Environmental Engineering Reference
In-Depth Information
approaches have been developed to deal specifi cally
with consequences in sets of local populations whose
dynamics are characterized by frequent extinction and
recolonization. We start this section by presenting the
scientifi c approach of metapopulation ecology and
genetics (section 7.3.1), and then illustrate how they
contribute to restoration ecology, focusing on the
' rescue ' of metapopulations (section 7.3.2 ), in analogy
to the rescue of populations.
persistence. On the other hand, migration rates should
be small enough to allow demographics of patches
within the metapopulation to proceed independently.
Spatiotemporal variation in demographics among
patches in a metapopulation is essential for species per-
sistence as it ensures the presence of source patches at
all times.
The structure of metapopulations may suggest that
they should be well buffered against loss of genetic vari-
ation, better than single local populations. Indeed,
compared to an unfragmented population of the same
total size, structures with several small fragments that
are ill connected (low levels of gene fl ow) initially retain
higher genetic variation (Frankham
et al
. 2005 ). This
is because different alleleles will be fi xed and lost in
different fragments; the chance that they are lost from
all fragments is therefore smaller than in a single large
population. However, due to the low effective popula-
tion sizes of the constituent fragments, all metapopula-
tion structures are likely to suffer higher levels of
inbreeding than a single large population. Due to the
larger extinction probability of smaller fragments, any
subdivided population structure will therefore eventu-
ally have lower total genetic diversity than a single large
population. The role of genetic rescue in restoration of
metapopulations could therefore be at least as impor-
tant as in restoration of unfragmented populations.
7.3.1
Metapopulation ecology and genetics
Metapopulation approaches are useful in the context
of restoration as they can provide guidance as to how
to optimize not only within-population, but also
among-population, processes that are critical for
species persistence across their respective ranges,
including patch connectivity and migration rates. The-
oretical models can be classifi ed according to the type
of spatial organization of suitable habitat patches in
the landscape matrix that they assume. Two extremes
are (1) the mainland-island models derived from island
biogeography (MacArthur & Wilson 1967) and (2)
Levins's models (Levins 1969) described below. Most
mainland-island models assume that there is one large
source patch (mainland) and numerous smaller habitat
patches (islands) that can go extinct and be recolonized
from the mainland patch. In contrast, Levins's model
assumes more evenly sized patches, any one of which
can go extinct and then be recolonized from nearby
patches. The spatial organization of most natural
metapopulations will share characteristics of both and
can be classifi ed in more detail. Not all sets of popula-
tions are characterized by frequent extinctions and
recolonizations, a key feature of metapopulation
dynamics in the strict sense (Hanski 1999). Also, large
patches do not necessarily act as sources. If their
intrinsic growth rates are low, for instance due to low
patch quality, they can act as sinks rather than sources
within the metapopulation. In that case, restoration
efforts that focus on large patches may in fact endanger
the metapopulation as immigrant sources are
neglected. Such knowledge is thus of vital importance
when deciding which patches are important to focus
on in restoration projects and programmes.
Migration rates play an important role in metapopu-
lation persistence. On the one hand, migration rates
should be large enough to ensure that recolonization
rates exceed extinction rates, key to metapopulation
7.3.2
Rescue of metapopulations
Similar to what we have seen for single populations
(section 7.2.4 ), metapopulation persistence is enhanced
by ecological and genetic rescue. The metapopulation
rescue effect refers to the positive effect of the number
of immigrants on occupancy of suitable patches,
reducing extinction risk. Practices that enhance disper-
sal (reinforcement, stepping stones and corridors)
therefore generally contribute to metapopulation
rescue, as they increase the proportion of suitable
habitat that is occupied and the successful colonization
rate (establishment effect). Even a very limited amount
of migration can have a profound effect upon the recip-
ient population. We illustrate this with some examples
given by Stacey
et al
. (1997) . White - footed mice (
Pe ro -
myscus leucopus
) have persisted in a remnant network
of woodlot patches in North America, connected by
migration routes. Populations linked by these high
levels of migration have higher growth rates than
populations linked by lower levels of migration and
