Mass mortality events among birds, fish and marine invertebrates have been increasing in frequency and magnitude over the past 70 years, a new analysis of 727 mass die-offs of nearly 2,500 animal species shows.
The number of individuals killed appears to be decreasing for reptiles and amphibians, and unchanged for mammals, researchers said.
Such mass mortality events occur when a large percentage of a population dies in a short time frame.
While the die-offs are rare and fall short of extinction, they can pack a devastating punch, potentially killing more than 90 per cent of a population in one shot.
However, until this study, there had been no quantitative analysis of the patterns of mass mortality events among animals, researchers noted.
"This is the first attempt to quantify patterns in the frequency, magnitude and cause of such mass kill events," said study senior author Stephanie Carlson, from the University of California, Berkeley.
The researchers at UC Berkeley, the University of San Diego and Yale University reviewed incidents of mass kills documented in scientific literature. The analysis focused on the period from 1940 to the present.
The researchers acknowledged that some of their findings may be due to an increase in the reporting of mass die-offs in recent decades.
But they noted that even after accounting for some of this reporting bias, there was still an increase in mass die-offs for certain animals.
Overall, disease was the primary culprit, accounting for 26 per cent of the mass die-offs. Direct effects tied to humans, such as environmental contamination, caused 19 per cent of the mass kills.
Biotoxicity triggered by events such as algae blooms accounted for a significant proportion of deaths, and processes directly influenced by climate - including weather extremes, thermal stress, oxygen stress or starvation - collectively contributed to about 25 per cent of mass mortality events.
The most severe events were those with multiple causes, the study found.
The study found that the number of mass mortality events has been increasing by about one event per year over the 70 years the study covered.
The study was published in the journal PNAS.
