Individuals and the communities they form are strongly influenced by the environments they live in. However, those environments are not static. Even if individuals are able to tolerate environmental shifts, if those shifts disturb the delicate balance between community members then large scale ecosystem shifts can arise. Particularly in the light of global climate change, understanding how communities will respond to changing environmental conditions is key to predicting if ecosystems will persist and what they will look like in the future.
|Left: Coral community in Moorea, French Polynesia. Right: An experimental nutrient addition plot at Isla Contadora.|
Coral reefs are culturally and economically important ecosystems distributed in tropical regions around the globe. Coral reefs have also recently undergone serious declines in almost all of those regions. These declines are potentially a result of anthropogenic changes to their environments at both local (increased sedimentation, nutrient runoff) and global (increased sea surface temperatures, ocean acidification) scales. Coral declines have also been associated with ecosystem shifts from coral dominated reefs to reefs dominated by fleshy macroalgae or algal turfs. I have been studying these transitions looking for threshold events that lead to catastrophic state changes and to see if there are internal mechanisms that stabilize reefs in both the coral dominated and algal dominated states. These stabilizing mechanisms would initially confer a resilience to coral reefs preventing algal overgrowth but in turn would also make recovery after an algal shift more difficult.
To address these questions I use a combination of observational and experimental methods. On the observational side I identify “naturally” occurring algal and coral dominated patches, assess their distributions and evaluate potential stabilizing mechanisms (such as variable rates of growth or herbivory). My experimental approach involves plots set up on reefs at Isla Contadora, Panama in the Tropical Eastern Pacific (TEP) where I have manipulated levels of sedimentation, nutrient inputs and herbivory to induce and monitor ecosystem shifts. Many reefs in the TEP seem to occur in either algal or coral dominated states (with a low frequency of intermediate patches) with repeated transitions between the states. By inducing a similar transition I hope to gain information about the nonlinear dynamics of ecosystem shifts that can help predict when those transitions will happen. Potentially this information can be used to inform conservation practices for other reefs and help understand how ecosystems more generally respond to changing environments.
|Increase in algal abundance in response to herbivore reductions and nutrient additions over 24 weeks.|