Understanding global and climate change for ecosystems at their range limits: Mangroves of South Africa
Mangroves are typically considered to be tropical ecosystems although their global latitudinal distribution extends generally between 30 °N and 30 °S. Along the South African coastline, mangrove ecosystems occur at a southern continental range limit (32°59’10’’S). The high energy wave action along the coast restricts mangroves to occurring in sheltered estuarine areas and this has facilitated the range extension of typically tropical species into higher subtropical, and even warm-temperate, biogeographic regions. However, estuaries in South Africa also face a unique set of threats, particularly in relation to ecological water requirements, as the climate is semi-arid with seasonal rainfall and there is high development pressure within catchment areas. Fortunately, there is a reliable long-term monitoring record for South African estuaries and current research is focussed on examining whether there are specific drivers of habitat distribution and changes over time. For mangroves in particular, research is focussed on determining factors that influence distribution limits as there have been recent reports of pioneer individual mangrove trees successfully establishing beyond their previous natural limits. This will be investigated using a modelling approach to examine the interactions between variables which explain the field observations. The major factors being considered are rising sea-surface temperatures and estuarine mouth dynamics, which relate to periods of connectivity and closure to the Indian Ocean. Another area of mangrove research that is currently underway includes the first comprehensive assessment of carbon storage potential for this region. This work will provide an interesting comparison to research carried out in the larger and more productive mangrove forests of tropical regions and could have implications for measuring ecosystem service provisioning for estuarine mangroves at subtropical latitudes. Finally, a long-term monitoring program is being set up to measure the responses of mangroves to sea-level rise. This will be the first project in Africa to relate surface elevation change in mangroves to measurements at tide gauges. In South Africa mangroves are celebrated for their natural beauty and the many ecological and economical benefits people obtain from estuaries that support these ecosystems. It is therefore a necessity to understand baseline processes that are influencing these ecosystems, so that responses to be climate or global change can be predicted, or mitigated if necessary.