Latitudinal gradient of thermal safety margin in an Australian damselfly: implications for population vulnerability.

The thermal tolerance of species may be exceeded by the predicted temperature increases and thus contribute to species extinction. However, the impact of temperature increases is thought to vary between climate regions and across latitudes. Here, we aim to establish the vulnerability of an ectothermic insect to a warming climate by estimating the thermal safety margin in Ischnura heterosticta damselflies. We measured the critical thermal maximum (CTmax) along a latitudinal gradient of 17° from 21 populations along the eastern coast of Australia. Our results showed that damselflies inhabiting tropical regions had higher CTmax than temperate damselflies. CTmax increased with increasing mean temperature and decreasing latitude. We further found a positive correlation between damselfly parasite number and temperature. Body size, body condition and sex had no impact on CTmax. Our projections showed that the damselfly thermal safety margin will be narrower in the tropics compared with temperate regions under a predicted 2.6°C annual mean temperature (future projected - current) increase for the years 2061-2080. Therefore, damselflies in the tropics are likely to be more vulnerable to climate change-driven extinction even though they have a relatively higher CTmax. Nevertheless, behaviour, temperature adaptation and thermal plasticity might mitigate predicted vulnerability.