untitled

disperser, changes in disperser motivation, or heterogeneous environmental effects

on dispersal tendency (Turchin 1998). Nonetheless, the simple model of diffusive

spread combined with exponential growth often does a tolerably good job of pre-

dicting patterns of population spread over time. These successful predictions suggest

that both rapid population growth at the wave front and some degree of randomness

in the pattern of movement contribute heavily to observed patterns of spread in many

wildlife species. Both the theory and empirical mechanisms underlying animal move-

ment across complex ecological landscapes are now developing rapidly, because both

have important conservation implications.

Dispersal also plays a key role in understanding the dynamics of species that are sub-

divided, for one reason or another, into discrete subpopulations. Provided that there

is some degree of dispersal amongst subpopulations, ecologists refer to the larger aggre-

gate as a metapopulation, or population of populations (Hanski and Gilpin 1997).

Metapopulations can occur in a variety of contexts. Bird species on continental islands

are an obvious example (Sæther et al. 1999). However, it is just as valid to think of

butterflies inhabiting grassy glades in a matrix of boreal forest as a metapopulation

(Hanski et al. 1994). Since 1980 there has been a surge in interest in metapopula-

tion dynamics, fueled in part by the recognition that human environmental impacts

often lead to fragmentation of natural areas, creating effective metapopulations from

populations that were continuously distributed in the not-so-distant past. Here we

outline some of the basic principles of metapopulation dynamics, particularly with

relation to the impact of further habitat loss.

There are many ways one can represent metapopulations, but the Levins model

(Levins 1969) and its subsequent modifications (reviewed by Gyllenberg et al. 1997)

have perhaps been the most influential. Let pbe the proportion of occupied sites, c

the probability of successful colonization, and ethe probability of extinction of an

occupied site. The rate of change in the number of occupied sites is calculated in the

following manner:

=cp(1 −p) −ep

dp

dt

104 Chapter 7

1910 1920 1930

Year

√Area

Fig. 7.8Radial spread
(measured as the square
root of area) of the
population of muskrats
introduced into the
countryside near Prague.
(After Skellam 1951.)

7.7 Dispersal and the sustainability of metapopulations

7.7.1Metapopulation
dynamics of a single
species