Environmental Engineering Reference
In-Depth Information
detect any potential unwanted results (i.e. the release of the vine from top-down
regulation) arising from the eradication of the herbivores.
h e eff ects of alien herbivore removal on native vegetation, under certain cir-
cumstances, might also have indirect negative eff ects, because of the presence of
other alien animals (Zavaleta
et al
. 2001). Rabbit removal on Macquarie Island
led to major increases in cover of the native tussock grass
Poa foliosa
(Copson and
Whinam 1998).
h e expansion of this tussock species, the preferred habitat of the
introduced ship rat (
Rattus rattus
), could expand the range of rats on the island and
consequently bring them into contact with burrow-nesting bird colonies on the
island, which have escaped rat predation so far (Copson and Whinam 1998).
Donlan
et al
.
(2002) used both large- and small-scale experimental manipula-
tions to investigate the impact and recovery of an island plant community follow-
ing the removal of exotic herbivores from the San Benito Islands, Mexico. h e
hypotheses tested were:
•
With herbivore removal, plant community structure changes due to the
release of top-down regulation.
The response by the plant community is predictable from the hierarchy of
•
herbivore preference.
Removal of European rabbits, donkeys, and goats began on San Benito West in
early 1998, while removal of rabbits on San Benito East was postponed until late
1999 to facilitate the comparison between the two islands. During the course of
the San Benito West eradication programme, the food preferences of rabbits and
exclosure plot studies were conducted on San Benito East. Results from the food-
preference trials accurately predicted changes in the perennial plant community:
the changes in relative abundance of the plant species were positively correlated
with the preference hierarchy on San Benito West (herbivores removed), and nega-
tively correlated on San Benito East (herbivores present). Despite the relationship
between herbivore food preference and changes in plant cover providing strong
evidence of a top-down eff ect, recovery of the ecosystem was shown to depend on
the bottom-up eff ects of resources such as water availability (Donlan
et al
. 2002).
In an attempt to preserve and restore an area of tropical dry forest on Hawaii, an
area of 2.3 ha (Kaupulehu preserve) was fenced in 1956 to exclude cattle and feral
goats. Cabin
et al
.
(2000) examined the eff ects of this long-term ungulate exclu-
sion from the Kaupulehu preserve and the recent control of rodents (
Rattus rattus,
R
.
exulans
,
and
Mus musculus
) by comparing the fl ora present to that of an adjacent
area subjected to continuous grazing since the fence was constructed. Compared
to the adjacent area, the preserve had a relatively diverse fl ora with substantially
greater coverage of native overstorey and understorey species. However, Cabin
et al
.
(2000) noted that the dominant herbaceous cover of alien fountain grass
(
Pennisetum setaceum
) and predation by rodents had thwarted the regeneration of
the native canopy trees within the preserve. h ese results once again indicate that
in restoration programmes, the removal of an alien herbivorous species should be
viewed as the fi rst critical step in the recovery of the ecosystem.
