Abstract
The Climate Variability Hypothesis (CVH) predicts that annual temperature variation plays an important role in predicting species' thermal breadths and, thus, their elevational distributions. Species at tropical latitudes are predicted to evolve narrow thermal breadths and have narrow elevational ranges because they experience low annual temperature variability. Species from temperate latitudes are predicted to evolve broad thermal breadths and have broad elevational ranges because they experience high annual temperature variability. The CVH has been an influential hypothesis in predicting community assembly, phylogeography, and potential vulnerability to climate change.To test the predictions of the CVH, I collected aquatic insect larvae from the orders Ephemeroptera, Plecoptera, and Trichoptera along elevational gradients in temperate and tropical Australia. I used single nucleotide polymorphism (SNP) sequencing to determine the proportion of shared alleles among populations as a measure of genetic differentiation. Partial Mantel tests were used to test the relationship between the 1-proportion of shared alleles and temperature while accounting for geographical distance. Temperature relationships were tested with the difference in mean temperature and the difference in the annual temperature range. The elevational range sizes of the tropical and temperate Australian species were compared with similar data collected previously in tropical and temperate Americas. The findings of this thesis did not support the predictions of the CVH in Australia. Five species in the tropics and six in the temperate region were used to test the relationship between annual temperature range and gene flow. One of five tropical and four of six temperate species had a significant negative correlation between annual temperature range and gene flow. Four species in the tropics and five in the temperate region were used to test the relationship between mean temperature and gene flow. Two of four tropical species and three of five temperate species had a significant negative correlation between mean temperature and gene flow.
In tropical Australia, nearly half of the taxa (41%) occupied close to the entire elevational gradient of 880 meters, indicating that these species were not restricted in their distributions as predicted by the CVH. Conversely, in temperate Australia, 58% of the taxa had elevational ranges less than half of the 1700m of available gradient, with most occurring within ranges of 500m or less. These findings suggest that temperate Australian species have more restricted elevational distributions than tropical Australian species, in contrast to the predictions of the iv
CVH. The patterns of small, realised niches of Australian temperate taxa are hypothesised to occur as poor competitive species are pushed up the elevational gradient into colder elevations with less competition pressure, leading to reduced elevational ranges.
The findings of this thesis suggest that the CVH alone has limitations globally as a generalised model for predicting species distributions along elevational gradients. Strong support for the predictions of the CVH has been found in the Northern Hemisphere. Still, this study and previous studies show that the CVH is either not supported or weakly supported in the Southern Hemisphere, as more nuanced patterns than the CVH can account for occur. In the Australian setting, this may be because the elevations of the highest tropical mountains are not sufficiently high for thermal limitation (Janzen’s “Mountain passes in the tropics may be higher”) to be a dispersal limiting process. Other factors affecting the generality of the CVH for elevational distributions may include differences in thermal conditions through evolutionary time and patterns of speciation following Quaternary glacial retreat.
As a general theory relating temperature regimes to species’ elevational distributions, the CVH has been evocative as a potential tool to predict responses to climate change. The results shown here suggest that the Hypothesis may not be sufficiently general to be applied this way and that any such predictions need to be made with extreme caution, particularly in the Southern Hemisphere.
| Date of Award | 2025 |
|---|---|
| Original language | English |
| Supervisor | Ben KEFFORD (Supervisor), LeRoy POFF (Supervisor) & Ross THOMPSON (Supervisor) |