Environmental Engineering Reference
In-Depth Information
habitats and having low dispersal capabilities, and T-
species such as
Amara plebeja
, occurring in unstable
habitats and having good dispersal potentials. The
turnover in populations of L-species was 1-3% per year,
whereas for T-species this amounted to 3 -10% per year.
Human-induced changes in the environment resulted
in a decline of L-species until extinction, whereas
T-species were not affected. Carabids, adapted to
extensive stable habitats, usually live there in hetero-
geneously structured metapopulations, and spreading
the risk of extinction over interconnected and differ-
ently fluctuating local groups is suggested to result
in almost unrestricted survival of the species in the
region. In later research, much effort has been put
into studies of birds and butterflies. The European
nuthatch (
Sitta europea
) is found to show all the
characteristics of a Levins-type metapopulation: the
distribution is dynamic in space and time, extinction
rate appeared to depend on patch size and habitat
quality, and colonization rate depended on density
of surrounding patches occupied by nuthatches
(Verboom
et al
. 1991). Another example is the cranberry
fritillary (
Boloria aquilonaris
), a butterfly of peat bogs
in western Europe. Baguette and Schtickzelle (2003)
have shown that the 14 patches of the metapopula-
tion on the Plateau des Tailles upland (Belgium) are
just below the minimum available suitable habitat for
the long-term persistence of a viable metapopulation,
and that the population network is unstable. There is
a huge number of examples showing that the meta-
population approach may be of central importance
when dealing with restoration ecology in modern-time
landscapes. Although a range of forms of local spatial
dynamics exist, these are qualitatively different from
the forms of population structure at the regional
level. One important characteristic of all metapopula-
tion systems is that the metapopulation itself can be
much more stable than the component populations,
because migration can buffer individual populations
from negative stochastic and genetic events in their
local environments.
In the present chapter I will first deal with some
background information on the dynamics and gen-
etics of metapopulations. Thereafter I will focus on
restoration problems, mainly related to re-introductions,
such as the identification of suitable habitat, the deter-
mination of connectivity between habitat patches as
related to dispersal problems, and the identification
of a minimal (meta)population size. I will also pay
attention to cascade effects, recognizing that popula-
tions do not occur as single monocultures; the indi-
viduals are also components of local communities.
6.2 Metapopulation dynamics and
genetics
6.2.1 The rescue effect
Though colonization and extinction of island popula-
tions, or of the local populations of a metapopulation,
are natural phenomena, literature aimed at nature
conservation may qualify the risk of extinction of a
local population as a negative aspect and start refer-
ring to a so-called rescue effect. Defined as 'immigrants
from other populations increasing the local popula-
tion size and therefore limiting the extinction risk'
(Brown & Kodric-Brown 1977), the term goes beyond
the more neutral term buffer and actually implies a
conservation or restoration approach. The rescue
effect was originally envisioned to occur in island
populations, rescued through input from the larger
mainland, but a similar effect may also occur in
metapopulations without a mainland. In this case, exist-
ing populations provide a kind of mutual aid to each
other: a reduced probability of population extinction
due to immigration.
Animals
Even a very limited amount of migration can have a
profound effect upon the recipient population (Stacey
et al
. 1997). We refer briefly to a few examples these
authors provided. All the studies on butterflies sug-
gest a pattern of evidence for important rescue effects
in butterfly metapopulations. Many species are clearly
predisposed towards a metapopulation structure be-
cause they occupy distinct habitat patches as a result
of specialized vegetation requirements during some
stage of their life cycle. Also, for small mammals such
as pikas (
Ochotona princeps
) migration among hab-
itat fragments appeared to be important in main-
taining genetic variation within the metapopulation and
preventing the loss of heterozygosity, a general meas-
ure of genetic diversity, in component populations.
Most movements of water voles (
Arvicola terrestris
)
