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Tiny, squiggly earthworms that burrow underground, and large white-tailed deer that trod the fields and forests above them, may not appear to have much in common. But a new study led by a University of Minnesota graduate student shows surprising connections between the two species. 

, shows that in northern forests, the presence of deer substantially increased the number of earthworms. 

That’s an important connection, scientists say, because earthworms are invasive to Minnesota, and have hugely hurt the health of forests. 

"Invasive earthworms are ecosystem engineers which negatively impact fundamental ecosystem properties such as nutrient retention and the diversity of native plant species,” said Lee Frelich, Director of the university’s Center for Forest Ecology and an advisor on the research. “Deer exacerbate these negative impacts by increasing earthworm populations.”

Researchers sampled earthworms in two long-running experiments in northern Wisconsin, where scientists have used fencing to exclude deer from certain areas, and also harvested trees in select spots to create gaps in the forest canopy. 

To count the worms, they poured a slurry of mustard and water over the soil, which causes worms to shoot to the surface. They compared their samples with data collected more than a decade ago when the experiments began. 

The presence of deer caused the number of worms to increase substantially. 

“It’s enough that it could make a significant difference in the function of the ecosystem,” Frelich said. 

The type of worms that seem to benefit most from deer, Frelich said, are the species that have the most detrimental impacts on forests.

Researchers have several theories as to why deer may increase the number of worms. One is pretty straightforward. 

“Deer are peeing and pooping in the forest. And that fertilizes the soil and favors earthworms,” explained Sam Reed, a PhD candidate at the University of Minnesota in natural resources science and management.

Reed and Frelich say other factors are also likely at play. Since deer are voracious plant eaters, plants could be reallocating more of their nutrients below ground to their roots, thereby benefiting worms. 

The researchers also found a link between earthworms and timber harvesting. They found that worm numbers decreased in areas where gaps were cut into the forest. 

That finding was less surprising, Frelich said. Worms require moist soil to thrive. Large gaps in the forest allows more sunlight to filter through, drying out the forest floor. 

For Frelich, who has studied the impacts of invasive earthworms on Minnesota’s forests for years, the main takeaway from the study is the surprising synergy between worms and deer. 

Worms decrease the density of plants in the forest, Frelich explained. Deer exacerbate that decline by eating more plants. 

“And then the deer increase the worms which furthers the damage to the plants, which allows the deer to do even more damage,” said Frelich. “So it's just like an endless cycle.”

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A new study led by a University of Minnesota graduate student shows that in northern forests, the presence of deer substantially increased the number of invasive earthworms — which can cause severe damage to forest plants and soil. 

New study shows surprising connection between deer and earthworms

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