Climatic envelope models focus on the climatic variables affecting species or species assemblages, and are important tools to investigate the effect of climate change on their geographical ranges. These models have largely been proposed in order to make successful predictions on species‘ persistence, determining which variables are likely to induce range expansion, contraction, or shifting. More recent models, including the ability and the cost for individuals to respond promptly to an environmental stimulus, have revealed that species may express phenotypic plasticity able to induce adaptation to the new environment. Consequently, understanding how species evolve to a changing climate is fundamental. From this perspective, investigating intraspecific responses to an environmental variable may contribute to better understanding and prediction of the effect of climate change on the geographical range and evolution of species, particularly in the case of widespread species. In this context, the present study aimed at establishing how environmental variables (focussing mainly on temperature) may have contributed to shape the spatial distribution, physiology, reproductive biology and connectivity of two species of Southern African sandhoppers (Talorchestia capensis and Africorchestia quadrispinosa, Amphipoda, Talitridae). Most of the work was carried out on T. capensis, due to its widespread spatial distribution. A first investigation of the biogeography of T. capensis and A. quadrispinosa, revealed that, for both species, spatial patterns of abundance, size and sex ratio were not explained by the Abundant Centre Hypothesis (greater abundance at the core of a spatial range), but rather guided by bio-physical forces. Precisely, the abundance of sandhoppers was driven by the morphodynamic state of the beach, salinity and temperatures, with strong differentiation among sites that reflected local environmental conditions. In support of these findings, strong population structure in the genetics of T. capensis was found (three main groups) when investigating its phylogeography and genetic connectivity. Although such defined structure may suggests cryptic speciation, the concomitant within-population variation in the COX1 region of mtDNA, also highlighted the importance of individual genetic variability. High individual variability was also found in the response of T. capensis to temperature, both in its physiology (thermal plasticity) and its reproductive biology (maternal effects). Since temperature is one of the main variables affecting the coastal marine systems of southern Africa and the metabolism of animals in general, its effect on the physiology and reproduction of T. capensis was therefore investigated. Thermal responses to increasing/decreasing temperatures were assessed for separated populations of T. capensis. Individual variability was reported in the oxygen consumption of T. capensis in response to temperature (high variation around the means, especially for increasing temperatures). Among population differences in thermal sensitivity were significantly correlated with air temperature variability experienced over the past 23 years, highlighting the importance of historical temperature fluctuations to the current thermal physiology of these sandhoppers. Temperature also had an important effect on the reproductive plasticity of T. capensis. Different temperatures induced mothers to adjust the size of their offspring (i.e. egg size), with larger eggs produced at lower temperatures. Interestingly, females showed strongly significant among individual variation in the size of the eggs. Given the importance of understanding rapid responses of organisms to climate change and considering the fundamental role played by phenotypic plasticity in evolution, the overall study revealed the significance of individual plasticity and variability in response to the environment and highlighted its importance. Particularly, studying the thermal physiology of separated populations and understanding within population reproductive plasticity in response to temperature, helped to clarify how differences among individual responses have important consequences at the population level, possibly explaining the widespread distribution of T. capensis.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:rhodes/vital:5846 |
| Date | January 2014 |
| Creators | Baldanzi, Simone |
| Publisher | Rhodes University, Faculty of Science, Zoology and Entomology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Doctoral, PhD |
| Format | 191 leaves, pdf |
| Rights | Baldanzi, Simone |
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