Massive mortality events (MMEs) occur when large numbers of populations die within a narrow time frame, due to disease, overhunting, habitat destruction, or environmental factors. The largest study ever performed on these die-offs have found that these events are becoming increasingly common among species of fish, birds, and marine invertebrates over the last 70 years. Stephanie Carlson of UC Berkeley was senior author of the paper, which was published in the Proceedings of the National Academy of Sciences.
"This is the first attempt to quantify patterns in the frequency, magnitude and cause of such mass kill events," Carlson said in a press release.
Carlson’s team hunted the scientific literature and analyzed 727 MMEs, spanning almost 2,500 species since 1940. Ultimately, the team found that while MMEs are increasing in fish, birds, and marine invertebrates, not all animals are experiencing the same bleak fate. Die-offs are becoming less common in amphibians and reptiles, while mammals are remaining fairly constant.
MMEs are so devastating because they can kill up to 90% of a population more quickly than the species can adapt. This results in a bottleneck effect of the species’ genetics, drastically reducing genetic diversity and thus, the species’ potential for long-term survival. It takes a considerable amount of time for the species to recover, if it is capable of doing so.
"The catastrophic nature of sudden, mass die-offs of animal populations inherently captures human attention," Carlson continued. "In our studies, we have come across mass kills of federal fish species during the summer drought season as small streams dry up. The majority of studies we reviewed were of fish. When oxygen levels are depressed in the water column, the impact can affect a variety of species.”
Disease was named as the number one cause of MMEs, as it was responsible for 26% of the die-offs. Environmental factors attributed to climate change, such as extreme weather, algae blooms, and thermal stress, amounted to 25% of all events. A total of 19% of MMEs were caused by human activity, such as pollution or habitat destruction. Since 1940, Carlson’s team found that these instances are increasing at a rate of about one per year.
"While this might not seem like much, one additional mass mortality event per year over 70 years translates into a considerable increase in the number of these events being reported each year," added co-lead author Adam Siepielski of the University of San Diego.
This study also taught the team what information is needed when documenting changes to the climate and environment, and how critical it is to account for the biodiversity. This will not only help create a more complete record of future MMEs, but might even help address concerns before they become major problems for animal populations.
"The initial patterns are a bit surprising, in terms of the documented changes to frequencies of occurrences, magnitudes of each event and the causes of mass mortality," explained co-lead author Samuel Fey of Yale. "Yet these data show that we have a lot of room to improve how we document and study these types of rare events.”