A global field study of reef fish has revealed a previously unknown fact: There is no such thing as too much biodiversity.
So conclude 54 scientists, including Octavio Aburto-Oropeza, a marine biologist at Scripps Institution of Oceanography, UC San Diego, who took part in an international research effort that combined visual survey data from nearly 2,000 reefs worldwide. Their goal was to assess the relative abundance and health of marine creatures across a spectrum of human encroachment.
“In lab experiments, you reach a plateau where an ecosystem reaches capacity and overall reproductive efficiency – what biologists call productivity – doesn’t increase with added species,” said Aburto-Oropeza, a postdoctoral fellow at Scripps’ Center for Marine Biodiversity and Conservation (CMBC). “In a natural setting, this doesn’t happen.”
The study, published in PLoS Biology, revealed that reefs near dense human populations had lower fish biomass and diversity. Using indicators of human effects including fishing, land use, and coastal development, the research team found that 75 percent of the world’s reefs are affected by humans. Biodiversity and productivity on these reefs was much lower than reefs in relatively pristine areas, leading to varying degrees of fisheries collapse and economic loss.
The surveys provided snapshots of fish present on rocky, coral, and kelp reefs in every ocean from the most pristine areas to those exposed to intense human activities ranging from fishing to dive tourism to sewage disposal. The team focused on three measures of productivity: quantity, size, and number of species of fish on each reef. They discovered that reefs with the most species also had the largest total quantity, or biomass, of fish and other marine organisms.
“Each species has a different function,” Aburto-Oropeza said. “More functionality on a reef makes it more productive.”
Aburto-Oropeza has been coordinating the Latin American participants in the worldwide study led by Camilo Mora, a former Scripps postdoctoral researcher now at Dalhousie University. Seventy reefs in the study are in the Gulf of California, where Scripps researchers have studied them for the past decade. Many of these are heavily exploited, and therefore have very low diversity, minimal productivity, and slim chance of recovery under current conditions.
The reefs inside one marine reserve, however, have an encouraging success story. Cabo Pulmo National Park was protected by the Mexican government in 1995, and by 2009 fish biomass on the reefs was five-fold larger than the average quantity in the gulf. Aburto-Oropeza’s current work includes a calculation of revenue lost by delaying further marine conservation in the Gulf of California since marine reserves were recommended in 1999.
Aburto-Oropeza argues that large scale ecosystem protection makes commercial sense. “We need to have entire systems working at full capacity to recover what we have lost,” he said.
Instead of a few species growing large, old, and dominant, more diverse reefs have higher turnover, growth, and export. As a result, diverse reefs support recreational economic activities such as diving, and fishing and harvest activities in their peripheral zones. Aburto-Oropeza believes that the link between diversity and productivity is fundamental, and extends to many exploited land and aquatic systems.
--Kait Frasier is a 2nd-year graduate student in the laboratory of oceanographer John Hildebrand