Size determinants in territories with inflexible boundaries: manipulation experiments on white-winged trumpeters' territories

Ecology, June, 1998 by Peter T. Sherman, Perri K. Eason

INTRODUCTION

Theoretical models of optimal territory size often differ in their predictions (reviewed in Schoener 1983). Similarly, empirical observations from the field generally demonstrate that territory sizes vary widely, even within a species and within habitats, to an extent apparently not predicted by current theory. To explain this variation, researchers have attempted to identify factors that may influence territory size.

Food abundance is most commonly broached as a potential determinant of territory size. Both empirical studies (see Hixon 1980, Temeles 1987, and the references cited therein) and some experimental studies in which food abundance was manipulated (Stimson 1973, Simon 1975, Hixon 1981, Mares et al. 1982, Hixon et al. 1983, Sullivan et al. 1983, Hart 1985, Mares and Lacher 1987, Sullivan 1990, Sullivan and Klenner 1992, Eberhard and Ewald 1994) have found that there is a rough, inverse correlation between territory size and food abundance.

It seems likely that results of those studies depend upon the type of territoriality exhibited by the study species: to observe changes in territory size with changes in food abundance, territorial borders must be flexible. Indeed, the studies cited were performed on systems in which territorial boundaries appear to have been relatively flexible. Flexible boundaries may occur when territories are not contiguous (Stimson 1973, Simon 1975, Hixon 1981, Hixon et al. 1983, Hart 1985, Eberhard and Ewald 1994), or in territories defended by animals that are not highly territorial, so that individuals tolerate a high degree of overlap between territories (Mares et al. 1982, Sullivan et al. 1983, Mares and Lacher 1987, Sullivan 1990, Sullivan and Klenner 1992).

In contrast, when territories have relatively inflexible boundaries, changing food abundance does not appear to affect territory size (Krebs 1971, Franzblau and Collins 1980, Norman and Jones 1984, Askenmo et al. 1994). Such territories commonly occur in highly territorial species where territories are contiguous. The lack of a close correlation between food and territory size for that type of territory may reflect a high cost of renegotiating boundaries with neighbors. It does not, however, necessarily indicate that there is no relationship between food abundance and territory size for territories with inflexible boundaries. Nonetheless, field studies on such territories often fail to demonstrate any correlation between resource density and territory size, and resources on territories often appear to far exceed the amount needed to feed the resident individual or group (see Verner 1977, Seastedt and MacLean 1979).

Several hypotheses have been proposed to explain that common observation. Some hypotheses propose that the observed surplus of food occurs because territory owners are defending some resource other than food. For example, an animal might defend an area large enough to provide access to safe nesting or roosting sites (Davis 1982), or expand its territory to obtain more mates (Schoener and Schoener 1982, Stamps 1983). In instances in which animals defend relatively small territories, an individual might reduce its long-term energy costs by increasing the size of the area that it is defending to prevent settlement of intruders in unoccupied space between territories (Burger 1981, Getty 1981).

Other hypotheses relate territory size in some way to food resources found within the territory. For example, it has been suggested that animals may defend areas containing a superfluity of resources in order to reduce fitness of conspecifics (Verner 1977), to provide sufficient quantities of certain preferred resources (Norman and Jones 1984), or to include adequate foraging areas within which risk of predation is relatively low (reviewed by Lima and Dill 1990). Alternatively, for habitats in which food abundance fluctuates either seasonally (Mares et al. 1982) or between years (Levins 1968, Pitelka et al. 1974, MacLean and Seastedt 1979, Millington and Grant 1983), an animal may defend the area required to provide adequate resources during periods when food levels are low (MacLean and Seastedt 1979).

Those hypotheses yield differing predictions that can be tested by focusing on individual territorial residents and variation in their food consumption in relation to natural or experimentally controlled changes in food abundance on their territories. The hypotheses that propose that territory size is not related to food density would be supported if a decrease in food on an animal's territory did not affect foraging of the resident. For example, if an animal is defending some resource other than food, such as nesting or roosting sites, its energy intake should not change significantly when food undergoes a natural decline, or when a limited amount of food is experimentally removed from its territory. Similarly, if an animal defends a large territory to reduce fitness of conspecifics or to prevent conspecifics from settling in unoccupied space between established territories, a reduction in abundance of food on the individual's territory should have no significant effect on its energy intake.