is an annual species that is abundant and widely distributed across southeastern Australia. Males and females emerge and mate in late spring/early summer (November/December), the males then die and the females become dormant until late summer/early autumn (February/March) after which they begin to oviposit on a wide range of grasses (Edwards 1973
). Observed emergence times for H. merope
in the vicinity of Melbourne, Victoria (37.60–38.54° S, 144.17–145.48° E) were based on opportunistically collected museum records and privately collected data courtesy of Kelvyn L. Dunn. We used the 10-year average of the earliest observed record per year from 1941–2005 as the emergence date. While the use of opportunistically collected data probably adds considerable noise to any signal of phenological shift, we accounted for any chronological bias by including the number of museum records as a covariate in the analysis of the trend through time.
The thermal dependence of development rate for eggs, larvae and pupae was determined from the offspring of 10 field-collected females, raised individually on the grass Ehrharta erecta
(Poaceae) from egg to emergence in glass vials (height 130 mm, diameter 25 mm) sealed with a moistened foam stopper. Temperature was controlled at 8°C, 12°C, 15°C, 25°C or 30°C with a 12 : 12 light : dark photoperiod. We observed the animals daily and recorded time to completion of each life cycle stage, the inverse of this being the developmental rate. We then fitted polynomial regressions to the relationship between development rate and temperature for each life stage, using the adjusted r2
method (Quinn & Keough 2002
We used historical weather data for 1945–2007 (Bureau of Meteorology, Australia) from Laverton (37.86° S, 144.76° E), a rural site close to Melbourne, to model the physiological response of H. merope
to temperature. This station is a ‘high-quality’ site, unaffected by changes in exposure, urbanization, instrumentation, etc., during the study period. Weather records (mean monthly maximum and minimum air temperature, wind speed and cloud cover) were translated into microclimates experienced by immature H. merope
using biophysical modelling software (Niche
; Porter & Mitchell 2006
). This software includes a microclimate model that translates weather-station records into near-surface air temperature and wind speed profiles and a soil surface energy balance (Porter et al. 1973
). The model includes a first-principles model of solar radiation (McCullough & Porter 1971
) and uses cloud cover records to predict long- and short-wave radiation loads. Individuals were assumed to be 3 cm above the ground in grass tussocks (75% shade), based on our observations of larval behaviour. Microclimate model predictions compared well with field microclimate measurements (figure S1, electronic supplementary material).
Animals were assumed to be at the 3 cm air temperature. Predicted daily cycles in animal body temperature were converted to constant temperature equivalents (CTEs) (as implemented in Mitchell et al. 2008
), which were then used as independent variables in the fitted equations for thermal dependence of development rate. There was a good agreement between modelled CTE-based development times and the development times measured under natural, fluctuating thermal conditions in grass tussocks in Melbourne over the winter of 2007 (table S1, electronic supplementary material). We encoded the equations for development rate and CTE calculations into the Ectotherm model of Niche
. The Ectotherm program tracked developmental stage on an hourly basis given a specified laying date (11 April—females lay from around March to April in Melbourne; Pearse 1978
) and outputted the time of completion of development based on observed meteorological data from 1944 to 2005. We calculated separate emergence dates for each year and then took the average value for each 10-year window as the final emergence date. Note that we did not solve an energy balance specifically for the caterpillars with the Ectotherm model but rather assumed that they were at the shaded 3 cm air temperature.
We compared observed temperature trends from the high-quality weather station in Laverton with output from extended climate model simulations, both including and excluding anthropogenic climate forcing for the single-model grid box overlying Melbourne and Laverton. Anthropogenic climate forcing included observed increases in greenhouse gases and estimated variations of anthropogenic aerosols, whereas natural external climate forcing included estimated changes in solar irradiance and volcanic aerosols. Multi-member ensembles of simulations from four different climate models with prescribed changes in both anthropogenic and natural external climate forcing were used to provide regional temperature data for 1944–2007 (table S2, electronic supplementary material). The range of possible trends owing to natural internal climate variability was estimated using the variability of regional temperature from extended control model simulations (including only natural climate variation with no changes in external forcing).