Scientists have recorded mass mortality events in nearly 2,500 animal species from the past 70 years, and found a rise in the events among birds, fish and marine invertebrates. The study also showed that the number of individuals killed appeared to be decreasing for reptiles and amphibians, and remained unchanged for mammals. This points towards disease, bio-toxicity and other stressors.
Researchers at UC Berkeley, the University of San Diego and Yale University, analyzed 727 studies to quantify patterns in the frequency, magnitude and cause of such mass kill events. They reviewed incidents of mass kills documented in scientific literature while focusing on the period from 1940 to the present. Researchers acknowledged that some of their findings could be due to an increase in the reporting of mass die-offs in recent decades. However, even after accounting for some of this reporting bias, scientists still noted an increase in mass die-offs for certain animal species.
Researchers found disease was the primary culprit, accounting for 26 percent of the mass die-offs. Direct effects tied to humans, like environmental contamination, caused 19 percent of the mass kills. Bio-toxicity triggered by events such as algae blooms accounted for a significant proportion of deaths, and processes directly influenced by the climate, including weather extremes, thermal stress, oxygen stress or starvation, which collectively contributed to about 25 percent of mass mortality events.
The study also found that the number of mass mortality events has been increasing by about 1 event per year over the 70 years the study covered. The 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 study appears in the Proceedings of the National Academy of Sciences.