Dramatic insect declines previously reported around the world are also occurring in Colorado. Researchers with the Rocky Mountain Biological Laboratory, or RMBL, report that flying insects in the mountains outside of Crested Butte have declined more than 60% since 1986.
The current research, published in the scientific journal Ecosphere, is noteworthy for the length of time covered and the relatively undisturbed mountain environment where it was conducted. The declines correlated with drier and warmer weather, suggesting an impact of climate change.
“Increasingly we are seeing insect declines in places that are more pristine, which is much more alarming,” said Julian Resasco, assistant professor of ecology and evolutionary biology at the University of Colorado.
While historically seen as agricultural pests and personal nuisances, insects and other invertebrates (no backbone) are increasingly recognized for the vital services they provide in nature: pollination, pest control, nutrient cycling and sustenance for birds and other animals higher on the food chain. The continued decline of insect populations could have profound consequences for the environment, humans and other animals.
“We rely on insects for ecosystem services. We need them to be abundant and diverse,” Resasco said.
Concern about declining insect populations surged in 2017 after researchers reported that flying insects in Germany had declined by more than 75% over 27 years. That was followed by several studies mostly, but not uniformly, reporting alarming declines in insect populations around the world. The reality and the causes of insect decline are ongoing debates among entomologists.
For their study, the RMBL researchers set up a tentlike trap in the middle of a 27-acre meadow at the Rocky Mountain Biological Laboratory, 9,500 feet above sea level near the abandoned mining town of Gothic. Surrounded by the peaks and meadows of the Elk Mountains, the setting is stunning — and far removed from intensive agriculture, urban growth, pesticide use and other human activities that have been blamed for insect declines.
“We thought that it was important for us to look at a site that is free from all those influences,” said David Inouye, co-author on the research paper, and a professor emeritus at the University of Maryland.
Two days a week, the researchers capture flying insects — mostly bees, wasps and flies. They count and dry the insects, weigh them and divide them into several broad groupings. Since 1984, researchers have captured and recorded data about the insects every week of every subalpine summer for 40 years.
“If you want to see a long-term trend, you need decades of data,” Inouye said. Insect populations can fluctuate several fold from year to year. Data collected over a longer period helps identify less dramatic long-term trends. The current study is the longest controlled study of insects in Colorado and one of the longest in the United States.
The project has lasted so long that it has relied on three generations of scientists. Authors on the paper include the now-deceased originator of the work, Michael Soulé; David Inouye, who is spending his 53rd season at the laboratory this summer; and David’s son, Brian Inouye, and daughter-in-law, Nora Underwood, both professors of ecology and evolution at Florida State University.
The paper analyzes 35 years of data, from 1986 through the summer of 2020. The researchers documented a 62% decline in the number of insects captured and a 49% decline in their total weight over the period. The insect decline was correlated with less winter snowfall, less summer rain and warmer temperatures.
Average annual snowfall at the laboratory fell sharply during the study period, to 344 inches per year from 463 inches. Abundant winter snow cover provides protective insulation to overwintering insects. Average summer rainfall did not change significantly during the study’s 35 years, but years of low summer rainfall had fewer insects. Summer rainfall promotes plant growth that feeds many insects. Average temperature rose about 2 degrees Fahrenheit during the study period and was correlated with the insect decline, although less so than precipitation.
“Changes in precipitation and warmer temperatures are expected to continue under climate change,” the researchers wrote in their report. “Thus, continued insect declines might be expected even in relatively undisturbed habitats.”
“We should be concerned,” Underwood said. “There are a lot of cascading effects of insects.”
Fewer insects can mean less food for other animals, fewer flowers pollinated and fewer nutrients recycled through the environment. Underwood does have faith in the resilience of nature and is not predicting an imminent insect apocalypse or deserts in the mountains. But she notes that the study documents big changes occurring to important players in the environment with likely, but unknown, impacts occurring as climate change continues.
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Underwood invokes the rivet hypothesis by famed biologist Paul Ehrlich, for whom both she and Brian worked during summers when he came to RMBL. An airplane has thousands of rivets holding it together. You can remove one rivet without causing any trouble. But if you keep removing rivets — or insects — eventually the plane will fall apart and crash. No one knows which is the crucial rivet, and maybe it is best to keep as many as possible.
David Inouye believes the insect declines in Colorado and around the nation may have already rippled through the environment. In 2019, researchers reported an alarming 29% decline in North American birds, a net loss of 3 billion birds, since 1970. Birds that feed on insects were a prominent portion of those losses. Around the Rocky Mountain Biological Laboratory, researchers have likewise documented a striking decline in white-crowned sparrows, an insect-eating bird whose distinctive call is heard less often than in past years.
Insects and white-crowned sparrows are just one of several changes that David Inouye has observed in his decades at the laboratory. Moose and fox now live there year-round, and a Wyoming ground squirrel has moved up from lower-elevation Almont, to Crested Butte and now the laboratory. Ticks and mosquitoes that can carry West Nile virus have also appeared around the laboratory in recent years. Wildflowers are blooming earlier.
“I think in the long term, most people are going to find those changes undesirable,” he said.
