Environmental Engineering Reference
In-Depth Information
Zavaleta
et al
. (2001) predict that the greatest potential for negative impacts on
native vegetation exists when herbivore eradication removes the disturbance that
is necessary to suppress establishment of late successional (tree or shrub) aliens.
For example, on San Cristobal Island, Galapagos, the removal of feral cattle from
degraded grasslands containing suppressed populations of exotic guava (
Psidium
guajava
) led to the rapid growth of this plant into dense and extensive thickets
(Eckhardt 1972).
h e removal of introduced pigs and goats on Sarigan Island provides another
example of the devastating eff ects that the removal of herbivores can have on the
fl ora. Due to logistical di culties, the management programme for the island
included only a minimal pre-eradication study. While the programme was suc-
cessful in removing the introduced ungulates, it failed in its ability to detect the
presence of the introduced vine
Operculina ventricosa
, which appeared to be a pref-
erential food item for the goats. h e release from grazing pressure enabled the
introduced plants to fully express their competitive superiority over the native
plants, resulting in their rapid invasion of the community as quickly as 2 years
after the removal of the alien grazers (Fig. 15.3) (Kessler 2002). Future monitoring
will be required to determine what eff ect the vine will have on the regeneration
and expansion of the native forest and its fauna. A more thorough pre-eradication
study incorporating simple fenced exclosure plots would have helped managers to
Fig. 15.3
Control of herbivores without taking into account introduced plants
may lead to undesired chain reactions, as occured on Sarigan Island, with the
invasion of
Operculina ventricosa
following goat removal. Photo: Curt Kessler.
