Environmental Engineering Reference
In-Depth Information
terraces running parallel to the coast. Average rainfall is ca. 2000 mm, most of
which falls between November-May. The island is covered by structurally simple,
broadleaved rainforest. Of the 51 tree species present, ca. 25 are common canopy
trees. The island is an Australian external territory of outstanding national and
international conservation signifi cance. BirdLife International has listed the island
as an Endemic Bird Area and there are two Ramsar Wetland Sites of International
Importance. Over 75% of the rainforest that originally cloaked the island still
remains, making Christmas Island one of the best-preserved insular tropical eco-
systems anywhere in the world. Rainforest dynamics are dominated by the activ-
ities of abundant land crabs, including the red crab (
Gecarcoidea natalis
). This
native omnivore regulates seedling recruitment and litter breakdown across the
island rainforest (e.g. Green
et al
. 1997, 1999, 2008).
h e YCA has been present on Christmas Island since at least the 1930s, but
supercolony formation is a relatively recent phenomenon, and has occurred mostly
since the mid-1990s. h e two supercolonies that triggered the emergency response
were detected incidentally in 1997, during long-term research on the eff ect of the
native redland crab on seedling recruitment (Green
et al
. 1997, 2008). By 2002,
YCA supercolonies had formed across 3000 ha of rainforest—about 30% of all
island forest. Most of these supercolonies had formed in the Christmas Island
National Park, which comprises 63% of the island.
On the forest fl oor,
Anoplolepis
has extirpated millions of red crabs from large
tracts of rainforest (O'Dowd
et al
. 2003), which has resulted in the formation of
distinctive forest states across the landscape, with altered resource levels and habi-
tat structure (O'Dowd
et al
. 2003). In the forest canopy, the ant forms new associ-
ations with herbivorous, honeydew-secreting Hemiptera (Fig. 11.2a; O'Dowd
et
al
. 1999; Abbott and Green 2007) that result in reciprocal increases in their popu-
lation sizes. h e combined direct and indirect eff ects of the YCA and several species
of scale insects have been rapid and multidirectional, aff ecting forest structure and
composition, species of special conservation value (O'Dowd
et al
. 1999, 2003),
ecosystem processes (Davis
et al
. 2008), and secondary invasions (O'Dowd and
Green 2009). h is demanded a coherent, coordinated response among scientists,
managers, and policymakers.
Although the identity of the ant species was unknown when expansive supercolo-
nies were detected in 1997, it was immediately obvious that this ant affected the
red land crab, seedling recruitment, scale insect populations, and litter decom-
position (Fig. 11.3). Even in the absence of identifi cation, the urgency of a man-
agement response was clear. Parks Australia North Christmas Island (PANCI),
the responsible management authority, was notifi ed immediately. This sense of
urgency was heightened by the subsequent authoritative identifi cation of voucher
specimens, followed by a literature search (O'Dowd
et al
. 1999) that revealed some
aspects of its biology and impacts, especially in the Seychelles (e.g. Haines and
