28
Jan
Documented Decline of Mayflies, a Keystone Species, Destabilizes Ecosystems
(Beyond Pesticides, January 28, 2020) In more bad news from the insect world, recent research reveals a precipitous decline in numbers of mayflies in territories where they have been historically abundant. Reported by National Geographic and published in the Proceedings of the National Academy of Sciences, the research finds that in the Northern Mississippi River Basin, seasonal emergence of burrowing mayfly (genus Hexagenia) adults declined by 52% from 2012 to 2019; in the Western Lake Erie Basin, from 2015 to 2019, the reduction was a shocking 84%. Neonicotinoid insecticides are a significant factor in this decline because mayflies are exquisitely vulnerable to their impacts, even at very low exposure levels.
Ephemeroptera to entomologists — and “mayflies” to the rest of us — are a keystone species, one on which other species in an ecosystem are very dependent, and without which, the ecosystem would undergo drastic change. The Latinate name is apt: mayflies are among the most short-lived organisms, with lifespans across the 2,000+ known species lasting from five minutes to one day to a few weeks. Like damselflies and dragonflies, members of an ancient group of insects, the 600 North American species, as do their global kin, make optimal reproductive use of their brief adult lifespans: each female can lay from as few as 50 to as many as 10,000 eggs on the surfaces of water bodies. Those eggs then sink to the lake or river floor, after which they emerge as nymphs and spend up to two years foraging on bottom sediments before appearing as adult mayflies. Read more about the unusual lifecycle of the mayfly here.
The plummeting mayfly “count” is especially alarming because mayflies are a critical, primary food source in aquatic and terrestrial ecosystems, and provide an important ecological service. As the research study notes, “Seasonal animal movement among disparate habitats is a fundamental mechanism by which energy, nutrients, and biomass are transported across ecotones. A dramatic example of such exchange is the annual emergence of mayfly swarms from freshwater benthic [lake or river bottom] habitats. . . . Annual . . . emergences represent the exchange of hundreds of tons of elemental nutrients, thousands of tons of biomass, billions of organisms, and trillions of calories worth of energy to the surrounding terrestrial habitat. . . . A single emergence event can produce 87.9 billion mayflies, releasing 3,078.6 tons of biomass into the airspace over several hours.”
Jason Hoverman, an ecologist at Purdue University, commented, “Mayflies serve critical functions in both aquatic and terrestrial ecosystems. Because of their important role as prey, reductions in their abundance can have cascading effects on consumers throughout the food web.” In a Beyond Pesticides Pesticides and You article, “Poisoned Waterways,” the authors put it squarely: “Without this critical keystone species, an important food source and nutrient recycler would be lost.”
In addition, because they thrive only in clean, high-quality water, mayfly populations serve as barometers of water quality, according to Kenneth Krieger, emeritus director of the National Center for Water Quality Research at Heidelberg University, and an expert on Lake Erie mayflies. The study co-authors conclude, “As ecological indicators, these losses may signal deterioration in water quality and, if current population trends continue, could cascade to widespread disappearance from some of North America’s largest waterways.”
Three phenomena account for most of this dive in mayfly populations: (1) dramatically increased use of neonicotinoid pesticides in recent years in these regions; (2) algal blooms, especially in Lake Erie, caused primarily by runoff of agricultural fertilizers and other nutrient-dense pollutants; and (3) the warming impacts of the climate crisis, which include higher water temperatures that can cause havoc with the development of these tiny creatures. Taken together, these factors are complexes of nasty synergies:
• nutrients in fertilizer runoff feed algal blooms that starve water of oxygen and generate toxic byproducts to which mayflies are susceptible
- warming waters increase stratification of waters, reducing mixing of nutrients and oxygen among the layers, which can then lead to transient or chronic hypoxia (low oxygen levels) that are lethal to waterway-bottom-dwelling mayfly nymphs
- the increasing concentrations of pesticides, and neonicotinoids in particular, in freshwater ecosystems exacerbate risks to mayfly populations
One of the most widely used classes of insecticides, neonicotinoids are highly toxic, damaging insects’ central nervous systems, and causing death even at low-level exposures. Neonicotinoids (neonics) are used on millions of acres of U.S. cropland, on nursery plants, and to treat seeds. Because neonics persist in soil and easily become airborne, they can travel to contaminate nearby waterways. Agricultural runoff increasingly includes neonic metabolites. Neonics in Great Lakes tributaries, for example, have registered at levels 40 times those established as protective by the Environmental Protection Agency’s (EPA’s) Aquatic Life Benchmark, according to a 2018 study.
In addition to their well-documented harms to bees, neonics represent a grave threat to insects such as mayflies. Indeed, in 2017, EPA identified mayflies as the most sensitive aquatic invertebrate to imidacloprid (a neonicotinoid) exposure. An EPA document said, “The potential exists for indirect risks to fish and aquatic-phase amphibians through reduction in their invertebrate prey-base,” e.g., mayflies. For more on the threats of these compounds, see the article, “Poisoned Waterways,” in Beyond Pesticides journal, Pesticides and You.
The loss of mayfly populations means more than just fewer insects for organisms higher on the food chain to consume; impacts can ripple through ecosystems. Although neonics are directly toxic to many insects, the role of pesticides in destabilization of ecosystems is not necessarily direct. In a Pesticides and You article, “Pesticide Use Harming Key Species Ripples through the Ecosystem,” the author explains the dynamics of trophic cascades. “Beyond direct toxicity, pesticides can significantly reduce, change the behavior of, or destroy populations of plants and animals. These effects can ripple up and down food chains, causing what is known as a trophic cascade. . . . The loss or reduction of populations at any trophic level — including amphibians, insects, or plants — can result in changes that are difficult to perceive, but nonetheless equally damaging to the stability and long-term health of an ecosystem.”
Such enormous losses of a keystone prey organism — mayflies — as the study researchers discovered represent a potential trigger of such trophic cascades in the identified watersheds. In the above-referenced article, the mechanisms of such a cascade are described. In that instance, the runoff of an agricultural pesticide (bifenthrin) caused significant downward population pressure on larval macroinvertebrates (including mayfly nymphs). That loss of periphyton-eating species (mayflies, stoneflies, and caddisflies) initiated a trophic cascade from the top down, causing algal blooms. In addition, the pesticide’s impact on the endocrine systems of macroinvertebrates caused an acceleration of the interval to metamorphosis, resulting in earlier emergence and smaller size than is typical.
As the authors wrote in “Poisoned Waterways, “Current federal aquatic life benchmarks for neonicotinoids are potentially underestimating risks. Experts find that standard test organisms used by EPA to establish these benchmarks are orders of magnitude more tolerant of neonicotinoid exposure than [are] other vulnerable species [such as Ephemeroptera], and recommend water levels to be well below the benchmarks set by EPA. In addition, chemical admixtures and potential synergistic effects are not considered in aquatic risk assessments, resulting in unknown risks to species. Stronger action is needed to restrict neonicotinoid contamination of waterways.”
As Beyond Pesticides wrote in 2017, “Action must be taken to protect vulnerable waterways from neonicotinoid contamination. The frequency of detections in U.S. waterways cannot be overlooked. Such routine detections, even at low levels, indicate that our waterways are being overloaded with mobile and persistent chemicals at highly elevated concentrations. . . . Thus far, little action has been taken to restrict the use of these chemicals in response to the independent scientific literature and EPA risk data that identify direct threats to aquatic invertebrates, as well as indirect threats to higher trophic organisms. . . . Federal benchmarks based on testing on insensitive species are not protective of more sensitive species. Given the toxicity of this class of insecticides to non-target, beneficial invertebrates, and the regulatory deficiencies, it is imperative that action be taken to limit their use and presence in waterways in the U.S.”
Given the plethora of harms caused by industrial and agricultural chemical contamination of our water, air, soil, food, and bodies, it is more important than ever that members of the public step up and speak up, contact their local, state, and federal elected officials, and do all they can to advance a shift to less- and nontoxic practices that will protect insects — and all living organisms. Stay current on these issues by following Beyond Pesticides’ Daily News Blog and journal, Pesticides and You, and find organizational resources here.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Sources: https://www.nationalgeographic.com/animals/2020/01/mayfly-insect-populations-in-decline/#close and https://www.pnas.org/content/early/2020/01/15/1913598117
