Environmental Engineering Reference
In-Depth Information
The mesopredator release effect
When an alien predator and an alien prey species co-occur, removal of the predator
can lead to the release of the prey from top-down regulation (Zavaleta
et al
. 2001).
This process of the rapid expansion of a prey population once top-down control
by a predator has disappeared is termed
mesopredator release
(Fig. 15.1b), and
can lead to negative effects if the increased alien prey population competes with or
consumes native biota (Zavaleta 2002).
On Amsterdam Island, the attempted reduction of the cat population was aban-
doned as it was alleged to have caused a compensating rise in the number of rats
and mice (
Mus musculus
), just as is predicted by the mesopredator release theory
(Holdgate and Wace 1961). However, through the collection of long-term data
(1972-2007) on the productivity of the threatened Cook's petrel (
Pterodroma
cookii
) breeding on Little Barrier Island, New Zealand, and the sequential removal
of cats (1980) and Pacifi c rats (2004) from the island, Rayner
et al
.
(2007) tested
the predictions of the mesopredator release hypothesis. h is study did in fact fi nd
that the removal of cats resulted in an increase in the predatory impacts of Pacifi c
rats, and more importantly, a decline in the fecundity of the Cook's petrel (Rayner
et al
. 2007). Furthermore, the removal of both cats and Pacifi c rats resulted in an
increase of Cook's petrel breeding success to a level above that recorded when both
introduced predators were present on the island. A further fi nding of conservation
importance was the altitudinal variation in the impact of rats on the Cook's petrel
productivity. h e observed spatial variation in the mesopredator release was attrib-
uted to the interactions between environmental gradients, resource availability,
and the nutritional requirements of Pacifi c rats. As noted by Rayner
et al
.
(2007),
local variation in the outcomes of mesopredator release has signifi cant implica-
tions for island restoration, and provides further support regarding the importance
of ecosystem level understanding to predict the potential impacts of introduced
species management on oceanic islands.
h e presence of a third predator in the prey-mesopredator-superpredator sys-
tem complicates matters further. h e managers of Bird Island, Seychelles, conser-
vation programme were aware of the potential dangers of removing a superpredator
from an ecosystem also containing a mesopredator, and as such took the cautious
approach of simultaneously removing both introduced cats and rats in order to
protect the local bird colonies. Unfortunately, the presence of the introduced
crazy ant (
Anoplolepis longipes
), in very low numbers on the island, was overlooked
(Feare 1999). It appears that the ant larvae could be an important prey item of
the introduced rodents, and the rat eradication led to a demographic explosion
of the ants (Feare 1999). h is resulted in the ants extending their range over the
island and impacting heavily on land crab and bird colonies. h is example once
again highlights the importance of pre-eradication, particularly diet, studies in
order to obtain a thorough picture of the trophic web interactions occurring in an
ecosystem.
When alien predators and prey co-occur, eradication of only the alien prey can
cause the predator to switch to native prey. In New Zealand, because rats are a
