Scientists have long known coral reefs are in danger, but a new study published today in the journal One Earth quantifies the loss of corals around the world. In-depth analysis shows that half of the reefs have disappeared since the 1950s. Scientists say climate change, overfishing and pollution are destroying these fragile ecosystems, putting communities and livelihoods at risk. Their study is one of the most comprehensive assessments of coral reefs and their associated biodiversity to date, highlighting the rapid pace of global coral collapse.
Tyler Eddy, a research scientist at Memorial University of Newfoundland who co-authored the study, said: "Coral reefs are decreasing all over the world – I think that's widely accepted. "When we look at the global scale, the decline in the number of coral reefs is not necessarily the same.
Coral reefs are biodiversity hotspots that provide habitat for fish, protect coastal communities, and generate billions of dollars for fisheries and tourism. Biologist Mary Hagdorn says corals die in part because they are very sensitive to changes in water temperature and acidity.
"Corals have bones, which makes them look like rocks," Hagdorn said, but they are animals with symbiotic partners. Coral polyps rely on colorful algae, called ancestral corals, which live in their tissues and produce the food they need to survive. When polyps are emphasized by changes in light, water temperature, or acidity, they break the symbiotic relationship and excrete algae in a process called bleaching. Corals have a short window to restore their symbiotic algae, but if the corals are stressed too long, their death is irreversible. "There is not a single reef on Earth that has not been affected by some aspect of this global and local threat," Hagdorn said.
Most coral assessments focus on specific areas or reefs, but Eddy and his colleagues from the University of British Columbia want a more comprehensive assessment of coral loss. Using a combination of databases containing thousands of coral reef cover surveys, marine biodiversity records and fisheries catch data, they assessed how each factor has changed over time. They are particularly curious about what dying corals mean for the "ecosystem services" of coral reefs — including providing habitat for a variety of marine species, protecting the coast from storms, and serving as a source of food and livelihood.
In addition to finding that half of all living corals have died since the 1950s, the researchers also found that the biodiversity associated with coral reefs has declined by 63 percent. Healthy reefs support thousands of different corals, fish and marine mammals, but bleached reefs lose the ability to support as many species as possible. Scientists also found that reef fish catches peaked in 2002 and have been declining since then, despite increasing fishing efforts. Studies have shown that the loss of reef species does not equate to some corals being more sensitive than others, leading some biologists to worry that some vulnerable coral species will disappear before they are recorded or preserved.
One of the challenges the team faced was to find detailed, accurate information about reef cover in the 1950s. To address this restriction, they relied on the 2018 Historical Coral Cover study coral cover estimates. In earlier studies, the study authors asked more than a hundred scientists what they thought coral reef cover would be in a given year, based on the available evidence.
Eddy and his colleagues also documented the impact of coral reef loss on coastal indigenous communities with close cultural ties to coral reefs. These communities have lost ecosystem services, including reef-related seafood that most of their diets rely on.
Ecologist Christina Hicks said the link between human communities and coral reefs is a particularly important part of the study. "It asked the question, 'Yes, we're losing ecosystems, which is tragic, but what do these losses mean for people?'" Coral reefs play a very important role in providing important micronutrients to indigenous and local communities, and if they lose them, it can lead to serious impacts.
Recent studies have not assessed the factors that have contributed to the decline in coral populations in recent decades, although overfishing and pollution from nearby onshore agriculture are common local stressors. Eddy and other coral experts agree that the biggest threat to coral reefs is climate change, noting that areas that contribute less to climate change tend to feel the most severe impacts. Every year, the ocean absorbs about a quarter of the carbon dioxide emitted by burning fossil fuels, making corals warmer, more acidic and less hospitable.
Hicks said: "There are many strategies to save coral reefs and reduce carbon emissions, and people often argue about what works best. "This study points out that it's even more important that we act now and that we act in all directions."