Where humans live, coral fails

The world’s coral reefs are in alarming decline, but what – or who – is most to blame?

A groundbreaking study published yesterday singles out human settlement, especially coastal development and agriculture, as the main culprit, even more so than warming sea waters and acidification linked to global warming.

The study focuses on the Caribbean, where declining reefs are endangering species of wildlife as well as tourism and fishing that are vital for the local economy, says lead author, Camilo Mora, of Dalhousie University in Halifax, Canada.

“The continuing degradation of coral reefs may be soon beyond repair if threats are not identified and rapidly controlled,” he said.

Teasing apart the complicated web of factors driving reef destruction – overfishing, runoff of pesticides and pollution, hurricanes, climate change – is crucial for devising the best conservation strategies.

There might not be enough time for second or third chances, Mora said.

But a welter of contradictory evidence, most of it gathered from single sites, has made it nearly impossible to figure out what causes what.

Which is why Mora and University of Miami marine biologist Robert Ginsburg decided to compare several large-scale databases that had never been systematically cross-referenced.

Focusing on corals, fishes and macroalgae, or seaweed, in 322 sites across 13 countries in the Caribbean, the study matched environmental and ecological data against patterns of human population density, coastal development and agricultural land use.

Also included were data on hurricanes, biodiversity, fish populations and coral disease.

Sifting through all these statistics showed clearly that the number of people is the main driver of the mortality of corals, along with declining fish biomass and increases in algae.

But different kinds of human activity resulted in different impacts, the study revealed.

Higher population density in coastal areas produces more sewage and depletes fish stocks, both of which are directly responsible for coral mortality.

But chemical discharges from agricultural land drives an increase in macroalgae, which is indirectly linked to coral loss.

Warmer sea surfaces are also contributing to coral decline, but not hurricanes, the study found.

“The human expansion in coastal areas inevitably poses severe risks to the maintenance of complex ecosystems such as coral reefs,” Mora said.

Within a reef, predators prey on plant-eating fish, herbivores graze on seaweed, which in turn interacts with living coral.

“A threat in any one group may escalate to the entire ecosystem,” Mora explained.

“The array of human stressors… are significantly affecting all major groups of coral reef organisms.”

The study also concluded that while Marine Protected Areas help restore fish populations, they do nothing to protect coral.

A fifth of the world’s marine reefs have already been destroyed and half are threatened because of human impact, whether directly or as a consequence of rising temperatures driven by climate change, according to the World Conservation Union (IUCN).

Coral reefs support some of the richest areas of biodiversity in the world, including many species that depend on reefs for shelter, reproduction and foraging.

Coral reefs also provide livelihoods for 100 million people and form the basis for industries such as tourism and fishing, worth $30-billion (