Consider bumphead parrotfish, so named because of a cartoonish bump on their heads. Iridescent green with a pink nose, they are as big as scrawny teenagers, and they are on the International Union for the Conservation of Nature’s Red List of threatened species.
But they’re not rare at Palmyra Atoll, a U.S.-administered island in the Pacific Ocean, where the waters are teeming with these coral-eating hulks. A decades-long conservation program there has led to a boom in parrotfish numbers, so much so that they are now harming local populations of corals and other species.
This is not an isolated case: Ecologists are facing similar dilemmas with elephants in a South Africa reserve that are killing trees in the savanna and with protected sea turtles in the Bahamas that are harming meadows of invaluable sea grass.
These instances show how even the best-thought-out conservation efforts can have unintended effects on the environment, benefiting some large-bodied species over less charismatic ones. That’s because reintroducing a species into an environment that lacks predators or other mechanisms to keep populations in check, or protecting a population that would otherwise disappear, can make the entire ecosystem unstable, said Douglas McCauley, an ecologist at the University of California at Santa Barbara. He recommends fixing all the damaged parts of an ecosystem at the same time.
“Ecosystems run a little bit like car engines,” he said. “If you wreck an engine, then try to run (it) after repairing only a few of the broken parts, you are likely to see a whole new set of parts start to break.”
From a dock on Palmyra one day seven years ago, McCauley watched a plume of foam approach on the ocean’s surface, heard the crunch of teeth on coral and soon saw a school of parrotfish move through the reef like a herd of buffalo feasting on grassland. To McCauley, then a graduate student studying reef ecology, it was a eureka moment. The parrotfish had to be a major player in the reef ecosystem and ought to be studied, he thought.
“The reef was basically being sucked up by them and pooped out,” McCauley said.
Once threatened, now flourishing
To appreciate the drama of what McCauley saw, one must understand Palmyra Atoll, an island of palm trees and white beaches that time assembled painstakingly atop an underwater volcano. Eons ago, the volcano had no living thing on it. Then, some coral larvae arrived on ocean currents. Fish larvae made the journey next, followed by plant life possibly carried over by sea birds from Hawaii a thousand miles away. Nature composed a symphony of life.
The atoll is made entirely of coral and is uninhabited except for occasional scientists such as McCauley, who arrive by a charter plane that lands on a runway made of crushed coral; there is little soil. The island, owned by the Nature Conservancy, an environmental organization, is far from civilization.
Still, human influence – in the form of overfishing of all species, including the parrotfish, which is somewhat of a delicacy – crept in. To protect the dwindling population, the U.S. government set up a marine-protected area at Palmyra in 2001 and made fishing illegal. The parrotfish flourished in the shallow reefs where their natural predators – gray reef sharks – were too big to reach. Large numbers of them happily chomped away at their primary food source – coral.
When McCauley saw the parrotfish destroying the reef, he dived in to inspect, following one fish around, noting what it ate and what type of coral it preferred. At one point, the parrotfish joined a group of five others, and they spotted McCauley. One male, weighing perhaps 100 pounds, charged full force at McCauley, who was both worried and struck by the wonder of the moment.
“It is the aesthetic pleasure of being knocked out of the way on a reef by a hungry school of buffalo-like bumphead parrotfish,” he explained.
Using data collected over four years at Palmyra, McCauley and his colleagues created a simple computer model of the reef. They found that each parrotfish ate about 2 tons of coral every year. At that pace, the fish would damage the shallow reefs within decades, causing them to shrink and become much less diverse. The findings were published last year in the journal Conservation Biology.
So, how should a species like the bumphead parrotfish – threatened and yet, in large numbers, destructive – be managed? Rather than protect particular species, a better approach may be to protect entire assemblages of species in an ecosystem, some conservationists believe.
Restoring the ecosystem
In the parrotfish case, that might mean protecting its predators. (Some parrotfish appear to avoid reefs that attract especially large numbers of sharks.) While this approach would not apply to Palymra, where sharks already are protected but too large to reach shallow reefs, it can be applied at a nearby atoll, Tabuaeran, whose reefs also have been damaged by parrotfish.
“We expect sharks are confining the bumphead to particular parts of the reef, controlling the impacts they have on the reefs,” McCauley said. “You need this rare bumphead, but in order to keep things more or less in balance, you also need the predators of this fish.’’
Creating a balance of protection is complicated. Ecologists know little about the intricate interactions between species in many ecosystems. Case in point is the parrotfish, which was once thought to primarily benefit reefs by eating algae and dead corals and creating space for new growth. Now, it appears the species both can benefit and harm the ecosystem.
Meanwhile, ecosystems constantly are changing. The world is in what many scientists are now calling the Anthropocene age – the age of humans, who are altering the chemistry of the oceans and atmosphere and hastening climate change. Humans are now the prime shapers of the planet, more so even than nature – building, growing new foods, creating marine reserves and catalyzing change. And humans are overfishing the oceans and removing top predators such as sharks.
As ecologist Daniel Botkin put it in his 1990 book Discordant Harmonies, nature is a symphony of several compositions, and ecologists are playing the role of conductors, “forced to choose among these (compositions), which we have barely begun to hear and understand.”
