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Earth is made of cycles. If you think back to high school Earth science class, you might remember the water cycle, the rock cycle, and the oxygen cycle, to name just a few. These natural processes continuously recycle our planet’s materials, maintaining the environment that hosts life as we know it.
The nutrient cycle is another crucial example of our planet’s constant churn. Part of the cycle consists of plants consuming nutrients from the soil, herbivores consuming nutrients from the plants, and then redelivering the nutrients back into the soil via their feces and urine. And a little poop can go a long way.
In a recent Ecology study, researchers from the Smithsonian’s National Zoo and Conservation Biology Institute (NZCBI) investigating the nutrient cycle in prairie grasslands revealed that prairie dogs and grasshoppers contribute more than much larger prairie fellows such as bison and cattle.
“While previous work has examined the roles of herbivores as nutrient cyclers, the comparison across the different herbivore body sizes is a relatively novel component of this study and I wanted to know more about that topic,” Ellen Welti, an ecologist at the Smithsonian Institution and senior author of the study, tells Popular Science..
Specifically, Welti and her colleagues studied how prairie dogs, grasshoppers, bison, and cattle contribute nutrients to grass and soil in 15 shortgrass prairie sites in northeast Montana. They found that prairie dogs were most beneficial to the prairie’s nutrient availability. Notably, Prairie dog towns (yes, that’s really what the areas they live in are called) feature the highest levels of carbon and nitrogen in the soil. This concentration increased when there was more of another small herbivore around—grasshoppers.
“There were way more grasshoppers on PD [prairie dog] towns than any other place in the grassland,” Welti explains. “There appears to be a mutualism here where grasshoppers benefit from the nutrient cycling of prairie dogs and are attracted to the high densities of plant tissue nutrients on PD towns.
Furthermore, the prairie dogs increased nitrogen, potassium, and magnesium in grass tissue. The researchers believe this could be due to their excretions and burrowing behaviors, that better integrate the nutrients in the soil. As for grasshoppers, they increased the soil’s phosphorus.
How are such small creatures able to contribute more than larger ones? Welti says that, “while ‘smaller’ herbivores are smaller on an individual comparison basis, they are generally much more abundant than ‘larger’ herbivores. In terms of total biomass/area, small herbivores may equal or have larger values to larger herbivores.”
What’s more, the researchers noted that grasshoppers’ life cycle is probably responsible for the increase in prairie plant biomass that usually takes place during midsummer before decreasing again. Baby grasshoppers, called nymphs, hatch in the spring and become adults—with an adult-sized appetite—during late summer and fall. This cycle aligns with the typical decline in plant biomass at the end of the growing season. Cattle and bison, on the other hand, are more or less constant amounts of plants during the summer.
Grasslands are some of the most at-risk terrestrial environments, and North American populations of bison and prairie dogs have sharply declined since European arrival. The study highlights the role of the grasslands’ small yet most helpful residents, with significant implications for conservation efforts.
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