Mass animal die-offs have been more frequent and often increasingly severe in recent years, say U.S. researchers who've studied more than 700 mass mortality occurrences over the last 70 years.
Of the 2,500 animal species experiencing such mass die-offs -- when a large portion of a population dies in a brief time frame -- fish, birds and marine invertebrates have seen the biggest increase in such events, the researchers say.
Such die-offs rose by about one event annually from 1940 to 2010, and the numbers of deaths increased for some species, a study involving Yale University, the University of California, Berkeley, and the University of San Diego found.
The finding should allow scientists to better assess trends in mass mortality incidents and provide an understanding of their causes, the study authors say in the Proceedings of the National Academy of Sciences.
"This is the first attempt to quantify patterns in the frequency, magnitude and cause of such mass kill events," says UC Berkeley researcher and study senior author Stephanie Carlson.
Disease was identified as the primary culprit in most die-offs, the researchers report, involved in 26 percent of such events.
Nineteen percent involved effects linked to human activities, including environmental contamination, while climate-influenced processes such as weather extremes, starvation, thermal stress or oxygen deprivation collectively were involved in around 25 percent of die-offs, they said.
The most serious die-offs occurred when multiple causes came together, the study found.
Fish ecologist Carlson says the impetus for the study came from observations by her and other researchers of fish die-offs in California streams and ponds during the state's extended drought.
"The catastrophic nature of sudden, mass die-offs of animal populations inherently captures human attention," says Carlson. "In our studies, we have come across mass kills of federal fish species during the summer drought season as small streams dry up."
While more fish, birds and marine invertebrates have been involved in mass die-offs in recent years, mammal die-off have remained more or less constant, the researchers said.
Curiously, die-offs among reptiles and amphibians seem to be occurring less often, although the researchers say that shouldn't be seen as necessarily good news.
"These are some of the most imperiled and threatened taxa that exist on the planet now," says San Diego University biologist Adam Siepielski, a study co-author.
"One possibility [for the reduced die-offs] is that because these animals are so threatened, their populations are so small that you can't get a mass mortality," he said.
The research was funded in part by the National Science Foundation and the Environmental Protection Agency.
