not the case. Indeed, the home-range area increases at a higher rate than metabolic rate and, in fact, scales almost linearly with body mass (3, 4). Yet parallel evidence from mammalian population density studies is consistent with a metabolic explanation of individual spatial requirements in that the reciprocal of population density (area per animal) appears to scale to the 3/4 power of body mass (5). As large mammals have home ranges bigger than would be predicted from their energetic needs, this implies a maintenance cost that goes beyond the acquisition of essential resources. On page 266 of this issue, Jetz andfco-workers (6) coalesce all of these findings by deriving a general model of mammalian spatial requirementsthat incorporates body mass, energy requirements, home-range size and, crucially, interactions with same-species neighbors. Cleverly, the authors use an equation from physics for collisions among gas particles to predict the frequency of interactions between home-range owners and intrusive neighbors. They show that large mammals require a home range that is larger than predicted by resource needs because they share resources with their neighbors to a greater extent than do small mammals (see the figure). This forced sharing is the result of body size-dependent processes, such as whether the mammal is able to traverse its home range often enough to exclude its neighbors.