Environmental Engineering Reference
In-Depth Information
be higher when the density is low because the same search eff ort may be required
but it is distributed over fewer individuals. Where a broad-scale control method is
employed (e.g. the use of fi re to control invasive plant species) and it is not neces-
sary to treat each individual separately, the costs per unit area may be the same
regardless of the density, though again, the costs per individual treated will be
greater at lower densities.
An ability to direct effort to where the benefi t:cost ratio is high requires that costs
and benefi ts are suffi ciently well known to justify any decisions. It requires some
understanding of how the abundance (density) of the particular invasive species
relates to the impacts that it has, the risks it poses in terms of further population
increase and range expansion, and the costs of management actions against it.
Despite control costs generally being lower where densities of the invasive species
are lower (though see section 5.3.6), locations with both higher abundances and
high value environmental or economic assets may receive greater effort. There may
also be situations where areas of high abundance are given low priority because
assets are already highly degraded. Effort may target infestations of an invasive
species that are located where they present a high risk of giving rise to new infest-
ations or where they are adjacent to high-value assets. Controlling such strategic-
ally located populations would yield a high return on investment.
an expanding distribution
On theoretical grounds, Moody and Mack (1998) showed that control of outlying
populations (nascent foci) of an invasive species is important in slowing the rate of
invasion. Concentrating effort at an invasion front will likewise probably be the
most cost-effective way of containing it, although this may depend on the level of
resources available (Taylor and Hastings 2004). Clearly, this requires knowledge of
the species' distribution and abundance (see section 5.3.3) and of dispersal mecha-
nisms, timing and distances, and a capacity to kill or remove outlying individuals.
Containment involves, in effect, placing barriers to range expansion by reducing
reproductive output, interrupting the dispersal process itself, or curtailing recruit-
ment. For any species a landscape will consist of habitats of varying suitability.
Areas of low-suitability or unsuitable habitat represent at least partial barriers to
range expansion that can be exploited in containment programmes. Their effect-
iveness will depend upon the dispersal distances achieved relative to the width
of areas of poor habitat. Containment effort may be directed so as to enhance
the effectiveness of the barriers. Effective natural barriers could include mountain
ranges, rivers and other water bodies, depending on the ecological requirements of
the invasive species involved.
