More Things Are Dying More Often
“Diversify your portfolio” is great advice—and not just for investments. The math that backs it up turns out to explain much more than stock market returns.
Biologists came up with the idea of a “portfolio effect” more than 15 years ago, as a useful model—borrowed from finance—to explain the relationship between diversity and stability in living systems. The more species there are in an ecosystem, the more stable it should be. A finance analogy is also helpful because animal populations sometimes crash like stock prices do. Now, a new paper in the Proceedings of the National Academy of Sciences (PNAS) may make it easier to investigate why these ecosystem crashes happen, by amassing into one place 75 years of data about fish, marine mammals, birds, reptiles, and others.
The research assembles 727 “mass mortality events,” from myriad species and ecosystems. Those events include 1,200 Alaskan walruses dead in 1978; 200,000 jays, thrushes, and warblers felled by a rainstorm in Michigan in 1976; and 380 million Gulf menhaden dead in coastal Texas since the ’50s. A mass mortality event is defined by 90 percent of a population being wiped out in an ecosystem, more than a billion individuals dying, or the carnage adding up to 700 million tons of dead animals.
The study's scientists concluded that on average the frequency of die-offs is up, and the magnitude of the events among sea mammals, fish, and birds has increased with time. They attribute this to disease, accumulated environmental toxins, and an increase in multiple smaller stresses, such as low-oxygen episodes in bodies of water or extreme weather. And just as economists might study survivor shocks after a financial cataclysm, scientists see aftershocks of mass mortality events ripple through neighboring animals. The 727 incidents included in the study allowed them to study the responses of more than 2,400 different species.
Biologists drawing inspiration from financial advisers is just one case of a mingling of economics and the natural sciences. Often the arrow points in the other direction. Economists’ axiom that markets seek equilibrium, for example, is a puzzle to many natural scientists who look at living systems and see only disequilibrium. Mark Buchanan, a physicist and Bloomberg View contributor, asked in his 2013 book, Forecast, if approaches from the physical sciences eventually would help economists predict the economy at least as well as, say, meteorologists predict the weather.
Both meteorologists and climatologists in particular have become quite adept at measuring temperature and precipitation around the world, detecting what they call “physical signals” of change. This new PNAS study sets up a framework by which scientists can look for analogous traces in ecosystems, too. "If you are changing the planet at both global and regional scales, you’d expect that there maybe would be a biological signal,” says Samuel Fey, a postdoctoral fellow at Yale who led the research. “Animals are responding to these changes.”
The team hopes their work will prompt governments and scientists to knit together existing monitoring networks, which would supply researchers with what they need most: more data about these rare events. But more data or not, the reality is clear: Mass die-offs are bad, and they're getting worse for some species. “It’s almost even more scary that we don’t have a better research program in place to study these kinds of events,” Fey says.
There’s no chance that the biologists, or the economists for that matter, will achieve true understanding of why crashes happen anytime soon. But it’s encouraging—and even kind of morbidly intriguing—that they can see similar patterns of catastrophe wherever they look.
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