number of populations, with some turnover.
The authors concluded that there was evidence for
both modes of extinction and also for variations in
dispersal rates over time. We may note that the fit
with equilibrium theory is thus uncertain. On the
other hand, their data show a degree of independ-
ence of population dynamics between the isolates,
combined with a demonstration of recolonization,
thereby fulfilling requirements of metapopulation
theory.
How does this view of extinction and recoloniza-
tion differ from that of the EMIB? In the latter, the
emphasis is on species number as a dynamic func-
tion of area and isolation. Whereas in the check-
erspot butterfly story the focus is on the status of
populations of a single species, with extinction seen
principally as a function of (temporary) alterations
in the carrying capacity of the system. It is consis-
tent with the metapopulation approach—although
not necessarily with all metapopulation models—
that rates of movement and of population gains
and crashes may vary greatly over time. As Haila
(1990) suggested, within a metapopulation system,
the dynamics of the islands are to some extent inter-
dependent. If the patches were much more isolated,
their population fluctuations would be entirely
independent, and one of the alternative theoretical
models would be required. If, on the other hand, a
more dispersive species was being considered, the
entire system might constitute a single functional
population, albeit one spread across a fragmented
habitat. Thus, metapopulation models form a
bridge between the study of population ecology
and island theories of the EMIB form (Gotelli 1991).
Relaxing the single population assumption of the
MVP model to allow for such ebb and flow between
local populations will lengthen projected persist-
ence times. Hanski et al. (1996) coin the term mini-
mum viable metapopulation(MVM) for the notion
that long-term persistence may require a minimum
number of interacting local populations, which in
turn depend on there being a minimum amount of
suitable habitat available.
Conservationists have suggested the creation of
metapopulation configurations for endangered
species as a means of maintaining populations
across areas of increasingly fragmented habitat. The
most celebrated example of this strategy is that of
the Interagency Spotted Owl Scientific Committee.
They proposed the creation and maintenance of
large areas of suitable forest habitat in proximity, so
that losses from forest remnants will be infrequent
enough and dispersal between them likely enough
that numbers of the northern spotted owl (Strix
occidentalis caurina) will be stable on the entire
archipelago of forest fragments. Much of the work
on metapopulations up to this point had been theo-
retical (Hess 1996), and Doak and Mills (1994)
expressed some concern that despite intensive
study of the owl, the metapopulation models have
to assume a great deal about owl biology. The
safety of the strategy cannot be demonstrated sim-
ply by running the models and so, in effect, the
experiment is being conducted on the endangered
species. Criticisms of the strategy led to a US dis-
trict judge ruling against it on the grounds that the
plan carried unacknowledged risks to the owl
(Harrison 1994).
Similar concerns were expressed by Wilson et al.
(1994) concerning plans to subdivide the small
captive flock of the Puerto Rican parrot into
three groups, following the argument that a260 ISLAND THEORY AND CONSERVATION
0.01 0.1 1 10 100Area (ha)1001010.10.01
Distance from source (km)Figure 10.3Occupancy of suitable habitat by the silver-studded
blue butterfly (Plebejus argus) in North Wales in 1990. Most patches
larger than 0.1 ha were occupied (filled circle), provided that they
were within about 600 m of another occupied patch. Beyond this
distance, no patches were occupied (open circle), regardless of patch
size. (Redrawn from Thomas and Harrison 1992.)