Across 115 degrees of latitude, spanning most of North and South America, higher ocean temperatures correlate with more intense predation by fish and shifts in invertebrate prey communities, according to a study published today (June 9) in Science. The results demonstrate one way that warmer ocean temperatures caused by climate change could affect marine ecosystems.
Few studies have looked at both predation intensity and effects on prey communities, and none have done so over such a large geographic scale, says Gail Ashton, a study coauthor and marine biologist at the Smithsonian Environmental Research Center. “Applying [this analysis] over such a large geography range is a massive step in our knowledge and understanding,” she says, adding that “the only way we were able to do this is through a massive collaboration.”
The study involved researchers at 57 institutions and took place at 36 nearshore sites in 11 countries along both coasts of North and South America. During the summer months, researchers at each site conducted three experiments. In the first, the researchers measured predation intensity by observing how much dry squid bait predators—mainly fish—consumed in an hour. In the second experiment, they submerged PVC panels below floating docks, half of which were caged to block access by fish larger than 1 cm, and assessed invertebrate community development on the panels over a three-month period. In the third experiment, they assessed how predation affected already-developed invertebrate communities by uncaging half of a group of panels after they’d been kept underwater for 10 weeks.
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The results showed that both predation intensity—measured as bait loss over one hour—and loss of invertebrate biomass on uncaged versus caged panels increased in warmer water temperatures. From high latitudes with mean summer water temperatures of 9°C down to tropical latitudes where water temperatures were 31°C, average bait consumption by predators increased from 0 to more than 10 percent. Across the same temperature range, the average difference in biomass between caged and uncaged panels went from close to 0 to more than 300 grams. “It’s significant that we found the kind of universal trend over such a large range of latitudes,” Ashton says.
She says that the near-zero difference in biomass between uncaged and caged prey communities at higher latitudes indicate that predators are not currently the most important factor affecting invertebrate communities in these areas. “I was surprised at the lack of effect at the higher latitudes,” she says. “I expected a smaller effect, but not quite as insignificant as we found.”
Jay Stachowicz, a marine ecologist at University of California, Davis, says the scope of the research is impressive. Stachowicz did not participate in the study but has collaborated with several of its coauthors, including one who was previously a graduate student in his lab.
“Through studies like this, we can start to get a global picture and try to develop something like a map of processes,” he says, adding that scientists have information on variables such as temperature and salinity distribution, but they lack data on how predation varies geographically, which he says is just as important: “There’s shockingly little large-scale understanding of that distribution.”
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However, Stachowicz notes that floating panels off of docks in the experiments excluded the effects of predators on the seafloor. “Crabs, snails, other sorts of things aren’t going to ever reach it. And we know those are pretty important predators in these communities,” he says. Ashton says the main reason for not including these predators was that there was too much variation in seafloor habitat types across such a large latitude range; using this method allowed the team to observe a standardized habitat.
Stachowicz says he’d like to see future research identify the mechanisms behind the increase in predation in warmer waters. “To what extent is it that there are more predators? To what extent is it that they’re more metabolically active? To what extent is it that they’re different species? Is it all of those things?” he wonders.
Ashton expresses a similar sentiment, saying, “It would be very cool to have a better idea of who the key predators are in each system.” Understanding variations in predator and prey species as well as environmental factors other than temperature could help explain some of the regional differences they saw, such as stronger impacts on prey in the North Atlantic and South Pacific, the authors write in the study.
A better understanding of how predation may shift as a result of climate change and increasing ocean temperatures could help ecologists pinpoint less harmful areas to fish in or help them identify specific conservation strategies that would be effective in marine protection areas, Ashton says. However, there is much yet to learn, she says.
More broadly, the study “tells us something about what might happen in the future with climate change,” Ashton says, but “it doesn’t tell us exactly how or what might change—it’s a piece of the very complex puzzle of how communities form and how they may change in the future.”