If predator-dominated Kingman represents the gold standard for coral reefs, how does the removal of large carnivores through fishing affect coral communities elsewhere, such as in Kiritimati? As the report from the Line Islands shows, overfishing can unleash a population boom of smaller fish. The reef might appear luxuriant for a time, but in a matter of decades its ecosystem can unravel from a wonderland of marine diversity into a sediment-choked ecological desert.
"Eliminating the top predators speeds the turnover rate of the entire reef community," Sala says. Through mechanisms not yet fully understood, this acceleration ultimately produces an explosion of microbes, some of which may cause coral death. Fishing out the large herbivores contributes to reef degradation. In the absence of grazers, large algae flourish, and their photosynthetic activity increases the availability of dissolved organic carbon in the system, boosting the growth of bacteria.
"It's bad for corals to be bathed in microbes," says Elizabeth Dinsdale, an expedition microbiologist. Ten times as many microbes populate the water above Kiritimati as live above Kingman. It's the difference, Dinsdale says, between swimming in a sewer and a chlorinated pool.
This research comes at a critical time for coral reefs, in trouble across the globe as rising greenhouse gas levels warm the oceans and boost the acidity of seawater. Elevated temperatures trigger mass episodes of coral bleaching. Rising acidity, the result of increased carbon dioxide absorption, threatens the very coral matrix. Pollution and overfishing make matters worse.
"Working out the relationships between overfishing and reef health is critical," says Sean Connolly, an Australian reef expert. "Protecting reefs from overfishing is within our power and might help mitigate adverse effects of other changes, such as global warming."
To Sala the message is clear: Overfishing is ecological sabotage. "It's like removing vital parts from a machine and expecting it to keep functioning," he says. At Kingman, the machine still has all its parts. And because the ecosystem is largely intact, it has stability and resilience and is able to recover from environmental stresses. Kingman Reef provides one of our last, best glimpses of what a coral reef should be: a postcard from the past for the benefit of the future.