Environmental Engineering Reference
In-Depth Information
regulatory issues and the dilemmas these are posing to practitioners. Finally,
we discuss some of the general constraints faced when trying to implement bio-
logical control projects. Case studies are presented to illustrate particular points
and principles.
tool for managing invasive species
One major theory which has been used to explain biological invasions is that
when a species is 'released' from its natural enemies, for example, through intro-
duction by some means into a new geographical area, that species becomes
invasive because the natural enemies no longer exert control. This theory has
been implicit in the ecological literature for many decades but was made expli-
cit through the 'Enemy Release Hypothesis' (ERH) (Keane and Crawley 2002).
Whilst this is not the only theory that attempts to explain biological invasions
(for a review see Mack
et al
. 2000) many now accept this as at least a core com-
ponent in generating invasions. There are also elaborations of the ERH, the
most cited being that related to plant invasive species. Here it is suggested that a
plant released from herbivore pressure is able to evolve more in competitive traits
(Blossey and Nötzold 1995).
Numerous studies since the beginnings of exact ecological science in the early
20
th
century have shown that natural enemies can suppress and/or regulate
populations of their host (e.g. see reviews in Cullen and Hassan 1988; Jervis and
Kidd 1996). All species in their native range host several natural enemies and
it is now well known that these natural enemy communities can sometimes be
diverse, especially on hosts such as the majority of plant and arthropod species.
h ese natural enemies can either be specialist or generalist, the long association
with a host generating a co-evolutionary relationship. It has been found that
specialist natural enemies tend to have a major impact on their host. h e species
composition of natural enemy communities frequently varies throughout the
native range of the host species, particular species being adapted to local condi-
tions within the host's range. But some natural enemies do have a wide geograph-
ical range themselves and some studies have shown that, in least in some cases,
natural enemies can limit the distribution of their host as well as abundance
(Harrison and Wilcox 1995).
Classical biological-control practice is based on this ecological theory and sup-
porting practical studies, but biological-control practitioners were using the notion
of the ERH back in the early 1900s. h e theory behind biological control is at the
core of the argument of why this technology is considered by many (see Hoddle
2002, for example) to be the best way of controlling many invasive species: host-
specifi c natural enemies are argued to be environmentally benign and in the case of
classical biological control, the agents are self-replicating and thus able to disperse
naturally in natural environments.
