Understanding historic and contemporary patterns of freshwater biodiversity is an important pre-requisite to predict the future distribution patterns and population trends driven by socio-economic pressures and climate change. The factors responsible for the past and present biodiversity patterns have long been studied by ecologists, bio-geographers and palaeontologists. Of the dozens of hypotheses put forward, three have consistently emerged as explanations of variation: species-area, species-energy, and history.
The species-area hypothesis refers to the existence of a positive relationship between the number of species present in a given area and the size of that area, explained by a combination of size-dependent extinction and speciation rates as well as the influence of the diversity of the habitat. For the first, the probability of extinction for a given species increases with a reduction in population size, which is assumed to be a function of the size of the area in which it lives. The second explanation suggests a positive effect of area on speciation rate by exposing species to greater ecological heterogeneity and more geographical barriers. The third explanation suggests that larger areas have more habitat types, thus allowing more ecological niches and consequently favouring the coexistence of more species.
The species-energy hypothesis, predicts a positive correlation between species richness and the energy available within the system, either as the factor determining available resources (e.g. primary production) or as the upper and lower physiological limits of species. In the former, one would expect an ecosystem variable such as net primary production to be an important predictor of species richness, whereas in the latter, autecological variables such as temperature tolerance would predominate. Historical hypotheses attempt to explain patterns in species richness as a function of recolonisation of systems and thus by the degree of maturity achieved since the last major climate change (e.g. retreat of Pleistocene glaciers ca. 10,000 years ago).
An important aspect of our research will be to quantify the ability of these overarching hypotheses to predict biodiversity changes across taxonomic groups and at increasingly smaller scales. The inclusion of factors related to the degree of fragmentation of water bodies, eutrophication processes and temperature regime changes will further allow us to evaluate the potential effects of these types of stressors on contemporary freshwater biodiversity patterns. A key contribution to our understanding of biodiversity trends will come from the integration of historical data sets (lake sediments, repeated plankton sampling over multiple decades) to determine the temporal scales on which patterns change.