Occurrences of animal mass mortality events (MMEs) and taxon-specific publication trends through time. Colored bars indicate the number of events during a 5-y interval (e.g., 1940 stands for the 1940–1944 period), and dashed lines show trends in the total number of papers published each year for each taxon. For all taxa, the increase in the number of MMEs is coincident with an increase in the number of publications. Graphic: Fey, et al., 2015

By Jane J. Lee
13 January 2015

(National Geographic) – We're not talking about a few dead fish littering your local beach. 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.

And according to new research, such die-offs are on the rise.

The study, published this week in the Proceedings of the National Academy of Sciences, is the first to examine whether mass die-offs have increased over time.

Researchers reviewed historical records of 727 mass die-offs from 1940 to 2012 and found that over that time, these events have become more common for birds, marine invertebrates, and fish. The numbers remained unchanged for mammals and decreased for amphibians and reptiles. (See "What's Killing Bottlenose Dolphins? Experts Discover Cause.")

Disease, human-caused disturbances, and biotoxins—like the red tides caused by algae that are prevalent along American coastlines—are three major culprits.

Big die-offs can permanently change food webs. Ninety-nine percent of the sea urchin Diadema antillarum disappeared from the Caribbean in 1983 thanks to a pathogen. The herbivore's vanishing act paved the way for an algal invasion of reefs, smothering corals.

Massive die-offs can also endanger human activities like farming by disrupting insects that pollinate plants, like bees.

"Such events can reshape the ecological and evolutionary trajectories of life on Earth," the study authors write. (See "Why Are Millions of Starfish 'Melting'?")

Mass Animal Die-Offs Are on the Rise, Killing Billions and Raising Questions

ABSTRACT: Mass mortality events (MMEs) are rapidly occurring catastrophic demographic events that punctuate background mortality levels. Individual MMEs are staggering in their observed magnitude: removing more than 90% of a population, resulting in the death of more than a billion individuals, or producing 700 million tons of dead biomass in a single event. Despite extensive documentation of individual MMEs, we have no understanding of the major features characterizing the occurrence and magnitude of MMEs, their causes, or trends through time. Thus, no framework exists for contextualizing MMEs in the wake of ongoing global and regional perturbations to natural systems. Here we present an analysis of 727 published MMEs from across the globe, affecting 2,407 animal populations. We show that the magnitude of MMEs has been intensifying for birds, fishes, and marine invertebrates; invariant for mammals; and decreasing for reptiles and amphibians. These shifts in magnitude proved robust when we accounted for an increase in the occurrence of MMEs since 1940. However, it remains unclear whether the increase in the occurrence of MMEs represents a true pattern or simply a perceived increase. Regardless, the increase in MMEs appears to be associated with a rise in disease emergence, biotoxicity, and events produced by multiple interacting stressors, yet temporal trends in MME causes varied among taxa and may be associated with increased detectability. In addition, MMEs with the largest magnitudes were those that resulted from multiple stressors, starvation, and disease. These results advance our understanding of rare demographic processes and their relationship to global and regional perturbations to natural systems.

Recent shifts in the occurrence, cause, and magnitude of animal mass mortality events



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