We’ve all heard the scary stories of foreign fish, bugs and mollusks hitchhiking on cargo ships and disrupting ecosystems in the Great Lakes. Fortunately, ballast water exchange rules put in place in 2006 seem to have put an end to that invasion route. But many of those invaders had already spread to the region’s many small lakes, and that’s where many battles are playing out now.
What do we mean by “aquatic invasive species?” The National Oceanic and Atmospheric Administration defines an invasive species as “an organism that causes ecological or economic harm in a new environment where it is not native.” Some nonnatives can co-exist in new environments without causing harm. And some natives can become invasive when they out-compete their neighbors and change the ecosystem.
These plants and animals typically do not have natural predators in their new environment; they can out-compete native species for resources such as food, light, prey, and habitat; they can spread disease; and all of this can lead to the extinction of native plants and animals, reduction of biodiversity, and permanently altered ecosystems. They can also change how humans use the environment. Anyone who tries to swim in a zebra mussel-infested lake can tell you that.
They come through several vectors: intentional planting, shipping, canals, aquarium release, bait release, aquaculture, and pet release. Human actions are responsible for the vast majority of these vectors.
Some of today’s favorite sport fish were enthusiastically stocked in the Great Lakes by individuals and governments beginning in the 1880s, including rainbow and Kamloops trout and sockeye and Atlantic salmon. Construction of the Erie Canal in 1825, the Welland Canal in 1829 and the St. Lawrence Seaway System in 1959 lay the Great Lakes system open to hitchhikers in the ballast water and on hulls of ocean-going ships. Asian carp, imported to keep aquaculture pens clean in the lower Mississippi system, escaped during floods and have been swimming north ever since. Managers are feverishly trying to keep them out of the Great Lakes. Aquarium and pet owners sometimes release problematic species into the wild. Fishers commonly dump unused bait when they leave a lake, despite the fact that it’s illegal to do so.
According to the DNR, about 8% of Minnesota’s more than 11,000 lakes are identified as infested. The most common and destructive invasives in inland Minnesota lakes include zebra mussels (Dreissena polymorpha), Eurasian watermilfoil (Myriophyllum spicatum), phragmites (Phragmites australis), non-native carp (Cyprinus carpio), and spiny water flea (Bythotrephes longimanus).
Researchers and natural resource managers track the progress of these undesirable beings and try to mitigate the damage they can cause. Sometimes it seems like a losing battle, but scientists have learned a lot in this confrontation, and experts now say the appearance of an exotic species in a body of water doesn’t necessarily mean the end of life as we know it. For example, with help from the Minnesota Aquatic Invasive Species Research Center at the University of Minnesota, the Coon Creek Watershed District was able to mow and spray invasive phragmites enough to reduce its coverage to six percent of its original area.
Left unchecked, the European strain of Phragmites forms dense stands that can lower water levels, change water flow, and clog drainage ditches; provides unsuitable shelter, food, and nesting habitat for native animals; and can limit human access to water.
Often, an invader may be in our favorite lake for years before we notice it. Then it doesn’t have a passport telling us where it came from or how many may have come with it or whether it is happily creating families and generations, or just barely hanging on. We also don’t know how existing creatures will respond to its presence, how it might find a niche without much disturbance of the aquatic community, or how an unexpected change in the local environment might allow it to explode and wreak havoc. Finding answers to these questions requires research, which requires money… and finding the solutions requires both funding and political will.
Containing the problem
Local, state and federal agencies collaborate to slow the spread of destructive invasives. Recognizing that “an ounce of prevention is worth a pound of cure,” control efforts have focused on people using lakes recreationally. Research conducted in the 1990s by Doug Jensen, then at University of Minnesota Sea Grant, showed that many boaters were willing to take actions to slow the spread; the few who wouldn’t said they were deterred by the lack of equipment at boat landings. Since then, hundreds of “Clean Drain, Dry, and Dispose” (C3D) stations have appeared on lakes around the country. In Minnesota they are funded by a yearly $10 million appropriation from the general fund. Combined with educational outreach projects, the stations have raised awareness to such a degree that Minnesota DNR research shows that 95 percent of watercraft arriving at a lake comply with state aquatic invasive species (AIS) transport laws. Compare that to the Minnesota recycling rate of about 44 percent!
Jensen is an international expert in AIS prevention. “Minnesota has 825,000 registered boaters, more than a million and a half anglers, thousands of public accesses, tens of thousands of private accesses,” he said in an interview. “It’s so easy to move invasive species from one location to another, if we hadn’t done this educational work, I’m convinced almost all of our lakes would have been infested.”
Jeff Forester, executive director of Minnesota Lakes and Rivers Advocates, has been an enthusiastic proponent of these conveniences.
“Cleaning stations have overhead and undercarriage lights, suction pumps to clean the bilge, wet-dry vacs, and high-pressure air hoses, Forester said in an interview. “They have grabbing tools, scrub brushes, scrapers, a universal plug wrench—everything a boater needs.” His group is planning to build such stations on 29 lakes this year, specifically to stop starry stonewort (Nitellopsis obtuse), a bright green macro-algae which is spreading quickly since it was first discovered in Minnesota in 2015. Money for that project comes from the lottery-funded Environment and Natural Resources Trust Fund.
These unwanted creatures can arrive at a lake from far away, perhaps a couple hundred miles. But research shows once an invasive species arrives, it can spread very quickly in about a 50-mile radius, usually carried by human lake visitors. “When the wind changes, fishers tend to move to another lake,” said Forester. The responsibility lies with people, he said. Although it’s theoretically possible for waterfowl and other animals to carry invasive organisms short distances, the evidence is that the spread occurs overwhelmingly through watercraft and fishing gear.
In addition to the C3D stations, Minnesota has 37 decontamination stations where trained workers use hot, high-pressure water to attack small-bodied organisms such as spiny water fleas.
Herbicides are commonly used to control aquatic invasive plants. These include glyphosate (the main ingredient in Roundup and Rodeo, banned in several European countries as a probable carcinogen) and formulations of 2,4-dichlorophenoxyacetic (2,4-D) acid (part of the Agent Orange defoliant used by the U.S. during the Vietnam War). Both are broad spectrum herbicides, and don’t distinguish between invasive plants and those native to the lake.
One study reviewed effects of herbicide treatments in 173 lakes in Wisconsin. The target was Eurasian watermilfoil (Myriophyllum spicatum L.). A feathery plant that forms dense mats on the surface, watermilfoil out-competes native plants, doesn’t provide appropriate habitat for native birds or insects, and drives swimmers crazy. A pre–post analysis of aquatic plant communities found significant declines in native plant species in response to lake-wide herbicide treatment. Modeling provided more detail. “Taken together, our results indicate that lake-wide herbicide treatments aimed at controlling M. spicatum had larger effects on native aquatic plants than did the target of control,” the researchers reported. “We conclude that unless there is strong evidence of high ecological, social, or economic impact for an invasive aquatic plant, aggressive chemical control at a lake-wide scale might do more harm than good.” Herbicides have another downside: when used repeatedly, they can induce herbicide resistance.
But it’s hard for people living along lakes and rivers to accept the appearance of invasive species without trying to do something. Tim Campbell, an AIS expert at University of Wisconsin Sea Grant, likened it to cancer. AIS is treatable, like most cancers, he pointed out. “It’s not uncommon for the cancer (like the AIS) to be there for quite some time before you see it. Understanding the best approach can take time but waiting to treat it often doesn’t sit well.”
Campbell said aquatic invaders seem to follow at least some of the rules of ecosystem behavior. For example, both native and nonnative plants are “commonly rare and rarely common.” It sounds like an oxymoron, but it means that in most locations, the abundance of both native and nonnative plants is low, but in a few locations, they are found in high abundances that can be problematic. “In most cases, they’re just part of the community,” Campbell said, “But in about ten-percent of cases they’re hyper-abundant,” and that’s where they cause the problems. Sometimes these invasives behave like “sleeper cells,” present for many years before they’re detected. For example, in Lake Mendota in Madison, Wisconsin, spiny water fleas (a tiny zooplankton that clogs fishing lines, preys on native zooplankton and is hard for fish to eat) hung out quietly until one cooler-than-average summer. “That gave them longer to reproduce, their population exploded, and now they are multiplying without cold summers,” said Campbell. Other changes can open the door to population growth, such as shoreline development which can add extra nutrients to the water. Campbell works with social scientists and experts on risk communication to understand how people feel about new populations of invasives. “A lot of people want to jump to chemical control,” he has observed. Other approaches include:
Physical (hand-pulling, digging, manual destruction or removal of nests, egg masses, or other life stages);
Mechanical (mowing and constructing barriers using tools or machines);
Biological (manipulation of natural enemies such as insects or diseases typically from the targeted species’ home range);
Rule changes (no-wake zones to minimize fragmentation of plants; limiting fertilizer use on lawns; tweaking permitting for different control methods).
“We’d like to see people use an integrated pest management approach,” Campbell said, “using all the tools in the toolbox: try some things and then adapt your approach for better results.”
What about biological control?
Biological control of invasive species has long been considered an environmentally benign management tool, and researchers celebrate the successes. After 12 years of research, the U.S. Department of Agriculture approved the use of three plant-eating insects for control of Purple loosestrife (Lythrum salicaria), which out-competes native plants, doesn’t help native animals, and can change the hydrology of wetlands. These insects have established reproducing populations and can be collected and released in new locations.
That said, scientists disagree about the safety of biological control. Some report that control agents are rarely found to be causing problems in areas far from where they were deployed; others say applicators do not routinely even consider the risk of expansion.
Successfully dealing with new species and their effects certainly needs to include the human members of an ecosystem. Jeff Forester with Minnesota Lakes and Rivers Advocates is on a mission to cultivate this resource. “Realtors need to understand how sensitive lakes are, what good development looks like; Chambers of Commerce should tie the health of main street to the health of the lakes; counties need to understand that water quality drives the property tax base. All these folks need to work together.”
He pointed to a recent success story. Every two years the Minnesota Pollution Control Agency releases its impaired waters list, which gets longer all the time. But last year, North Center and South Center Lakes in Chisago County were removed from the impaired list. This came after ten years of work among the lake association, the Lake Improvement District, and the Chisago Soil and Water Conservation District. Educating people, building relationships and trust takes years, and Forester says more money should go to supporting that work. Then, he said, when an invasive plant or animal shows up, the structure is there to respond appropriately.
Essentially the difference between native and nonnative species is that one grows in a place as a result of natural forces, and the other as a result of human actions. Timothy Seastedt, professor emeritus at University of Colorodo, suggests that these days, environmental changes, including climate change and human movement, are happening so fast that “it may soon be a moot question whether a plant is ‘native’ or ‘nonnative.’” Until that day, water lovers will fight back.