Environmental Engineering Reference
In-Depth Information
visiting the islands. While it was originally thought that golden eagles did not stay
on the islands long enough to constitute a threat to the local prey, the discovery
of a nest on Santa Cruz Island revealed this was not the case: the presence of fox
remains in the nest confi rmed eagle predation. Despite these fi ndings, the question
remained: how could the eagle threaten fox survival through predation, if there
was not enough local prey on the island to allow the continuous presence of the
eagles? h e remains of piglets also found in the nest provided the answer. Feral pigs
(introduced in the 1850s) were present on Santa Cruz Island, and by producing
piglets all year round, they provided visiting eagles with enough resources for them
to colonize the island. Even the irregular and low predation rate of eagles on a spe-
cies such as the fox that is ill-adapted to avian predation (both in terms of behav-
iour and life-history strategies), was su cient to drive the fox populations towards
extinction (Roemer et al . 2002). Roemer et al . (2002) were able to show that in
the absence of pigs, any introduction of eagles, however large, will eventually lead
to colonization failure and fox population persistence. However, when pigs are
present, a single eagle pair will be able to colonize the island and build a popula-
tion that is so large that foxes will go extinct, while pigs will remain at moderate
densities. With the use of models to mimic diff erent control strategies (control of
pigs only, of eagles only, or of both species, with diff erent strength) and compari-
son of their relative e ciency, Courchamp et al . (2003b) revealed that the eradi-
cation of pigs (in absence of eagle control), the intended course of action on Santa
Cruz, would lead to the extinction of the fox. In theory, the most e cient solution
would be to remove both eagles and pigs. Each removal project was faced with its
own di culties; the removal of the large pig population was logistically di cult,
whereas the protected status of the golden eagle meant that it had to be removed
from the island via live trapping methods (Courchamp et al . 2003b). h isscenario
illustrates the many challenges, not always apparently obvious, that conservation
workers face in the attempt to conserve threatened species.
15.2.3.2. Interactions resulting from inconspicuous aliens
Release from introduced herbivores
Introduced herbivores are sometimes removed from an ecosystem where they
cause damage that may threaten local fl ora and fauna. However, such manage-
ment actions have shown mixed results with regards to the restoration of the native
vegetation (Coblentz 1978, 1997; Van Vuren and Coblentz 1987). Surveys fol-
lowing the eradication of rabbits and goats ( Capra hircus ) from Round Island,
Mauritius, revealed that in the short term the general predicted effects of eradica-
tion were upheld: increases in plant biomass and tree recruitment (Bullock et al .
2002). However, unpredicted effects (such as differential population responses of
reptiles and increasing rates of establishment and infl uence of non-native plants)
also occurred, leading Bullock et al . (2002) to predict that new ecological commu-
nities (not necessarily dominated by local plants) are likely to develop on Round
Island as a consequence.
 
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