untitled

The first term represents colonization of new sites, and the second term represents

extinction. Provided that c>e, this model predicts that the proportion of occupied

sites will converge over time on the following equilibrium:

peq= 1 −

This can be clearly seen in the simulation shown in Fig. 7.9. The stability of the

metapopulation at equilibrium belies a constant turnover of subpopulations. A

substantial fraction of sites (45% in fact) go extinct per unit time. This extinction

rate does not get translated into a dangerous collapse of population because of a stream

of colonists from the remaining occupied sites. As we shall discuss in Chapter 17,

local extinction is expected to become common when subpopulations have been reduced

to low numbers, simply due to chance demographic events or rapid genetic loss. Hence,

fragmentation of the environment into numerous small patches creates a situation in

which local extinction risk is a very real possibility.

Empirical data consistent with the metapopulation scenario are accumulating. One

of the best-documented examples is Hanski and co-workers’ studies of the Glanville

fritillary (Melitaea cinxia), an endangered butterfly inhabiting a number of forested

islands in the eastern Baltic Sea, off the coast of Finland (Hanski et al. 1994; Saccheri

et al. 1998). Field studies have concentrated on one island, Åaland, in particular.

The spatial distribution of butterflies on Åaland is quite patchy, in keeping with

the patchy distribution of the larval food plants. The Finnish team repeatedly

censused the number of butterfly larvae at each of several hundred locales. As the

larvae are colonial and quite conspicuous, it is relatively straightforward to ascertain

whether local extinction has taken place in the small grassy meadows. Results of the

repeated censusing demonstrated that extinction was common amongst these local

subpopulations, in accordance with metapopulation theory (Fig. 7.10).

As we might expect, many factors influenced the risk of extinction, including

size of the local subpopulation, degree of genetic variability, and the degree of

isolation from neighboring sites (Hanski et al. 1994; Saccheri et al. 1998). A high

degree of turnover of local populations was normal, with the overall prevalence

determined by the probabilities of colonization versus extinction. Unfortunately, recent

population trends suggest that the Glanville fritillary may be fighting a losing battle

against extinction.

e

c

DISPERSAL, DISPERSION, AND DISTRIBUTION 105

0.8

0.6

0.4

0.2

0

0 5 10 15 20

Time, t

Proportion of occupied sites,

p

Fig. 7.9Dynamics
over time of a
metapopulation
with colonization rate
c=0.90 and extinction
rate e=0.45.