Abstract
Climate change involves increases in mean temperature and changes in temperature variability at multiple temporal scales, but research rarely considers the effects of temperature variability across scales. All climate change scenarios predict global increases of mean temperatures, as well as changes in the magnitude of diel and annual temperature variability.The overall aim of this thesis was to use latitudinal and elevational gradients in temperate and tropical regions to understand the potential for aquatic invertebrates to adapt to the impacts of climate change. Within this over-arching aim the following objectives were addressed: 1) What are the predictors of thermal breath in tropical and temperate freshwater insects, and how do these relate to existing general theories on thermal tolerances? 2) How can information on thermal breadth be used to determine vulnerability to climate change? 3) Are thermal traits related to other organismal traits? 4) Is there a phylogenetic basis for thermal tolerance and what is the role of cryptic species? and 5) How does organismal performance relate to thermal conditions?
The Climate Variability Hypothesis (CVH) (Janzen, 1967) predicts ectotherms in temperate regions tolerate a wider range of temperatures than those in tropical regions in response to greater annual variability in temperate regions. We measured thermal regimes and critical thermal limits of freshwater insects along elevation gradients in streams in temperate and tropical regions of eastern Australia and determined which variables were most correlated with thermal breadth. Consistent with the CVH, thermal breadth tended to increase with increasing annual temperature range. We also found some support for a related hypothesis, the Climate Extreme Hypothesis (CEH), particularly for predicting upper thermal limits. These results suggest that temperate organisms may be able to tolerate wider ranges of temperatures than tropical organisms.
Understanding the impact of evolutionary experienced temperatures on the overall temperature tolerance range of species is critical to predicting effects on species survival. The CVH evokes annual temperature variability experienced over evolutionary time to determine species’ thermal breadth, resulting in narrow temperature tolerance ranges among organisms in thermally stable habitats like the tropics. In contrast, the Thermal Constraint Hypothesis (TCH) (Payne and Smith 2017) proposes that acute contemporary effects of high temperature generate physiological trade-offs, resulting in narrow Tbr among temperature specialists in warm habitats. We experimentally measured the temperature tolerance of aquatic macroinvertebrates and related them to the thermal conditions of their habitats via comparisons between temperate locations (high seasonal variability) and tropical locations (low seasonal variability) across elevational gradients in three continents. Our results show that temperate species have wider Tbr than tropical species. This supports the conclusion that the narrow Tbr of tropical organisms compared to temperate is determined by adaptive evolution (Janzen, 1967) and not by physiological trade-offs based on thermodynamics (Payne & Smith, 2017).
Understanding the potential impacts of climate change requires understanding how temperature variability affects individual species’ thermal tolerance. We compared sublethal critical thermal limits of three aquatic macroinvertebrate morphospecies in temperate (high seasonal variability) and tropical (low seasonal variability) locations in Australia. We used single-nucleotide polymorphisms (SNPs) to analyse geographic patterns and temperature tolerance of morphologically identical species. We found that thermal tolerance was related to phylogenetic associations, suggesting an important role for evolutionary mechanisms in driving thermal tolerance and the potential to use phylogenetic associations to predict vulnerability to climate change.
Our results suggest that it is not possible to make broad global assumptions about the thermal vulnerability of species. We must therefore account for local temperature regimes, as well as local climate change predictions, as different regions are impacted differently by changes in annual variability, annual mean temperatures as well as extreme events. Without understanding the impact of climate change induced changes on annual temperature variability, predictions on extinction risks of aquatic insects, and following indirect effects of temperature changes on higher tropic levels, might be underestimated.
| Date of Award | 2025 |
|---|---|
| Original language | English |
| Supervisor | Ross THOMPSON (Supervisor), LeRoy POFF (Supervisor) & Ben KEFFORD (Supervisor) |