Scientists blame disease for mass die-offs of animals

Overall, disease was the primary culprit, accounting for 26 percent of the mass die-offs. (Credit: rom82/Flickr)

Mass die-offs of animals may be increasing in frequency and—for birds, fishes, and marine invertebrates—in severity as well, according to a study that spans more than 70 years.

Despite the ecological importance of individual mass mortality events, in which a larger than normal number of individuals die within a population, little research has been conducted on patterns across mass mortality events.

Researchers say the new study of 727 mass mortality events since 1940 will help assess trends in mass mortality events and their causes.

“The initial patterns are surprising, in terms of the documented changes to frequencies of occurrences, magnitudes of each event, and the causes of mass mortality,” says Samuel Fey, a postdoctoral fellow in the ecology and evolutionary biology department at Yale University.

“These data also show that we have a lot of room to improve how we document and study these types of rare events.”

The magnitude of the die-offs has increased in birds, fishes, and marine invertebrates, held steady among mammals, and decreased in frogs and amphibians.

The researchers acknowledge that more scientific research has been done on mass mortality events in the last few decades but say accounting for this “discovery bias” doesn’t explain all of the increases that appear to be associated with a rise in disease emergence, biotoxicity, and multiple interacting stressors.


Overall, disease was the primary culprit, accounting for 26 percent of the mass die-offs. The impacts of direct human activity, primarily from environmental contamination, caused 19 percent of such events.

Another major cause was biotoxicity triggered by events such as algae blooms, the rapid increases of algae in water systems. Processes directly influenced by climate—such as weather extremes, thermal stress, oxygen stress, or starvation—also contributed accounted collectively for about 25 percent of mass mortality events.

The most severe events were those with multiple causes, the paper, published in the Proceedings of the National Academy of Sciences, shows.

“This study should improve our understanding of the continuum of mortality patterns and processes that exist between background mortality levels and species-level extinctions,” Fey says.

Researchers from University of California, Berkeley, and from University of San Diego are coauthors of the study.

Source: Yale University