untitled

abound in the natural history literature, as we discuss in Chapter 18. The importance

of habitat loss may be magnified in the future, if man does not learn to limit habitat

fragmentation and prevent further alienation of subdivided patches of habitat.

Through natural or human-influenced patterns of disturbance, suitable patches of

habitat for breeding can become widely separated. Individuals setting up breeding

territories must locate these suitable patches in an unfavorable matrix before they

can set up a territory to attract mates (Lande 1987). Under many circumstances, the

dynamics of territory occupancy by breeding pairs and their offspring are logically

similar to those of a classic metapopulation (Noon and McKelvey 1996). Extinction

is amplified when the probability of territory (patch) colonization is low relative to

the probability of territory vacancy (local extinction) arising due to mortality. A key

difference, however, is that successful breeding requires that both a male and a female

independently discover a suitable territory site. At low probabilities of discovery, some

individuals may never find mates. This Allee effect (after Allee (1938) who defined

the process) could cause extinction if the overall level of territory occupancy falls

below a critical level (Lande 1987, 1988; Courchamp et al. 2000a).

It is possible to imagine a metapopulation of local protected areas, each of whose

internal dynamics are determined by the fragmented territory model (Lamberson

et al. 1992, 1994). Under these circumstances, similar levels of species persistence

could be obtained by a few large reserves, even if they are widely spaced, or a large

number of small reserves, each of which is much more vulnerable in isolation to extinc-

tion. Depending on which circumstances prevail, the management priority would switch

from maintaining territory quality within specific reserves to enhancing dispersal across

a large network of reserves (Noon and McKelvey 1996).

A good example of this kind of situation involves the northern spotted owl (Strix

occidentalis) of the western USA (Lande 1988). Spotted owls require substantial tracts

of old growth forest for their breeding territories, but 80% or more of the mature

forest in the northwestern USA has been logged over the past half century. As a con-

sequence, local populations of owls are increasingly isolated from each other by large

areas of clear-cutting. Moreover, variation in forest structure at the local level can

influence territory occupancy. Concern about the long-term viability of northern

spotted owl populations led to debates about appropriate management schemes for

the public forest lands in the Pacific Northwest (Doak 1989; Lamberson et al. 1992,

1994; Doak and Mills 1994). The solution was both to control the future loss of mature

forest and to manage the spatial pattern of forest utilization in such a way as to

DISPERSAL, DISPERSION, AND DISTRIBUTION 107

0.8

0.6

0.4

0.2

(^005101520)
Time, t
Proportion of occupied sites,
p
Fig. 7.11
Metapopulation
dynamics over time for
the same parameters as
in Fig. 7.9 (c=0.90,
e=0.45), now with
substantial habitat loss
(H=0.3) among the
previously inhabitable
sites. This leads to
a reduction in the
metapopulation
equilibrium from
50% to 20%.
7.7.3Fragmented
territorial systems