From tree to sea, humans have chain effect

Researchers have discovered one of the longest ecological interaction chains ever documented, and they say it shows the long-ranging effects of human activity. Meanwhile, graduate student Hillary Young was studying palm tree proliferation’s effects on bird communities and native habitats. Palmyra is a unique spot where scientists can compare largely intact ecosystems within shouting distance of recently disturbed habitats. Diverse animal life—huge grey reef sharks, rays, snapper, and barracuda—fill the water, while seabirds flock from thousands of miles away to roost in the forests of the tropical idyll. 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 says. 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. “This is an incredible cascade,” says researcher Rodolfo Dirzo, a professor of environmental science and senior fellow with the Stanford Woods Institute for the Environment. “As an ecologist, I am worried about the extinction of ecological processes. This dramatically illustrates the significance of such extinctions.” The study also suggests that these cascades are going largely unseen. “Such connections do not leave any trace behind,” says 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 puts 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.” By way of comparison, researcher Robert Dunbar, a professor of earth sciences and Stanford Woods Institute senior fellow, recalled the historical chain effects of increasing demands on water from Central California’s rivers. When salmon runs in these rivers slowed from millions of fish each year to a trickle, natural and agricultural land systems lost an important source of marine-derived fertilizer. These lost subsidies from the sea are now replaced by millions of dollars’ worth of artificial fertilizer applications. “Humans can really snip one of these chains in half,” Dunbar says.