untitled

Well-studied examples of metapopulation dynamics in vertebrates are less com-

mon. Long-term studies of pool frogs along the coast of Sweden demonstrate a

steady pattern of subpopulation turnover (Sjögren Gulve 1994). Similar patterns of

extinction and recolonization have been shown in a number of other systems: cou-

gars inhabiting chapparal shrub patches in urban southern California (Beier 1996),

pikas living on mine tailings in the Sierra Nevada mountains of California (Smith

and Gilpin 1997), sparrows on windswept islands off the coast of Norway (Sæther

et al. 1999), and beavers inhabiting isolated ponds in Canada (Fryxell 2001). There

is no doubt that the preconditions for metapopulation dynamics exist. The unresolved

question is how common they might be.

Levin’s (1969) simple metapopulation model can be readily modified to predict the

effect of habitat loss. Let Hreflect the proportion of sites destroyed by man, so that

any propagule that lands on a degraded site cannot persist. The dynamics of this

degraded environment are depicted as follows:

=cp(1 −H−p) −ep

As a result of habitat loss, the equilibrium level of occupancy is reduced (Fig. 7.11). If

His large enough, the metapopulation may not be able to persist at all. This is a simple,

but graphic, way to look at the potential costs of habitat degradation. Empirical exam-

ples of habitat degradation leading to extinction are largely anecdotal, but nonetheless

dp

dt

106 Chapter 7

Fig. 7.10A map of

Åaland, showing sites of

local subpopulations of

the Glanville fritillary

butterfly. Sites with

suitable host plants

occupied by larvae in

1995 are shown by

filled symbols, whereas

open circles depict

unoccupied sites. A

subset of 42 occupied

sites were studied in

detail, shown by large

circles. Seven of

these subpopulations,

depicted by triangles,

had gone extinct by the

next sampling period.

(After Saccheri et al.

1998.)

7.7.2Habitat loss

and metapopulation

collapse

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