Coral reefs may look benign but, really, they're war zones. Pollution and climate change have tipped the scales against corals, in favor of sponges.
Violence is everywhere in nature, but it doesn’t always involve teeth and claws. On coral reefs around the globe, corals and sponges have been locked in a slow-moving war. For millenia, chemical weapons have reigned supreme in territorial disputes where wins or losses may be tallied in just millimeters.
In recent decades, though, human activities have wrought a dramatic shift in the balance of power. Overfishing, nutrient pollution, excess sediment runoff from land, and the impacts of rising greenhouse gas levels - warming waters, falling ocean pH, and rising sea levels - all have combined to push corals into an unprecedented retreat. It's estimated that at least one fifth of corals globally have been lost, and in the Caribbean, that number is more like four fifths.
"When you look at the landscape, it's like looking at a mossy, overgrown city, like something out of those apocolyptic movies," says Dr. Mark Patterson, a coral researcher at Northeastern University who's been studying Caribbean reefs for decades. "You see the corals that used to be there and, in your mind's eye, you just blur your eyes and imagine they're all alive. And then you open your eyes and see they're all covered in algae and that they're dead. It's depressing."
Sponges and algae have exploited corals' decline, moving in to fill the void. Sponges have usurped coral's title of "primary habitat-forming organism," meaning that sponges now provide the hiding spots and varied topography that reef inhabitants need. Sponges are also incredible water filters, pumping up to 50,000 times their volume in water each day. But relatively little is known about how sponges change the water they pump - what they might add, remove, or alter - and how that impacts the ecosystem around them.
One thing we do know: sponges can't build reefs. In fact, many sponges are what's known as bio-eroders, meaning they literally eat away at the calcium-based reef structures laid down by corals. On healthy reefs, coral growth slightly outpaces the forces of bio-erosion. It's a delicate balance that keeps reefs growing - slowly, but growing.
On unhealthy reefs, that balance is replaced by a spiraling decline. Corals stressed by pollution grow more slowly, or die altogether. Ocean acidification - the drop in ocean pH caused by absorption of excess carbon dioxide from the atmosphere - also makes it more difficult for corals to build their calcium carbonate skeletons. As corals slow down or die, sponges and other bio-eroders move in, exacerbating the problem.
And reef growth is even more imperative in the face of rising sea levels. Corals are actually not just animals; microscopic algae live inside corals and feed them. There are also other plants that live on reefs. Get too deep, and those plants can't get enough sunshine to sustain themselves and the reef ecosystem. Healthy, growing corals might be able to keep up with accelerating sea level rise; eroding ones certainly won't.
Altogether, some estimates suggest that coral reefs could be all but gone by the middle of this century. Mark Patterson and Anne Cohen, a coral researcher at Woods Hole Oceanographic Institution, both say the "hope" that won't come to pass, but acknowledge that avoiding that will take a lot of effort, and soon. Both say that protecting corals from the immediate pressures of fishing and pollution are the best bet for giving corals a fighting chance against the global impacts of climate change.