Written by: douglas main
Ephemeral populations are dwindling dramatically, and billions of ephemera are dying in droves, as seen on weather radar, reflecting the plight of insects around the world.
In Minnesota, a hexagenia limbata that dug holes had just landed on its blades. Between 2012 and 2019, ephemera populations in the upper Mississippi River basin fell by 52 percent. Photo by Michael Francis, Earth Scenes, Nat Geo Image Collection
Every summer, ephemerals in lakes and rivers erupt suddenly, and they fly in groups into the skies of North America. These insects are especially abundant in the Northern Mississippi River Basin and the Great Lakes, and are aquatic during the juvenile period, after which they will molt and feather into sub-adults and adults. When ephemera assemble, they sometimes form an army of up to 80 billion adult insects, and because of their large numbers, they are sometimes shoveled up by snowplows in waterside towns.
The proliferation of ephemerals has provided food for a variety of animals, such as perch and other important economic freshwater fish, as well as birds and bats. But new research suggests that ephemeral populations are declining. A recent study published in the Proceedings of the National Academy of Sciences found that ephemeral populations have declined by more than 50 percent throughout northern Mississippi and the Lake Erie region since 2012, with pollution and algal blooms likely being the two biggest culprits.
"We were surprised to find that the number of ephemera is decreasing year by year. This is so unexpected," said Phillip Stepanian, a biometeorologist at the University of Notre Dame, the study's lead author.
A large swarm of ephemera appeared on weather radars used to track rain and snow, and for a long time meteorologists dismissed these signals as a "noise," he said. But, as an experienced meteorologist, Stepanian realized that these signals could provide some very useful information, such as the number and movement of animals like birds and insects.
The animation shows an ephemeral swarm that erupted on Lake Erie in late June, and according to weather radar monitoring, the number of ephemera may exceed 2 billion.
In the paper, Stepanian and colleagues used radar to estimate the number of ephemera and validated the method by comparing it with the number of ephemerals in sediments at the bottom of rivers and lakes.
Studies show that between 2015 and 2019, the number of burrowing ephemera in the genus Hexagenia in western Lake Erie decreased by 84 percent. In the northern part of the nearby Mississippi River Basin, ephemeral populations fell by 52 percent between 2012 and 2019.
The drastic decline in ephemeral populations can have a significant ecological impact, as insects are an important link in the food chain and prey for a variety of predators. They also transfer tons of nutrients from water to land, a valuable ecological service.
Ephemerals go through two periods of metamorphosis. When they change from juvenile to sub-adult, they inhabit shores or on objects like lampposts. After a few hours, they dry out, molt again, become adults, fly to their mates at dusk to complete mating — and then die. Photo by Phillip Stepanian
"Ephemerals play a key role in both aquatic and terrestrial ecosystems," said Jason Hoverman, an ecologist at Purdue University.
"As predators, their position is crucial, and their reduction in number will have a knock-on effect on consumers across the food web."
Possible causes
There are several possible reasons for the decline in ephemeral populations. First, in recent years, levels of neonicotinoid pesticides have risen in many freshwater systems in Lake Erie and the Midwest, chemicals that are toxic to many insects. According to a 2018 study, the levels of such pesticides in the Tributaries of the Great Lakes are 40 times higher than the baseline level of protection for pesticides and aquatic organisms set by the U.S. Environmental Protection Agency.
Second, Lake Erie is plagued by algal blooms, which are caused by runoff rich in fertilizers and other pollutants. Algal blooms can lead to oxygen-starved "dead zones" that are toxic to seafloor-dwelling organisms such as ephemerals. Third, as the climate changes, so does the water temperature, which can affect the animal's life cycle and potentially reduce oxygen circulation in the lake.
Kenneth Krieger, honorary director of the national center for water quality research at Heidelberg University, said ephemera can serve as an overall indicator of water quality, which is why the decline in ephemeral populations is worrying.
For humans, ephemera groups may be a bit annoying, or they may also find it magical. As predators, they provide food for many creatures, such as fish and birds. Photo by Phillip Stepanian
"Other aquatic insect species may be experiencing the same decline for the same reason," added Francisco Sanchez-bayo, an ecologist at the University of Sydney in Australia. "The number of insect-feeding birds, frogs, bats and fish in these areas will also decrease, which is an inevitable consequence."
The Revelation of Insects
Other insects face the same dilemma: studies around the globe have shown that the number of many insects is on the decline. A study published last April in Biological Conservation showed that 40 percent of insect species are declining and could become extinct in the coming decades.
Neonicotinoid insecticides are notorious for their toxicity to aquatic insects, and ephemera seem to be particularly sensitive to them. Another recent study found that the use of neonicotinoid in a lake in Japan led to a decline in aquatic invertebrates, as did the number of two species of fish of great commercial value that fed on them.
Ephemeral populations declined decades ago and have since recovered, but the continued decline in recent years is troubling, hoverman said.
"A growing body of research shows that insect populations have fallen dramatically, and this study adds another example," he said.