What could be a more natural part of Michigan's ecosystem than the earthworm? It improves our soil structure, keeps our gardens healthy and compost piles composting, and is ever popular fishing bait. Earthworms seem to be such an innate part of Michigan life that many may be surprised to learn they're not actually from around here.
"Before European settlement of Michigan, earthworms were not native to this state," says Michigan State University Professor of Fisheries and Wildlife Dan Hayes. "It actually has had a real profound impact on the forest, because the amount of leaf litter on the forest floor effects the wildflowers and nutrients, and earthworms obviously change how that leaf litter is processed."
So if earthworms are non-native species that have changed local ecosystems, what makes them any different from a species like sea lamprey, which the U.S. and Canadian governments spend $18 million a year controlling? What makes one species "invasive" and another simply non-native?
"The two terms do get confused a lot," Hayes says. Simply put, most invasive species are non-native, but not all non-native species are considered invasive. In fact, like the earthworm, many species that are non-native to the state bring benefits to Michigan communities and economies.
Non Natives: The Benefits and Objections
Earthworms may have simply hitched a ride with European settlers to Michigan, but some non-natives are actually brought here and maintained purposefully, like Chinook salmon and coho salmon in the 1960s. At that time, an invasive fish called the alewife was so abundant it would wash up on beaches in droves, which was bad news for Michigan tourism.
"People had to use bulldozers to move them," says Hayes. "If the beach is knee deep in dead alewives, I don't think you're going to go swim."
Without any natural predators around to control the alewives, the Chinook and coho salmon were stocked in the Great Lakes to do the job. And it worked. Plus, in addition to the benefit of controlling an invasive species that threatened local economies, the salmon also created the economic impact of great fishing opportunities.
But not everyone believes the economic value of some non-native species makes them worth supporting.
"A lot of times people value economics; other people put a value on the fact that a species is native or not," Hayes says. "A lot of people would have preferred to never have seen Chinook or coho salmon stocked."
In Hayes's view, however, that preference doesn't necessarily line up with reality, considering invasives are already a part of local ecosystems.
"Some things you can't undo," he says. "Sure, the natural system would be better, but we can't get it back. So now what do we do?"
Invasive or Naturalized: Making the Call
While the benefits of non-native salmon and earthworms may be as obvious as the damage caused by invasive species like zebra mussels and sea lamprey, what about species whose impact is less clear? What is the criteria for determining which non-natives are invasive?
"The term 'invasive' is kind of a human construct," Hayes explains. "We see an invasive organism as one that is overly aggressive, and the positive benefits [of other non-natives], those are determined from a human point of view."
Consider the dandelion. For some, one unwanted spot of yellow in a yard is an invasion. Other people don't mind a yard full of them. But either way, dandelions don't pose an uncontrolled threat to the ecosystem. So while the plant may be non-native and a nuisance for some, it's considered naturalized.
Brown trout is another naturalized, non-native species in Michigan that people often assume is native. When the now-extinct grayling fish was waning in the cold water streams of the northern Lower Peninsula in 1883, brown trout were stocked there because, like the grayling, they are large fish that are good for fishing and eating.
"There was an empty niche," Hayes says. "I suspect they helped in the demise of grayling, but with the grayling gone, there wasn't much fishing economy on those rivers. And the brown trout has really helped fill in those empty niches that were there."
Often, it's economics that makes people really sit up and pay attention, such as the impact sea lamprey had on Michiganders' ability to generate income from the lakes in the absence of the lake trout the sea lamprey preyed upon.
Time can also be a factor. A species may be invasive initially, but disease or pests may eventually adapt to become predators, naturalizing the non-native species into the ecosystem.
While there is an abundance of criteria in determining what makes a non-native species invasive or naturalized, it's important to distinguish between the two, says Hayes, in order to focus on combating those that truly pose a threat.
"It's important in terms of what we react to," he says. "When people hear something non-native has made it into their lake, they get kind of panicky."
Instead, Michiganders should channel their energy into preventing the unintentional introduction of new non-natives into the ecosystem that could be potentially invasive, and relying on wildlife authorities to raise the alarm when a species poses a threat.
So even while taking care to close pathways to potential invasives such as not moving firewood, not releasing exotic pets or plants into the wild, and power-washing boats and trailers when moving to a new body of water, there's no conflict between these activities and enjoying the benefit of fishing for Chinook salmon in the Manistee River or adding earthworms to your compost pile in Ann Arbor. These naturalized non-natives can be as beneficial to Michigan communities and local economies as species that have been around for tens of thousands of years.
This story is a part of a statewide Invasive Species Community Impact Series edited by Natalie Burg. Support for this series is provided by the Michigan Department of Natural Resources.