Temperature tolerance at multiple scales

Submitted by editor on 10 June 2016. Get the paper!
A chameleon grasshopper (Kosicuscola tristis) and alpine skink (Pseudemoia sp.), share a spot in the sun. As ectotherms, their body temperature is directly linked to environmental conditions, making them good candidates for studies on thermal tolerance. Photo: Rachel Slatyer.

by Rachel Slatyer

The distribution of a species is determined, at least to some extent, by the environmental conditions it can tolerate. This link provides a means of predicting and explaining species distributions, and forecasting how these distributions might be altered with continuing climate change. Defining tolerance limits for a species is not straightforward, however, as these limits are not necessarily static traits of a species – they can vary both among populations. Differences among populations could be driven by spatial variation in selection; latitudinal gradients in temperature are a classic example.

Our aim was to test the relationship between interspecific and intraspecific variation in thermal tolerance. Our study focused on three species of small, flightless grasshopper (genus Kosciuscola) endemic to the mountains of south-eastern Australian. Mountain environments are interesting systems for studying thermal constraints, as temperature can vary enormously over very small distances. Ambient temperature decreases with elevation, akin to latitudinal temperature gradient. At the same time, small-scale (e.g. metres, or even centimetres) temperature variation tends to increase towards higher elevations. Individuals occupying different elevations are thus likely to experience quite different thermal conditions.

For each species in our study, we tested heat and cold tolerance in one high-elevation population and one low-elevation population. We found that while there was significant differences in cold tolerance among species, variation in heat tolerance was only apparent within species. Our results emphasise that thermal tolerance traits can vary independently between species and populations and, as such, both levels of variation should be considered when linking these traits with species distributions. Counter-intuitively, we also showed that individuals from treeless alpine habitat were generally more heat tolerant than their lower-elevation counterparts. This result supports microclimate data and recent models that both suggest individuals at higher elevations might be more exposed to temperature extremes than those living at slightly lower elevations where vegetation cover is greater.

Treeless alpine habitat provides little shelter from the sun. High solar radiation means that near-ground temperatures can approach 50°C in summer, even though ambient air temperature is much colder. Together, these two factors mean that individuals living in alpine habitat could be exposed to more extreme high temperatures than those living lower down the mountain. Photo: Rachel Slatyer.