Environmental Engineering Reference
In-Depth Information
There are two general strategies used in eradication that have different effects on
the question of increasing scale. The fi rst type is where a single operation is applied
with the expectation that all the pests will be killed (e.g. in aerial baiting for rodents
where baits are presented across the whole landscape over a short period (Cromarty
et al
. 2002)). The second type is where repeated control events, often using differ-
ent techniques as densities decline, are applied until no target pests remain (e.g. for
weeds especially where seed banks ensure the need for repeated control (Panetta
2007) or for ungulates where the control is by shooting (Cruz
et al
. 2005). There
are some intermediate scenarios such as in eradications of rabbits, where aerial
baiting has so far almost always failed to kill 100% and the operations have had to
detect and rapidly mop-up survivors to achieve eradication (Parkes 2006c).
It is not clear that increasing scale causes any intrinsic increase in risk of failure
of aerial baiting of rodents. It may be just a matter of more helicopters and more
bait. However, we can speculate that (a) increasing scale increases risks correlated
with increasing habitat complexity which means that some pests do not encounter
baits; and (b) if any rodent has a minute but positive chance of survival that sheer
numbers (correlated with scale) will increase the risk that some will survive despite
exposure to baits.
However, scale aff ects the second strategy more obviously because recruitment
and immigration are inevitable during the spaced control events. Recruitment
can be managed by more intensive control applied more frequently to meet the
fi rst rule of eradication, but immigration can be a major problem as scale increases
and the target pest has to be managed in 'bite-sized' units. A comparison between
two successful feral pig eradications on large islands, both having access to similar
control methods, illustrates this. On 58,465 ha Santiago Island (Galapagos) feral
pigs were eradicated without the use of fencing to divide the island, but it took 30
years (Cruz
et al
. 2005). In contrast, on 24,900 ha Santa Cruz Island (California)
feral pigs were eradicated within 1 year after the island was fenced into fi ve blocks
and the problem of 'back-fi lling' dispersal into previously cleared blocks thereby
resolved (Morrison
et al
. 2007).
Scale is important in terms of the benefi ts of eradication. Island biogeographic
principles (MacArthur and Wilson 1967) would suggest that increasing the size
of islands from which pests have been removed increases the biodiversity benefi ts
achieved or made possible, although no one to our knowledge has analysed these
benefi ts across insular eradications.
One particularly diffi cult issue on very large islands or continents is how to deal
with uncertainty about the distribution of the pest or weed. This is of course not a
problem on smaller islands as the sea is assumed to be the boundary in most cases.
Delimitation of distributions is a critical and expensive issue for weeds because,
however successful a project is at removing known infestations, undetected plants
