New York, Jan 13 (IANS): Mass mortality events among birds, fish and marine invertebrates have been increasing in frequency and magnitude over the past 70 years, shows an analysis of 727 mass die-offs of nearly 2,500 animal species.

The number of mass mortality events has been increasing by about one event per year over the 70 years the study covered, the findings showed.

"Going from one event to 70 each year is a substantial increase, especially given the increased magnitudes of mass mortality events for some of these organisms," said study co-lead author Adam Siepielski, assistant professor of biology at the University of San Diego.

The researchers reviewed incidents of mass die-offs documented in scientific literature. The analysis focused on the period from 1940 to the present.

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 die-offs.

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.

At the same time, the number of individuals killed appears to be decreasing for reptiles and amphibians, and unchanged for mammals, the findings showed.

The study was published in the journal Proceedings of the National Academy of Sciences.