Bird poop and manta rays: intricacies in human impacts on natural ecosystems

It is always fun to read about interesting research that you can relate to your own. I get especially excited when I can gain insight about the ecosystem I study from research in other ecosystems. That was the case when I recently came across a neat study about manta rays that touched on several areas of interest to me including anthropogenic impacts, nutrient cycling and species interactions.

The study took place on islands of the Palmyra Atoll in the Pacific, comparing manta ray abundance at a relatively pristine site versus a more human impacted site. As expected, the researchers observed more rays at the pristine island, but the long chain of interactions responsible is really an intriguing part of the study. By assessing the whole ecosystem, including terrestrial inputs, the researchers found that birds were more abundant at the pristine site because they prefer to nest in native trees, instead of the invasive, human-introduced palms that dominated the impacted site. These birds added a significant amount of nitrogen to the soil of the island via their guano (droppings). All the nitrogen entered the water via runoff, increasing plankton abundance at the pristine site. Manta rays eat plankton, and therefore, through this long chain of species interactions, rays were more abundant at the pristine site.

I found this study particularly cool because it brings up several relevant topics in ecology and conservation. First, it is often hard to predict how humans are altering ecosystems, due to the complex interactions between organisms and physical processes (like run-off) that influence these interactions. Second, marine and terrestrial ecosystems are often thought of as separate entities, but it is very important to consider how they impact each other, especially because of our tendency as humans to settle on (and therefore alter) coastlines. Third, reminds us of the danger of introduced species. Even if an introduced species doesn’t have a direct negative impact, it could be altering species interactions several links down the food chain, as demonstrated in this study. Finally, the part that relates most to my research is the importance of understanding how nutrients cycle through and between ecosystems, and how nutrient inputs (or lack of them) can change how organisms interact or occur in a certain habitat.

For a more detailed summary of this study check out the popular press article, on ( or for ALL the details check out the journal article by McCauley et. al in Scientific Reports: (


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