Disturbance of long ecological chains: how planting palm trees can affect manta rays

Douglas McCauley and Paul DeSalles did not set out to discover one of the longest ecological interaction chains ever documented. But that’s exactly what they and a team of researchers — all current or former Stanford students and faculty — did in a new study published in Scientific Reports.

Their findings shed light on how human disturbance of the natural world may lead to widespread, yet largely invisible, disruptions of ecological interaction chains. This, in turn, highlights the need to build non-traditional alliances — among marine biologists and foresters, for example — to address whole ecosystems across political boundaries.
This past fall, McCauley, a graduate student, and DeSalles, an undergraduate, were in remote Palmyra Atoll in the Pacific tracking manta rays’ movements for a predator-prey interaction study. Swimming with the rays and charting their movements with acoustic tags, McCauley and DeSalles noticed the graceful creatures kept returning to certain islands’ coastlines. Meanwhile, graduate student Hillary Young was studying palm tree proliferation’s effects on bird communities and native habitats.

Over meals and sunset chats at the small research station, McCauley, DeSalles, Young and other scientists discussed their work and traded theories about their observations. “As the frequencies of these different conversations mixed together, the picture of what was actually happening out there took form in front of us,” McCauley said.

Through analysis of nitrogen isotopes, animal tracking and field surveys, the researchers showed that replacing native trees with non-native palms led to about five times fewer roosting seabirds (they seemed to dislike palms’ simple and easily wind-swayed canopies), which led to fewer bird droppings to fertilize the soil below, fewer nutrients washing into surrounding waters, smaller and fewer plankton in the water and fewer hungry manta rays cruising the coastline.

Equally important is what the study suggests about these cascades going largely unseen. “Such connections do not leave any trace behind,” said researcher Fiorenza Micheli, an associate professor of biology affiliated with the Stanford Woods Institute. “Their loss largely goes unnoticed, limiting our understanding of and ability to protect natural ecosystems.” McCauley put it another way: “What we are doing in some ecosystems is akin to popping the hood on a car and disconnecting a few wires and rerouting a few hoses. All the parts are still there — the engine looks largely the same — but it’s anyone’s guess as to how or if the car will run.”