Environmental Engineering Reference
In-Depth Information
was the most limited habitat attribute, whereas food
for otters appeared to be available in adequate quant-
ities (Bich 1988). This study, therefore, recommended
that no otter re-introductions should be made until
riparian zones were rehabilitated and protected,
since re-establishment of stream-bank vegetation
was deemed essential to provide escape cover for re-
introduced otters. Similarly, Howells and Edwards-
Jones (1997) studied the feasibility of re-introducing
wild boar (
Sus scrofa
) to Scotland through an assess-
ment of suitable woodland habitat that could support
a minimum viable population of the target animal. This
species has been the focus of early attempts to re-
introduce it into Britain. Based on a review of
S. scrofa
ecology, the authors identified woodland habitats
suitable for supporting wild boar. Only long-established
woodlands containing some stands of semi-natural
origin and larger than 500 ha in size were considered.
None of the woodlands could be considered optimum
habitat for wild boar and none was large enough to
support a minimum viable population of 300 animals.
The study concluded that the goal of (re-)establishing
a self-sustaining population of wild boar in Scotland
was unrealistic in the short term.
Habitat destruction and modification can also be
brought about in the form of invasive species. Such
invasions often result in dramatic changes in eco-
system structure or function (Gordon 1998, Hobbs &
Mooney 1998). Invasive species may not only lead to
changes in ecosystem properties but can also hamper
re-introductions through predation. Recent attempts
to recover razorback suckers (
Xyrauchen texanus
), an
endangered piscivorous fish species, by re-introducing
them into their native range of mainstream Colorado
River have not been successful because of predation
on the native young suckers by non-native fishes
(Johnson
et al.
1993). In another study, Bergerud
and Mercer (1989) reviewed 33 (re-)introductions of
caribou that took place in eastern North America
between 1924 and 1985. Twenty introductions
resulted in sustained populations and 13 failed, the
majority as a result of predation by wolves. The
fate of these 33 introductions is consistent with the
view that predation (natural and hunting) is a major
factor in the decline of caribou in eastern North
America following European settlement. In Europe,
meanwhile, attempts to re-introduce black grouse
(
Lyrurus tetrix
) and capercaillie (
Tetrao urogallus
)
7.4 The re-introduction site
Re-introduction in the core of the historic range is
sometimes indicated to be better than along the
periphery (Griffith
et al.
1989, Wolf
et al.
1996).
However, Lomolino and Channell (1995, 1998) found
that 23 out of 31 species of endangered mammals per-
sisted along the periphery, not in the core or central
portion of their historic range. In addition, persistence
was greater for insular than for continental popula-
tions. According to Lomolino and Channell, the range
periphery, in comparison with core sites, encom-
passes a much more diverse collection of habitats
and environmental conditions. They referred to the
California condor (
Gymnogyps californianus
) as an
important case in point. Which range should invest-
igators adopt as the raptor's historic range? Recent
efforts include release to a site in northern Arizona,
well outside the condor's present range, but also pro-
viding protection from anthropogenic threats. From
their review of re-introductions of Marsupialia in
Australia, Short
et al.
(1992) came to the conclusion
that the success rate of island (re-)introductions (60%)
was far greater than those in mainland Australia
(11%), even though the successful island (re-)intro-
ductions were all to islands with no historic record of
the occurrence of the (re-)introduced species. Success
of (re-)introduction of these macropods appeared to
depend critically on control or exclusion of exotic ter-
restrial predators such as foxes and cats. Peripheral
sites should thus not automatically be discarded as
suitable re-introduction sites.
A crucial aspect of any re-introduction plan is an
assessment of the availability and quality of the re-
introduction site. Re-introductions can have a chance
of success only if the habitat and landscape require-
ments of the species are or could be satisfied, and are
likely to be sustainable. The area should have
sufficient carrying capacity to sustain growth of the
re-introduced population and support a viable self-
sustaining population over time. Identification and
elimination, or reduction to a sufficient level, of
previous causes of population decline and/or habitat
transformation should take top priority. In a habitat
suitability study for an otter re-introduction project
in Utah, it was found that 94% of the studied streams
were unacceptable for re-introductions. Escape cover
