When coral reefs die, what will replace them?
It’s a question that feels almost inappropriate. The ongoing decline of what some people call “the rainforests of the sea” is one of modern humanity’s great shames; to think of the aftermath risks resignation, even acceptance, of habitat degradation and climate change — and, make no mistake, much can still be done to protect corals.
But nothing can change the fact that many reefs have been destroyed; barring a miracle, many more will yet be lost. And confronting the aftermath may help people make the best of a tragic situation. We might even find something to treasure.
“While we are seeing corals continue to decrease in abundance across the world,” says James Bell, an ecologist at New Zealand’s Victoria University of Wellington, “other things are taking up the space that corals occupied.”
Several years ago, Bell and colleagues suggested that post-coral reefs might not, as conventional wisdom held, become underwater barrens dominated by algae. Though that’s happened in some places, particularly the Caribbean, reefs elsewhere sometimes follow a different trajectory. Even as corals die, sponges proliferate. Coral reefs become sponge reefs.
Though most people know sponges from their kitchen sinks, phylum Porifera in fact contains between 5,000 and 10,000 species. Like corals they’re technically animals, descended from an ancient trunk of life’s evolutionary tree — and the fossil record suggests that, when Earth’s oceans warmed and acidified 200 million years ago, sponge reefs indeed replaced coral reefs.
Present-day warming is far more rapid affair, but something similar has happened off the coast of Brazil and the Indonesian island of Sulawesi. Sponges are surprisingly resilient — thanks in part, as Bell and other researchers described this spring in Global Change Biology, to lipids and fatty acids that counteract the effects of heat stress on cell walls.
In a study published last month in Ecology, Bell’s team moves from physiology to interactions. Sponge reefs don’t support the abundance and richness of organisms that coral reefs do, but there’s still much life to be found in them, albeit only hazily understood. If not quite rainforests, they might at least be forests.
The researchers model the interactions supported by reefs: flows of carbon and other nutrients, the transition from coral-adapted to sponge-adapted organisms, and so on. Though models can’t capture all the complexities and uncertainties surrounding sponge reefs, they illuminate some key patterns. Sponges are filter-feeders, straining organic matter from water they absorb and expel; as a result there’s less matter for other organisms, but sponges also break down and become detritus on which other creatures can feed.
Seaweed can thrive among them, and their presence slows down the erosion of dead corals on which sponges form — but eventually, decades or centuries hence, those skeletons will crumble. Sponges will continue to grow, but the structural complexity provided by corals, the nooks and crannies that are niches for yet more creatures, will vanish. What sort of life will those reefs support? And how much? These are open questions.
At the moment, says Bell, it’s important to both protect surviving coral reefs and study emerging sponge reefs. He also pointed to the existence of sponge reefs at depths beneath which corals now grow. It’s possible that these communities will move higher as corals decline; they might contain seeds of the future. “So we should protect these reefs as well,” Bell says, “and do much more to understand how they work.” In time we might even appreciate them.
Sources: Bell et al. “Climate change alterations to ecosystem dominance: how might sponge-dominated reefs function?” Ecology, 2018.
Bennett et al. “Elucidating the sponge stress response; lipids and fatty acids can facilitate survival under future climate scenarios.” Global Change Biology, 2018.
Image: Photo: Holly Bennett / VUW
About the author: Brandon Keim is a freelance journalist specializing in animals, nature and science, and the author of The Eye of the Sandpiper: Stories From the Living World. Connect with him on Twitter, Instagram and Facebook.