We usually think of a food web as containing a high number of prey items at the bottom supporting relatively few predators that sit at the top. But what if it doesn’t always have to be like that? Researchers diving in the remote and near pristine waters of French Polynesia have found an ecosystem that turns this on its head, where predators dominate and vastly outnumber their prey.
Beneath the waves surrounding Fakarava atoll in the Pacific, ecologists found a curious food web that is both incredible and unusual. In a tiny canyon they found a truly awesome sight: close to 900 reef sharks congregating in an area just 0.175 square kilometers (0.07 square miles) in size. They had stumbled upon the highest ever concentration of gray reef sharks, which made up 78 percent of the sharks patrolling the small channel, dominating an ecosystem that is seemingly paradoxical. There are too many predators being supported by the number of prey fish living in the same pass.
Calculating how much food would be needed to support such high numbers of sharks, the researchers found they would require over 90 tons of fish to keep them going, yet when they surveyed the canyon they discovered that the fish biomass production was a measly 17 tons per year, way below the shark’s basic needs. So the obvious question then arose: how the sharks manage to survive living in such astonishingly high densities? The answer, it seems, could give us a glimpse at how a normal reef should look in pristine seas, away from the influence of humans.
The grouper use the canyon to spawn. Laurent Ballesta/Andromede Oceanologie/Blancpain Ocean Commitment
It turns out that the channel in which the sharks haunt is also a favorite spawning ground for grouper fish, which visit every year between June and July to lay their eggs and reproduce. This, the researchers suggest, brings in the biomass from other local food webs, concentrating it in one single small location for this short part of the year, and allowing the sharks to maintain such massive densities. It was previously believed that fish spawning aggregations only represented a minor food source for the marine predators, but it now seems that instead they can underpin the entire food web, and maintain an inverted trophic pyramid.
“Our findings confirm that in pristine remote coral reefs, sharks can be numerous, sometimes even outnumbering their prey at local scales,” explains Johann Mourier, who coauthored the paper explaining this unusual ecosystem, published in Current Biology. He suggests that as French Polynesia represents near-pristine reefs, having escaped shark fishing and being protected since 2006 forming the world’s largest shark sanctuary, these types of food web could be the norm in fully functional unspoilt reefs.
And the sharks feast upon the grouper. Laurent Ballesta/Andromede Oceanologie/Blancpain Ocean Commitment/ Mourier et al. 2016