May 21st, 2012 by
Manta ray

Giant manta ray Credit: Vlad Karpinsky

You’ve heard me talk a lot about how everything is connected. It’s a theme in the natural world that I find completely fascinating, so you can imagine how much I enjoyed this recent article from Yale Environment 360 about finding a connection between trees and giant manta rays. Yes, I said giant manta rays. These creatures can grow to more than 25 feet across, but even small changes in the ocean food web can significantly impact their populations. So, I bet you’re thinking, “If these creatures are so tied to the deep blue sea, what could they possibly have to do with decidedly land-bound trees?”

U.C. Berkeley marine biologist Douglas McCauley found himself asking a similar question. He and his colleagues were tracking Pacific manta rays around the Palmyra Atoll, a ring of islands halfway between Hawaii and American Samoa that are mostly uninhabited and undeveloped. The majority of the atoll is covered in thick native forests, broken in some places for sections of coconut palms, which are not native to the islands, but were brought there by humans. As McCauley and his team traveled the waters around the atoll, monitoring their mantas, they noticed an odd pattern: No matter which rays they followed, sooner or later, they ended up in water near Palmyra’s native forests. But never near the coconut palms.

To find out why the rays seemed to so strongly prefer the native-forested coastlines, the team tested the waters near both types of vegetation and found a lot more fish in those around the native forests than the palms. It seemed that the marine life really did prefer the waters around the areas of native forest — an ecosystem they couldn’t even touch. The scientists discovered that the reason behind this starts not with a manta ray or even a tree, but with a bird.

Birds on Palmyra Atoll

Birds nest in native trees on Palmyra Atoll Credit: Island Conservation

Many bird species take advantage of the undisturbed habitat on Palmyra Atoll, and migratory species use it as a rest stop in the middle of the Pacific Ocean. For seabirds like the red-footed booby and white tern, there is no lack of fishing in the surrounding waters. These birds have their choice of habitat on the island: coconut palms or the native forest. The majority opt for the native forest because while palm fronds may be pretty, the branches of native trees form a more stable canopy for the birds’ rest and protection. Where more birds rest, there are, understandably, more bird droppings, which contain a lot of nitrogen — a natural fertilizer. During their tests, McCauley’s team found that the soil in the native forests had a nitrogen level five times higher than the palm forests. The trees and plants absorb the nitrogen, which falls back to the ground in the form of leaves and twigs, creating a nice, nitrogen-rich layer on the forest floor. When it rains — and Palmyra gets about 175 inches of rain each year — the nutrients from the soil get washed out into the ocean, feeding all the organisms that live there, including plankton. McCauley’s team found that there were three times as many zooplankton off the coast of the native forests than off the palm-forested coastlines. And what eats plankton? Manta rays.

It all comes full circle. Birds catch and eat fish from the ocean, then rest in the native forests on land. Their droppings fertilize the soil, helping the trees grow and concentrating the nitrogen in the soil, which washes out to sea and feeds everything there, ending up right back in the fish that will be caught by another bird.

While we often realize the connections between flora and fauna in the same ecosystem, or even between land-based ecosystems, it is easy to forget how easily an action onshore can influence something deep in the sea — and vice versa. McCauley’s study is a fantastic example of a connection we rarely make: That marine and terrestrial ecosystems, though sometimes miles apart, are anything but independent.