Environmental Engineering Reference
In-Depth Information
there is clear evidence for a similar rescue effect in
several other small mammal species. Pool frogs (
Rana
lessonae
) that occur in natural metapopulations along
the Baltic coast of Sweden reproduce only in distinct
water bodies. Over a 6-year period, populations isolated
by greater than 1 km became extinct, whereas less iso-
lated populations tended to persist.
In the aforementioned examples of metapopulation
ecology, genetic aspects have not been considered. We
now refer to a number of studies on the genetic rescue
of metapopulations. Populations of blacktail prairie
dog (
Cynomis ludovicianus
), which appeared to consist
of several family groups called 'coteries', remain spa-
tially distinct from each other because of a rigid social
structure. Regular infl ux of male immigrants reduces
inbreeding within local populations and elevates levels
of heterozygosity, potentially ' rescuing ' populations
from inbreeding depression and possible extinction
(Stacey
et al
. 1997). Also, for small mammals such as
pikas (
Ochotona princeps
) migration among habitat
fragments appears to be important in maintaining
genetic variation within the metapopulation and
preventing the loss of heterozygosity in component
populations. In metapopulations of the checkerspot
butterfl y
Melitaea cinxia
, many local populations are
small and inbred. Inbreeding depression is strong,
especially under less favourable environmental condi-
tions, and inbred populations suffer increased extinc-
tion risk (Nieminen
et al
. 2001). Since most populations
within the metapopulation are small, the total number
of migrants and thus the potential number of recolo-
nizers within the metapopulation is low, increasing
metapopulation extinction risk and calling for
enhanced migration and metapopulations rescue.
Similar observations have been made for plants. For
example, in metapopulations of the white campion
(
Silene latifolia
) effects of inbreeding are mitigated by
higher gene fl ow into patches with genetically related
than into patches with genetically unrelated individu-
als (Richards 2000). The effect is probably due to the
higher success of outcross pollen in patches with
genetically interrelated individuals than in patches
consisting of unrelated individuals, pointing at a role
of connectivity through pollen-mediated gene fl ow
on persistence of isolated patches. In 27 populations
of the marsh grass
Parnassia palustris
, distributed
over three metapopulations, between-metapopulation
crossings result in higher seed production than within-
metapopulation crossings, especially for smaller popu-
lations. This
genetic rescue
is probably due to alleviation
of the low pollen compatibility within metapopulations
due to low genetic diversity (Bossuyt 2007).
7.3.3
Corollaries and consequences
Several generalities arise from metapopulation theory
(cf. Maschinski 2006). Persistence is enhanced by
higher numbers of suitable patches, larger average
patch size, larger fraction of large patches at the inte-
rior, higher within-patch reproductive rates, enhanced
patch connectivity, higher immigration rates and lower
emigration rates. Metapopulation models help to
predict the fate of a metapopulation as a whole, to
assess the value of individual patches for species' per-
sistence, and to manage migration by stepping stones,
corridors and repeated introductions. They can also
serve to estimate minimum viable metapopulation
sizes (MVM), which is the minimum number of inter-
acting local populations necessary for long-term per-
sistence of a metapopulation. The MVM is thought to
be governed by an equilibrium between the rates of
extinction and of recolonization of suitable habitat
patches. Metapopulations that are not at extinction-
recolonization equilibrium are heading for extinction.
Even though most relevant models suggest that the
largest patch in the metapopulation is the most impor-
tant determinant of overall persistence, in any particu-
lar restoration programme it is often diffi cult to judge
the relative benefi ts of enhancing size per patch
(decreasing local extinction probability of subpopula-
tions) versus constructing more patches (increasing
metapopulation viability). Nicol and Possingham
(2010) point out that the optimal metapopulation
structure strongly depends on the specifi c metapopula-
tion parameters and suggest stochastic dynamic pro-
gramming as a tool to design optimal restoration
strategies.
7.4 SPECIES INTERACTIONS
AFFECTED BY HABITAT
FRAGMENTATION
Habitat fragmentation of plant populations not only
affects individual plant species themselves, but also
their interactions with potential mutualists such as
mycorrhizae and pollinators, as well as antagonists
such as herbivores and pathogens (see also Chapter 6).
This is because the spatial scale of population processes
