Australia’s Dengue Risk: Human Adaptation to Climate Change - Recent Advances and Issues in Environmental Science - page 136

climate layers and elevation (the ANUDEM generated DEM is described above) to
model the range of Ae. aegypti . Species record sites and the climate change layers for
eight environmental parameters were derived from the climate change scenarios for
2030 and 2050 as well as the elevation layer. We utilized the medium sensitivity which
corresponds to a global warming of 2.6°C for a doubling of CO 2 from 280 to 560 ppm
[27]. The GARP procedure was implemented using half of the species record sites as a
training data set for model building and the other half for model testing. Optimization
parameters included 100 models for each run with 1,000 iterations per model and 0.01
convergence limits. The best subsets procedure [28] was used to select five models
which were added together using TNTmips to produce predicted range maps for each
species.
Theoretical Temperature Limits for Ae. aegypti Extrapolated Across Australia
Previous studies of the distributional limits of Ae. aegypti were used to develop dis-
tribution maps for Australia. Christophers [5] hypothesized a climate limit of 10°C
winter isotherm based on historical global collection data and laboratory-based experi-
ments. We also evaluated the hypothetical limit from Otero and colleagues [6], who
used a complex stochastic population model that incorporates the lifecycle parameters
of Ae. aegypti to suggest a 15°C annual mean isotherm. Both these values were incor-
porated into distributional maps of Australia using TNTmips.
Climate Limit of Dengue Transmission in Australia
Dengue transmission maps were developed using data from historical dengue out-
breaks in Australia [12]. This work found that these dengue epidemics ceased when
the outside temperature reached 14-15°C wet bulb isotherm and that a single param-
eter of 14.2°C annual mean wet bulb isotherm (T W ) best approximated the limit of
the 1926 epidemic--probably as a result of reducing the mosquitoes' feeding activity
and the ability of the virus to replicate. This 14.2°C annual mean wet bulb isotherm
value was mapped onto Australia for the current climate using TNTmips and three
seasonal increments: the annual mean, the warmest quarter (December-February), and
the coolest quarter (June-August).
Distributional Projections of Ae. aegypti : GARP Modeling
Distribution sites for Ae. aegypti in Australia (234 sites) were collated and displayed
in a single map using GPS coordinates (Figure 1). Ecological niche models were
built with desktop GARP to produce a best subset model that showed agreement
with the full complement of Ae. aegypti collections in Australia (Figure 2A). In this
projection, much of northern, eastern and southeast Australia was projected to pres-
ent a suitable niche. This model closely tracks an annual rainfall pattern of less than
300 mm. However, the excluded region around central Australia included two Ae.
aegypti positive collection sites (Meekatharra in central Western Australia and Boulia
in Queensland): both collection localities are small regional centers on main inland
transport routes.