Environmental Engineering Reference
In-Depth Information
of concern. However, even when habitat suitability is
ensured, many re-introductions nevertheless fail. In
some cases it has been hypothesized (Law & Morton
1996, Lundberg
et al.
2000) that this could be caused
by 'community closure'; that is, the feasible and
persistent community to which the lost species once
belonged is no longer 'open' for reinvasion. This
approach could, for example, help explain the results
of a study on dispersing prairie voles (
Microtus
ochrogaster
) by Danielson and Gaines (1987). In this
experiment, voles were introduced into enclosed
resident populations of the same species, of southern
bog lemmings (
Synaptomys cooperi
), of cotton rats
(
Sigmodon hispidus
) or into an empty enclosure.
The results indicated that colonization by dispersing
voles was negatively affected most by resident con-
specifics. Introduced female voles were more strongly
affected than males during the growing season but not
during the non-growing season when reproductive
activity was typically low. Resident bog lemmings also
negatively affected colonization by dispersing voles,
but after the colonization phase coexistence was
possible. Cotton rats did not affect colonization by
dispersing voles. Further investigation is required to
reveal to what extent this kind of interspecific inter-
action within guilds plays a role in re-introduction
attempts.
There are, however, many other factors involved in
re-introduction becoming either a success or a failure.
Wolf
et al.
(1996) evaluated 80 translocations of
birds and mammals in Australia, New Zealand and
North America, and compared the results with a sim-
ilar analysis carried out in 1987 by Griffith
et al.
(1989).
The analysis revealed that approximately 58% of all
translocations conducted with thousands of indi-
viduals of threatened, endangered or sensitive birds
and mammals have failed to establish self-sustaining
populations. Furthermore, keystone species play a
critical role in communities, and their effects are
generally much larger than would be predicted from
their relative abundance. The importance of keystone
species is essentially recognized through removal
experiments (Paine 1966; see also Chapter 2 in this
volume). A keystone species often referred to is the
sea otter (
Enhydra lutris
), living in the north Pacific.
Sea otters feed on sea urchins (
Strongylocentrotus
franciscanus
), which in turn feed predominantly on
kelp (macroalgae; e.g. Mate 1972). If keystone species
become threatened or go extinct in their habitat it can
be expected that the system changes dramatically
and that, next to trying to re-introduce the keystone
species into its habitat, the changes may have be-
come so extensive that re-introduction becomes very
difficult. Recently it has been shown through model-
ling work that even (random) removal of species can
lead to cascading extinctions far beyond the target one
(Borvall
et al.
2000, Lundberg
et al.
2000), and that
cascading extinctions are positively related to species
abundance and connectance (Law 1999). If extinctions
are followed by community closure, re-introductions
are even more difficult. If ordinary (i.e. non-keystone)
species can already have such effects, what can we
expect if keystone species become extinct? No clear field
data are available at present, but this question stresses
the need for the conservation of keystone species while
they are still present in their original habitat.
In the remainder of this chapter I will highlight some
of the important aspects of the art and science of
re-introducing species that largely determine either
success or failure.
7.2 Source populations
Individuals to be re-introduced can come from vari-
ous sources and as a first step a careful assessment
should always be made of the taxonomic status of the
candidate sources or provenances. Even though the
species concept as a basic taxon unit is controversial,
individuals should ideally be of the same subspecies
as those that were locally extirpated. Genetic studies
should be carried out, if possible, to determine the
relative degree of taxonomic and genetic similarity
between possible substitutes and the pre-existing
population. Genetic analyses may also permit predic-
tion of the likelihood of hybridization taking place with
other taxa in the target or release area or region. For
animals, it is preferable that source animals derive from
wild populations. For plants and animals, the source
population should ideally be closely related genetically
to the original native stock and also show ecological
characteristics (morphology, physiology, behaviour,
habitat preference, etc.) similar to the original popu-
lation or subpopulation.
If a subspecies has become extinct in the wild and
in captivity, a substitute form may be chosen for
