Environmental Engineering Reference
In-Depth Information
Haines 1978). The events of 1997 crystallized observations of an ant 'infestation'
made much earlier, in 1989. At this time the ant was identifi ed as
A
.
gracilipes
, but
the discovery of what in hindsight was a small YCA supercolony occurred prior
to most of the subsequent research on red land crabs, and the supercolony and its
potential signifi cance were almost forgotten until the discoveries in 1997.
h e task of engaging key stakeholders began. Local PANCI managers were
immediately convinced of the threat. However, persuading the administrative
o cers in Darwin (Parks Australia North) and Canberra (Environment Australia)
was more challenging, because unlike the local staff , they had no fi rst-hand experi-
ence of YCA supercolonies and their impacts, especially on red crabs. Furthermore,
the notion that a single invasive ant species could extirpate tens of millions of the
dominant red crab was met in at least one instance with open scepticism. Others
felt that the ant invasion could be transient—an irruption soon followed by col-
lapse and recovery (cf. Simberloff and Gibbons 2004). Rapid, quantifi ed assess-
ment of impacts at several sites on the island (O'Dowd
et al
. 1999) helped tip the
scales. Public interest generated through the media also helped to maintain focus
on the problem.
Initial scoping of the distribution of the supercolonies was ad hoc, involving
infrequent but epic treks through remote tracts of the rugged island. h isillustrated
that supercolonies were widespread, but reinforced the need for a systematic island-
wide survey (IWS) of the invasion (O'Dowd
et al
. 1999). h e IWS was based on a
grid of 1024 waypoints spread across the island (including rainforest, built envir-
onment, and areas cleared for phosphate mining) on a grid of 364 m intervals. h is
interval coincided with an existing network of overgrown 'drill-lines' bulldozed
across much of the island plateau in the 1960s for phosphate exploration. Drill-
lines were crucial because they provided ready access for fi eld crews conducting the
survey. Survey also depended on the existing Christmas Island GIS system. Each
waypoint was off set into undisturbed forest and fi eld crews used hand-held GPS
units to locate them. At each waypoint, a 50 m transect was set out on which YCA
activity was recorded using card counts (Abbott 2005); red crab burrow density
was used as an indicator of impact at each waypoint. Supercolonies were defi ned
operationally (rather than biologically) as those areas where there were su cient
ants to cause death of the red crab. Data were displayed in ARCView to show the
spatial distribution of ant supercolonies in relation to crab burrow densities.
h e initial IWS took 3 months to complete and revealed three key fi ndings.
First, YCA were widespread and occurred at 47.6% (359/754) of waypoints in
undisturbed rainforest (Fig. 11.4a). Second, supercolonies were found at 24.0%
Fig. 11.3
Timeline from 1995-2007 showing key events leading to the aerial
control campaign of the yellow crazy ant in 2002 and its aftermath. Key dates
and events in the rapid build-up, conduct, and evaluation of the aerial campaign
are telescoped to the right.
