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MacMillen 1976; Hixon 1980; Hixon et al. 1983). To understand why mem-
bers of these species display flexibility in their territorial behavior, one must
start with the concept that a territory must be economically defensible (Brown
1969). Carpenter and MacMillen (1976) showed theoretically that an animal
should be territorial only when the productivity of its food (or whatever its
limiting resource is) is between certain limits. When productivity is low, the
costs of defending a territory are not returned through exclusive access to the
limiting resource. When productivity is high, requirements can be met with-
out exclusive access. The model developed by Carpenter and MacMillen
(1976) is broadly applicable because it expresses clearly the limiting conditions
required for territoriality to exist and it incorporates limits on territory size
from habitat heterogeneity, or patchiness. Some approaches to modeling terri-
torial behavior, such as Ebersole's (1980), Hixon's (1987), and Kodric-Brown's
and Brown's (1978), do not express limiting conditions for territorial behavior
but tacitly assume, a priori, that territoriality is economical. Understanding
the limiting conditions for territorial behavior is important to understanding
spacing behavior and home range variation in many species. Using economic
models is a good approach to understanding limiting conditions for territori-
ality as long as the limiting resources do not change as conditions change
(Armstrong 1992). Otherwise, the limiting resources must all be known
clearly for the different conditions under which each is limiting. For example,
if a small increase in the abundance of food leads to another resource becom-
ing the limiting resource, that new limiting resource must be understood as
well as the importance of food is understood. Researchers must also under-
stand how an economic modeling approach fits into a broader picture, such as
how animals use information from the environment to make decisions and
how they perceive information (Stephens 1989).
When productivity of the limiting resource for an individual is very low
and close to the lower limit for territoriality, the individual must maintain a
territory of the maximum size possible. Such an individual should be com-
pletely territorial and not share any part of its territory. As productivity of the
limiting resource for an individual approaches the upper limit for territoriality,
however, its territorial behavior should change in one of two predictable ways.
If necessary resources are evenly distributed in defended habitat, then the indi-
vidual should maintain a smaller territory than in less productive habitat
(Hixon 1982; Powell et al. 1997; Schoener 1981). If the individual's resources
are distributed patchily and balanced resources cannot be found in a small ter-
ritory, then it might exhibit incomplete territoriality. The individual might
maintain exclusive access only to the parts of its home range with the most im-
