Mass deaths among birds, fish, marine invertebrates on the up over past 70 yrs

Scientists have recorded mass die off's in nearly 2,500 animal species from the past 70 years, and found a rise in the events among birds, fish and marine invertebrates.

An analysis of 727 studies, which also showed that the number of individuals killed appeared to be decreasing for reptiles and amphibians, and unchanged for mammals, points towards disease, biotoxicity and other stressors.

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 percent 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, the study authors noted.

The study was led by researchers at UC Berkeley, the University of San Diego and Yale University, and senior author Stephanie Carlson, an associate professor, said that the attempt was the first to quantify patterns in the frequency, magnitude and cause of such mass kill events.

The researchers reviewed incidents of mass kills documented in scientific literature. The analysis focused on the period from 1940 to the present. They 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 percent of the mass die-offs. Direct effects tied to humans, such as environmental contamination, caused 19 percent 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 percent 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.

This study suggests that in addition to monitoring physical changes such as changes in temperature and precipitation patterns, it is important to document the biological response to regional and global environmental change. The researchers highlighted ways to improve documentation of such events in the future, including the possible use of citizen science to record mass mortality events in real time.

The study is published in the Proceedings of the National Academy of Sciences.