Mass die-offs of birds, fish and marine invertebrates may be increasing both in frequency and severity, according to a new study which looked at 727 mass mortality events over the past 70 years.
Reptiles and amphibians, on the other hand, are seeing a decrease in death toll, while mammals seem to be unaffected either way.
Much is already known about the importance of individual mass mortality events, defined as a larger than normal drop in population, but prior to this study, little research has been conducted on patterns across mass mortality events.
“This is the first attempt to quantify patterns in the frequency, magnitude and cause of such mass kill events,” said study senior author Stephanie Carlson.
The research team, comprised of scientists from the universities of Sand Diego and California-Berkeley, reviewed all the documented cases of mass mortality events since 1940 to the present.
They reported that even though the past several decades saw more research done on mass die-offs, this “discovery bias” is not enough to explain the consistent pattern they uncovered.
A closer analysis indentified disease emergence, human activities, biotoxicity and climate-related stressors as the main culprits.
Disease came in first, accounting for around 26 per cent of the die-offs. Anthropogenic contamination of the environment contributed 19 per cent. Another major cause was biotoxicity, triggered by such events as algae bloom – a rapid increase of algae in water systems. And processes directly influenced by the climate – such as weather extremes, oxygen stress and starvation -collectively caused around 25 per cent of the mass kill events.
The gravest events were those with several causes. The group had observed such die-offs in their studies of fish in California, which made them interested in this topic in the first place.
“In our studies, we have come across mass kill 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 the water column, the impact can affect a variety of species,” claimed Carlson.
The study also found that each year the rate of mass mortality events has been increasing by one individual event for the past 70 years.
“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,” explained co-lead author Adam Siepielski.
“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 authors suggest that monitoring and documenting biological response to global changes in the environment is as important as monitoring temperature fluctuations and precipitation levels. One of the proposed means for improving such documentation is involving citizens, capable of carrying out the required procedures.
The study was published in the Proceedings of the National Academy of Sciences on the 12th of January.