“Mirages are associations among variables that spontaneously come and go or even switch sign, positive or negative. Ecosystems are particularly perverse on this issue. The problem is that this kind of system is prone to producing mirages and conceptual sand traps, continually causing us to rethink relationships we thought we understood.” – George Sugihara, McQuown Chair Distinguished Professor of Natural Science at Scripps
Last week, the Scripps Institution of Oceanography issued a press release announcing a new mathematical tool to help avoid misleading conclusions for species management. The tool, called “convergent cross mapping,” looks to address a fundamental question scientists face every day: Given two events, how do we know if the relationship is causal or correlative. If, for example, a fisher catches more fish when wearing a particular ball cap, it doesn’t necessarily follow that the ball cap produces more fish.
The new technique, developed by Sugihara and his colleagues at Scripps, takes multiple variables into account and avoids the centuries-old “correlation does not imply causation” issue that has plagued single-factor studies.
When discussing fisheries, it’s all to easy to draw a causal link between events that seem to be related. For example, it has been postulated by some scientists and fisheries managers that crashes in certain fisheries are caused by global climate change, and that may well be the case, but it might also be the case that the drivers are far more complex. Failing to acknowledge this fact, can lead to developing “solutions” that don’t truly address the totality of the problem.
“Studying ecosystems in this piecemeal way makes it hard to find quantitative relationships, the kind that are useful for management and stand the test of time,” says Ethan Deyle, a Scripps graduate student and co-author on a paper published in the Proceedings of the National Academy of Sciences discussing convergent cross mapping.
The authors of the paper looked at the collapse of the sardine fishery in California in the 1940s and concluded the forces behind the collapse are “a dynamic and interconnected moving target.” Rather than evaluating various factors such as climate and anthropogenic stressors independently, scientists ought to be looking for the more complex cumulative effects. This approach is fundamental to ecosystem-based management, a fisheries management approach increasingly being embraced by fisheries managers.
“Sustainable sardine fishing based on ecosystem-based management should adapt to dynamic changes in the ocean environment, and future policies should incorporate these effects to avoid another [fishery collapse],” says Deyle.