Principles of containment and control of invasive species - Invasive Species Management - page 63

strategy only for organisms that are expanding their range and where it may be
possible to curb or halt the rate of range expansion. It is conceivable that a 'per-
manent' barrier to range expansion could be put in place, or that an effective bio-
logical control programme alters the demographics of the invasive species so that
the likelihood of range expansion is radically reduced. However, there are few,
if any, examples of this being effective, and many where it has not. Historical
examples of attempts to impose barriers to range expansion such as the construc-
tion of thousands of kilometres of fences to prevent the spread of European rab-
bits ( Oryctolagus cuniculus ) in Australia were notorious failures (Rolls 1969; Noble
1997). The 'end-point' of containment programmes is most likely to be that an
invasive species 'is being contained' rather than that it 'has been contained'.
h e eff ort and resources expended in order to maintain containment and control
strategies can indicate success where these decline over time. Rarely, an extremely
successful containment strategy may be replaced by an eradication strategy (Mack
and Foster 2004).

5.2.1 When to contain, when to control
Once eradication has been rejected as a goal, the options are containment, control, or
do nothing. The total area occupied by an invasive species and the complexity of its
distribution will infl uence whether containment is possible. This is because a contain-
ment strategy involves putting most effort into managing a species at the periphery of
its range. The greater the area the species occupies, the more disjunct its distribution
and the more convoluted the range boundary becomes, the longer will be the poten-
tial management periphery. Control then becomes a more likely strategic option.
Consequently, containment is most likely to be viable during only the early stages of
an invasion (Fig. 5.1), consistent with the general benefi t of early intervention in pest
management (McNeeley et al . 2005). Whatever the strategy, the earlier management
action is commenced, the more economical and successful it is likely to be.

5.2.2 Feasibility of containment
Three sets of factors determine both invasion potential and the feasibility of con-
tainment:

• The characteristics of the invasive species;

• The characteristics of the environment that is being invaded;

• The management regime that is imposed on that environment (Grice 2006).

Together they will determine the resources that would be required in order to
contain the species.
Species that are highly fecund, highly mobile and/or readily dispersed, and that
have short generation times and broad ecological tolerances, are more likely to
rapidly invade suitable environments (Rejmánek et al . 2005). 'Propagule pres-
sure', defi ned as 'the number of individuals released into a region to which they
are not native', is proposed as a major determinant of the establishment success of
an introduced species (Lockwood et al . 2005; Dehnen-Schmutz et al . 2007). It is