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Study finds link between deer, spread of earthworms

Posted 4/26/23

REGIONAL— A newly-published study from the University of Minnesota has found a link between invasive earthworm populations, white-tailed deer and tree harvesting in northern forests.The …

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Study finds link between deer, spread of earthworms

Posted

REGIONAL— A newly-published study from the University of Minnesota has found a link between invasive earthworm populations, white-tailed deer and tree harvesting in northern forests.
The findings, recently published with free access in the journal Ecology, show invasive earthworm populations increase with the presence of deer and decrease with tree harvesting. Invasive earthworms, which are not native to Minnesota, are known to harm soil and regenerating trees.
“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 an adjunct professor in the Department of Forest Resources and Director of the Center for Forest Ecology. “Deer exacerbate these negative impacts by increasing earthworm populations.”
The team of U of M researchers, in collaboration with the U.S. Forest Service, the Wisconsin Department of Natural Resources and others, sampled earthworms in two long-term experiments in northern Wisconsin. Both experiment sites were established in the mid-2000s and and were fenced to exclude deer. Tree cover was also removed in some cases, to simulate logging which typically leaves large gaps in the forest canopy.
Nearly 13 years after each experiment started, the team tested how the absence of deer and the tree canopy gaps impacted earthworm populations. The researchers also used earthworm data collected prior to the experiment’s establishment to test how earthworm populations changed over 13 years. Earthworms were sampled by pouring a slurry of mustard powder and water on the ground, which causes earthworms to come shooting to the surface — a method anyone can use at home.
The researchers found:
In both experiments, invasive earthworms were lowest in areas where there were no deer and with a canopy gap overhead.
In addition, earthworms increased the most in areas outside of deer fencing and far away from the center of canopy gaps.
The largest and most influential invasive earthworm species were increased by deer and decreased by canopy gaps.
The research team has several theories as to how these disturbances might be linked. First, deer could be changing the soil in a way that is favorable for earthworms, through their droppings and urines. Another theory is that since deer voraciously consume and kill flora, plants could be reallocating nutrients below ground to their root systems to avoid being eaten, which could indirectly favor earthworms. When it comes to tree harvesting and canopy gaps, researchers theorize canopy gaps could decrease earthworm populations because there is less moist, high-nutrient leaf litter underneath a canopy gap, which earthworms use for food and shelter.
Future research is needed to focus on the mechanisms behind how deer and tree harvesting might change invasive earthworm populations. In addition, disturbance ecology as a whole should be broadly testing how disturbances might be influencing one another and how ecosystems respond to multiple, overlapping disturbances.