Billions of animals are dying, and while a so-called mass mortality event is not unheard-of in the natural world, the rate and numbers are accelerating for numerous species, a new study has found.
“The increase in mass mortality events (MMEs) appears to be associated with a rise in starvation, disease, biotoxicity and events produced by multiple interacting stressors,” said Adam Siepielski, an assistant professor of biology at the University of San Diego and the study’s co-lead author, in a statement. “The most alarming and interesting result was the sheer magnitude of some of these mortality events. Billions of individuals dying are just huge numbers to comprehend. The study provides yet another example of the challenges to life that organisms are confronted with on a planet increasingly dominated by the influence of humans in the environment.”
While less drastic or final than extinctions, and still relatively rare, such events can rip a hole in the web of life.
“We're not talking about a few dead fish littering your local beach,” National Geographic explains. “Mass die-offs are individual events that kill at least a billion animals, wipe out over 90 percent of a population, or destroy 700 million tons—the equivalent weight of roughly 1,900 Empire State Buildings—worth of animals.”
Siepielski and a team of researchers from the University of San Diego, Yale University and the University of California Berkeley published their findings this week in Proceedings of the National Academy of Sciences. They analyzed 727 documented MMEs from 2,407 animal populations between 1940 and 2010 and found that the magnitude of these events has been intensifying for birds, fishes and marine invertebrates, even as it decreases for reptiles and amphibians and holds steady for mammals. In particular, Siepielski had observed die-offs in California ponds related to the ongoing drought—part of his inspiration for the larger inquiry.
“Over the past few years, many of those have died off over a matter of weeks,” Siepielski, who specializes in aquatic ecology and evolutionary biology, told UT San Diego. “Literally you’re not seeing only one species die, but you’re seeing entire food webs disappear from lakes or ponds.”
In 19 percent of the MMEs, environmental contamination from human activity was the cause, while 15 percent came from toxic algae blooms, UT San Diego said. However, the die-offs with the biggest scope stemmed from multiple causes, the researchers found.
Siepielski cautioned that an increase in detection and recording of these events does not necessarily coincide with an increase in the number of events, although he and his team did work to filter out that possibility and still found the events to be increasing.
It is “important to recognize that this increase may also reflect an increase in detectability, reporting and awareness of such events,” Siepielski said in the statement. “Determining whether this perceived increase is a real phenomenon or due simply to increased awareness will be an important challenge going forward.”
Such events, while relatively rare, deserve much more scientific study, the researchers said.
“This is the first attempt to quantify patterns in the frequency, magnitude and cause of such mass kill events,” Stephanie Carlson, a senior author of the study who is an associate professor at the UC-Berkeley’s Department of Environmental Science, Policy and Management.
“The initial patterns are a bit surprising, in terms of the documented changes to frequencies of occurrences, magnitudes of each event, and the causes of mass mortality,” said Samuel Fey, a postdoctoral fellow in the Ecology and Evolutionary Biology Department at Yale and the co-lead author of the study. “Yet, these data show that we have a lot of room to improve how we document and study these types of rare events.”