Environmental Engineering Reference
In-Depth Information
possible release. Such substitutions are actually a
form of benign introduction. Selection of a suitable
substitute should focus on extant subspecies and
consider genetic relatedness, phenotype, ecological
compatibility and the conservation value of potential
candidates. For example, a local population of ibex
(
Capra ibex
) that became extinct in Czechoslovakia was
replaced by re-introductions of Austrian
C. ibex
and
Turkish
Capra hircus aegagrus
and
C. ibex nubiana
from the Sinai desert (reviewed in Stanley-Price
1989). The inevitable hybrid forms dropped their kids
in the middle of the winter, 3 months earlier than pure
C. ibex
, resulting in the death of all offspring. This
case illustrates the need to assess both hybridization
risks and ecological compatibility (Seddon & Soorae
1999). In general, there is a need for information on
whether the introduction can literally be considered
a re-introduction or whether it entails a risk of effects
like those related to unintended invasions by aliens
(see Chapter 2 in this volume).
Removal of individuals for re-introduction should
not endanger the wild source population, and indi-
viduals should only be removed from a wild popula-
tion after the effects of translocation on the donor
population have been assessed and evaluated. When
removals from source populations are large relative
to its size, problems may arise (Stevens & Goodson
1993). Sometimes a species may become so threatened
in the wild that it is taken into captivity, and the loss
of wild animals may leave only captive populations.
Examples include the Arabian oryx (
Oryx leucoryx
),
the Przewalski horse (
Equus przewalskii
) and the
Sorocco dove (
Zenaida graysoni
; Stanley-Price 1989).
In such cases there is still the potential to breed spe-
cies in captivity although the results of genetic and
phenotypic changes such as genetic drift, inbreeding,
domestication, increased tameness and the loss of
behavioural traits will tend to preclude the chances
for successful re-introduction and subsequent
in situ
conservation. However, many attempts are and should
be made to conserve and restore critically threatened
species through the re-introduction of captive-bred ani-
mals into suitable habitats. Recent examples include
programmes for the black-footed ferret (
Mustela
nigripes
), the golden lion tamarin (
Leontopithecus
rosalia
) and the red wolf (
Canis rufus
). Unfortunately,
the success rate of re-introduced captive-bred indi-
viduals is highly variable and often very low (James
et al.
1983).
A special hazard to successful re-introductions of
animals is the risk of disease introduction. The guide-
lines of IUCN (1995) prescribe that prospective release
stock should be subjected to a thorough veterinary
screening process before transport from the original
source. There are many examples of devastating
effects of diseases introduced unintentionally. From 1893
to 1906, 332 elk (
Cervus canadensis
) were released
in the Adirondack region of New York. Additional
animals were released in 1916 and 1932. The releases
initially appeared successful, and in 1906 the popu-
lation was estimated at 350 elk. However, the elk slowly
disappeared, and there has not been an authenticated
report of elk in the Adirondacks since 1953. The para-
sitic round worm
Pneumostrongylus tenuis
was the
likely cause of the failure of elk to survive in the
Adirondacks (Severinghaus & Darrow 1976). How-
ever, it is not sure when the round worm appeared
for the first time in this area.
7.3 Founding numbers, diversity and
population structure
In general, the number of individuals that are released
in re-introduction attempts is small. This means that
founding groups are susceptible to the same dan-
gers of increased extinction risks as small, natural
populations: environmental fluctuations, demographic
stochasticity and inbreeding. Therefore, to achieve
the highest possible success, a primary goal of re-
introduction should be to maximize the initial rate
of population increase in order to shorten the period
during which the introduced population is exposed to
these risks. This can be brought about by releasing a
high number of individuals in a high-quality habitat.
Komers and Curman (2000) investigated how the
rate of increase of more than 30 newly re-introduced
populations was affected by various population char-
acteristics such as population size, sex and age struc-
ture in Artiodactyla (even-toed ungulates). Their
results were in line with the general notion that
re-introduction success increases with the number of
animals released (Fig. 7.1). The function became
asymptotic at about 20 animals. When fewer than
20 animals were released, the variance in growth rate
increased substantially and, of a number of factors,
only the age structure explained a significant portion
of this variance. The population growth increased
