Mass Die-Offs Increase for Birds, Fish

According to a new study, mass die-offs have been increasing for birds, fish and marine invertebrates over the last several decades, affecting nearly 2,500 animal species.

A mass die-off occurs when a large percentage of a population perishes in a short amount of time. Though these events are rare and don't necessarily drive a species to extinction, they still pack a devastating punch and are capable of killing more than 90 percent of a population in one fell swoop. There have been 727 mass mortality events within the last 70 years, and this study is the first to calculate their frequency and magnitude, as well as determine their causes.

The findings were published in the journal Proceedings of the National Academy of Sciences.

In an analysis of previous studies and data, focusing on the period from 1940 to the present, researchers found that disease was primarily to blame for the downfall of many animal populations - accounting for 26 percent of the mass die-offs. Climate processes, including weather extremes, thermal and oxygen stress, or starvation came in second overall, contributing about 25 percent to mass mortality events.

Humans played a part as well, with activities such as environmental contamination causing 19 percent of the mass kills. In addition, biotoxicity triggered by events such as algae blooms were a significant contributor, though the most devastating die-offs were those that had multiple origins.

And these catastrophic killings show no signs of stopping. According to the results, the number of mass die-offs has long been on the rise, increasing by about one event per year over the 70-year study period.

"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," study co-lead author Adam Siepielski, an assistant professor of biology at the University of San Diego, explained in a statement.

Interestingly, while birds, fish and marine invertebrates are suffering, the number of mass die-offs for reptiles and amphibians seems to be decreasing, while it remains unchanged for mammals.

"The catastrophic nature of sudden, mass die-offs of animal populations inherently captures human attention," added senior author Stephanie Carlson.

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