Environmental Engineering Reference
In-Depth Information
7.1
INTRODUCTION
tion generally suffer increased extinction risk for several
reasons, including demographic, environmental and
genetic factors and their interactions, and often it is not
easy to identify the primary cause. Population viability
analyses (Menges 2000) and estimates of
minimum
viable population
(MVP) size (Traill
et al
. 2007 ) - con-
cepts that are being applied to conservation and resto-
ration - depend on whether genetic variation is included
or not. Yet, the role of genetic factors in the persistence
or extinction of threatened species has been long
debated. This debate has been inspired by the fact that
in some species, populations that have gone through a
bottleneck of even a single pair, resulting in high levels
of inbreeding and low genetic variation, nonetheless
are able to persist (Groombridge
et al
. 2000 ). Frankham
(2005) considers it risky to use such examples to argue
that genetic factors do not play a role in determining
the fate of small isolated populations, as they in fact
may represent exceptions: a much larger fraction of
genetically impoverished populations might have gone
extinct compared to similar-sized populations with
higher levels of variation or lower levels of inbreeding,
as indeed shown in experimental studies (Reed &
Frankham 2003). For instance, genetic variation as
measured by heterozygosity appears to be on average
35% lower in threatened (IUCN red list) plant and
animal taxa than in related, nonthreatened, taxa
(Spielman
et al
. 2004a). Low genetic variation also
reduces opportunities for populations to tolerate chang-
ing or fl uctuating environmental regimes or the ability
to evolve and adapt to them (Bijlsma & Loeschcke 2005;
Bakker
et al
. 2010). Without considering genetic
factors, extinction risks tend to be underestimated and
restoration strategies may be inappropriate.
In this chapter we will fi rst deal with local popula-
tion ecological and genetic processes that are relevant
for
restoration ecology
and show how knowledge of
these processes can help in planning and managing
succesful
ecological restoration
(section 7.2 ). We
will then elaborate on these processes in the spatial
context of metapopulations and discuss the challenges
of restoration from a metapopulation perspective
(section 7.3). Next, we will discuss the implications of
changes in the spatial structure of the habitat for
trophic interactions between populations that respond
at different spatial scales and its relevance for restora-
tion ecology (section 7.4). Finally, in section 7.5, we
refl ect on new challenges implied by restoration as
compared to conservation of populations. In Box 7.1
we present commonly used measures of genetic varia-
A population is defi ned as a group of potentially inter-
acting organisms of the same species occupying a
given space at the same time. The physical environ-
ment (the local sites) in which the populations of a
species are found is termed the species'
habitat
. In
restoration ecology, population processes should be
approached in the context of processes taking place at
the landscape scale (see Chapters 2 and 5). Therefore,
in this chapter considering populations of both plants
and animals, and also plant-animal interactions, we
will focus in particular on the dynamics of spatially
structured
metapopulations
, which are defi ned as
assemblages of local populations connected by mutu-
ally dispersing individuals in a network of habitat
patches. Within their habitat patches, species interact
with a subset of the entire
biotic community
in the
ecosystem. Interacting species obviously perceive the
spatial scale of discontinuities in the landscape differ-
ently, depending on their trophic level, degree of
habitat specialization, body size and dispersal ability,
among other variable traits and factors. Habitat
destruction and fragmentation, or other impacts on
the spatial structure of habitats, can therefore alter the
extent and outcome of interactions between popula-
tions of different species in a community.
Population ecology and the ecology of metapopula-
tions are somewhat different, and make different con-
tributions to restoration ecology. Population ecology
focuses on understanding local abundances and can,
among other things, be used to predict population per-
sistence over time. Metapopulation studies (see section
7.3, below) explicitly link population ecology to bioge-
ography, link the regional occurrence of species and
specifi cally contribute to restoration ecology in terms
of optimizing among-population processes that are
critical for species persistence; specifi c interventions
may include determining the best ways of adding, rein-
forcing, reintroducing and connecting populations.
Both fi elds require detailed biological knowledge of a
species and long-term data sets providing not only
mean values but also spatio-temporal variances in
demographic parameters (vital rates) at different life
stages of populations under a range of relevant envi-
ronmental conditions.
The fate of populations is not only determined by
demographic and environmental parameters, but also
by genetic factors. Populations that have become small
and isolated due to habitat destruction and fragmenta-
