MANHATTAN, Kansas — Ellen Welti has a Ph.D. in, essentially, grasshoppers.
And yet she was still mystified about why the number of grasshoppers in a long-protected and much-studied patch of Kansas prairie was dropping. Steadily. For 25 years.
After all, the grass that the springy bugs feast on had actually grown more robustly as it absorbed mounting levels of carbon dioxide in the atmosphere.
So why were the grasshoppers faring increasingly worse?
“We thought that this is a pretty nice habitat for grasshoppers,” she said.
The insects dwell on the Konza Long-Term Ecological Research site. Their home sat in preserve, shielded from development, from farming, from just about everything people do to the planet.
“It doesn’t have a lot of the pressures we usually associate with insect decline. Like, there’s no pesticide spraying,” Welti said. “The size of the habitat is not shrinking. It’s the big natural reserve.”
And yet the numbers showed an alarming decline — nearly 2% a year for about a quarter century.
“It’s like compound interest,” said Michael Kaspari, a research professor at the University of Oklahoma who advised Welti on the paper. “You add that up over 25 years and we’re talking about a significant decline in one of the most important insects on the prairie.”
Welti and her research team found that all those human-boosted levels of carbon dioxide in the air changed the grass. Bigger leaves, sure, but all that volume diluted the nutrients grasshoppers need to thrive.
Basically, climate change had converted the grasshopper’s staple into insect junk food.
“That was a surprise for me,” Welti said. “But thinking more about it and knowing about grasshoppers and how sensitive they are to these plant nutrients, it makes a lot of sense.”
A normal amount of grasshoppers on the prairie can eat as much grass as a herd of cattle. Grasshoppers are a huge part of the tallgrass ecosystem. But they’re also limited. They really just eat the grass, so when it changes, it can have a huge impact.
To figure out just what was going on, Welti went back to the Konza research station to take a deeper look. Researchers there have tens of thousands of grass samples from the last 40 years.
“Ecology can happen rather slowly,” said Amanda Kuhl, a research assistant charged with collecting and cataloguing the samples. “Sometimes it can take decades to notice a trend, or tease things apart.”
In one of the storage rooms at the Konza research building on the campus of Kansas State University, brown paper bags full of grass and dirt samples fill the walls from the floor to the ceiling. And those bags are just some of the samples from last year.
“Once they get these cleaned up they’ll go back into a drying oven to remove all the moisture,” Kuhl said. “I’ll weigh them and that’s how we get our biomass numbers.”
The facility also has thousands of small envelopes full of ground-up samples. They can be used to run chemical analysis.
The biomass numbers paired with the chemical analysis led Welti to what was causing the grasshopper declines: bigger, but less nutritious grass.
“At first, it looks like there’s overall more food availability for grasshoppers,” she said. “But when we got back the plant chemistry, what we could see is that this increase in plant biomass really corresponded to decreases in the quality of the biomass.”
It’s something known as nutrient dilution.
“Every bite of those grasses is giving less and less back to all the other living things that depend on the plants,” Kaspari said.
The change is driven by an increase in the amount of carbon dioxide in the atmosphere from burning fossil fuels. Every time the plants take in air it contains far more carbon than ever before. They can’t get rid of it so they stick it in their tissue. That equals bigger plants.
The problem is that the amount of other nutrients in the soil, such as nitrogen, potassium and sodium, aren’t really changing.
It’s like switching out a plate of kale for a plate of iceberg lettuce. The plates might have the same amount, but one has a whole lot more of the good stuff bodies need.
Kaspari said the fact that the problem is rooted in too much carbon dioxide in the atmosphere is particularly daunting.
“You can remediate habitat destruction,” he said. “You just stop doing it and you reforest, or reclaim the grassland. But the answer to nutrient dilution is actually reducing the CO2 in the atmosphere.”
Welti said the findings were pretty clear. Where they found large, but less nutritious, grass, they found grasshopper declines.
And unlike humans, who can use fertilizer to increase nutrients in our food or take a multivitamin to supplement any gaps, grasshoppers are stuck with what is there naturally.
Welti said the findings are about more than the bugs. Fewer grasshoppers will lead to fewer birds. Fewer birds could lead to more rodents.
The decline of one species of grasshopper could strengthen another, potentially leading to swarms that damage crops.
And the underlying problem, nutrient dilution due to climate change, will impact all animals that rely on eating plants. From grasshoppers and leaf beetles up to bison and cattle. And eventually humans.
“Even (at the Konza Prairie),” she said, “climate change can strongly affect the community and even lead to insect declines — which is pretty depressing.”
— Brian Grimmett reports on the environment, energy and natural resources for KMUW in Wichita and the Kansas News Service. You can follow him on Twitter @briangrimmett or email him at grimmett (at) kmuw (dot) org. The Kansas News Service is a collaboration of KCUR, Kansas Public Radio, KMUW and High Plains Public Radio focused on health, the social determinants of health and their connection to public policy.
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