Signs of Life

An indicator species is an organism whose presence, absence, or abundance can tell us something—about their habitat, about the climate, about air or water quality, even about how our own actions affect the environment. Minnesota’s landscapes are full of indicator species, all telling us different things in different ways.

The term goes back a few decades, but the concept stretches back to the proverbial canary in a coal mine and almost certainly longer: Humans have long taken cues from keen natural observations.

A somewhat malleable term in the biological sciences, indicator species nonetheless provoked ample suggestions of examples from biologists at the Minnesota Department of Natural Resources, and in one case from the Minnesota Pollution Control Agency, who responded to the phrase and shared expertise and anecdotes.

Some of the scientists spoke of species whose physical sensitivity to conditions make them good environmental indicators, or whose needs are so specialized that they indicate specific habitat types. Others referred to indicators as ways to establish scientific and legal definitions that can lead to greater understanding and species protections. Some experts cited species that indicate what we have lost in nature, others what we have regained. And virtually all of them took the discussion beyond single species to communities of species, to surrounding habitats and landscapes, to natural and human history—the whole complex web of life itself.

The indicator species that follow are rich, varied, and illuminating for Minnesota nature watchers. We’re certain they’ll tell you something about the place where we live.

River Otter
If you see river otters inhabiting a stream, you can draw some conclusions about the place, says DNR furbearer specialist Jason Abraham.

“River otters typically survive on medium and smaller-size fish,” he says. “When you’re finding those, generally what you have is a clean, stable river and healthy fish base, and that all adds up to a good watershed.”

Across the state, otters are thriving, and the sleek, water-loving mammals have returned to many places in southern Minnesota where they hadn’t been seen for decades (see “Otter Odyssey,” May–June 2019). Abraham says they’re a visible sign of environmental improvements, especially a reduction in point-source pollution—waste discharged directly into waterways.

“Water quality is a huge driver,” he says.

Otters also indicate the success of reintroduction programs in southwestern Minnesota and northern Iowa and efforts to reconnect waterways through measures like dam and dike removal.

“Otters are really a transient species, so connectivity is big for them to move into new areas.”

Because otters are so mobile, however, Abraham cautions against using an otter sighting alone as evidence of strong watershed health. “What you’re really looking for are long-term sightings, over and over again at the same site,” he says.

That’s more easily accomplished than it used to be.

“You can see it in the population levels over the years, with the surveys from the ’70s all the way through today, how otters started to repopulate the state,” he says. “It’s a story that we’ve been watching with our own eyes, and it’s pretty cool.”

Leadplant
When DNR regional ecologist Megan Benage is looking for signs that a patch of land might be one of the state’s few remaining native prairie remnants—never plowed, never “broken”—she keeps an eye out for certain plant species as indicators. One of them is leadplant, a handsome shrub with woolly leaves and clusters of purple flowers.

“Leadplant is what I consider to be an old-growth prairie plant,” says Benage. “When I see it, and then I start seeing it with its plant friends—‘associates’ would be the science-y word—like prairie rose or harebell or things like that, then I’m like, ‘Aha-ha: We’re probably in a remnant.’”

Leadplant can be hard to establish in a planted, or reconstructed, prairie, says Ben-age, so its presence can often signal ancient prairie. But it can grow in reconstructions, so it’s not foolproof as a sole indicator, and that’s why Benage is always looking for those indicator associates that highlight a prairie community and might include prairie phlox, prairie smoke, ground plum, prairie turnip, silverleaf scurfpea, and wood betony.

“The more plants that you find that you wouldn’t necessarily expect to find in a reconstruction,” she says, “the more certain you are that you’re in a remnant.”

American Goshawk
Some hawk species can be seen in many places because they are generalists, adaptable to various habitats. Not so the American goshawk. This large raptor, found in remote forests in north-central and northeastern Minnesota, has very demanding habitat needs, so its presence speaks volumes.

“We think of it as an indicator species because among species that use mature and older forest, its habitat requirements are one of the most difficult to meet,” says DNR nongame wildlife specialist Gaea Crozier.

The home range for a pair of goshawks spreads about three miles from their nest, with three distinct “zones.” They’ll choose an older forest stand for their nesting zone, with a closed canopy and large-diameter trees that can support a large stick nest. Around that is an area biologists call the post-fledging zone, which contains larger patches of mature and older forest. The rest is the full home range where the goshawks forage for prey.

Another important goshawk need is solitude and space.

“They are quite sensitive to human disturbance,” she says. “If there are too many people near their nest, too much activity, they will abandon the area.”

The American goshawk is listed as a species of special concern in Minnesota, and Crozier and others would like to keep it from slipping to threatened. Because the goshawk has strong site fidelity, often using the same areas year after year, “A good strategy is to protect the known locations where they’re nesting,” Crozier says.

The DNR and its federal partners at the Chippewa and Superior National Forests, in the heart of goshawk range, keep an eye on goshawks and, where possible, manage for their needs.

Marsh Arrowgrass
Calcareous fens are a very specific and rare type of peat-rich wetland created by constantly upwelling groundwater, and in Minnesota they’re subject to unique legal protections. In order to identify calcareous fens, a team of DNR scientists assembled a score sheet of indicator plants as part of a standardized process that also includes hydrology and chemistry analysis. Among those plants, marsh arrowgrass has a particular yen for fens.

“Marsh arrowgrass gets a really high score, because in Minnesota it’s a plant that we would expect to find only in calcareous fens,” says Megan Benage, a regional ecologist on the team.

Not actually a grass, marsh arrowgrass is a slender and inconspicuous plant that grows tiny, feathery flowers along a single reddish or purplish stem. Both it and its more common relative seaside arrowgrass are known for having a unique aroma similar to cilantro when crushed, and according to the Minnesota Wildflowers website they are “often smelled before they are seen.”

If you come across a peculiar whiff in a wetland, you may have found marsh arrowgrass and, with it, strong evidence that you’re in a calcareous fen.

Pugnose Shiner
The pugnose shiner is an unassuming member of the minnow family that hides amid aquatic plants in clear lakes and streams in the north-central part of the state. DNR native fish biologist Brett Nagle is paying close attention to this little fish, listed as threatened in Minnesota, as an indicator of lake health.

“You only find them in clear lakes with diverse native plants,” says Nagle. “They like stem density and a lot of cover. This is a species that’s really dependent on plants.”

The pugnose shiner, described in fisheries research as “timid” and by Nagle as “cute,” avoids direct light as it forages amid aquatic vegetation, which in turn needs light to thrive. “The clearer the water is, the deeper sunlight penetrates. And even a minor change in clarity can really affect the plant community.”

The fish is vulnerable to invasive species like Eurasian milfoil and curlyleaf pondweed, which can dramatically alter plant communities, and to development.

As cabins are built, shorelines mowed, docks installed, and native aquatic plants cleared, pugnose shiners lose out, says Nagle, who describes conversations with landowners that illustrate the challenges these fish, and these lakes, face.

“I’ll tell them it’s a really rare fish, and you find them in the best lakes. ‘That’s really cool,’ they say. And then their next question will be, ‘How do I get rid of all these weeds around my dock?’ Pugnose shiners are in a space where there’s conflict between human interests and undisturbed habitat.”

American Marten
If you’ve seen an American marten in Minnesota, chances are you were way up north, near the Canadian border. “It’s that older mixed conifer forest they’re after,” says DNR furbearer specialist Jason Abraham.

Tree-loving members of the weasel family, martens need the structural complexity of older forests, he says, with a lot of downed logs to support a healthy rodent prey population, mid-level branches to use for transport and hunting, and, higher up in the trees, cavities for nesting and protection. They also require their “bread and butter” prey species, the red-backed vole.

“If you see martens, then you probably have all of those components in one place,” Abraham says.

Because they are so tied to old-growth forests, the presence of martens can indicate how well we’re preserving and managing their habitat. Before the late 1800s, the marten was common across northern Minnesota, but it almost disappeared from the state by 1920 due to heavy logging. Since the 1950s, public land protections and improved forest management practices have led to a dramatic recovery for the species. Trapping harvest records show a stable population.

Pickerel Frog
“Amphibians as a whole are basically bioindicators, because they, unlike some other species like mammals and reptiles, spend so much time in the water, and they need it for everything from their habitat to their reproductive process,” says Jeff LeClere, amphibian and reptile zoologist for the DNR’s Minnesota Biological Survey. Though some amphibians breathe through lungs, they also exchange water and gases through their semi-permeable skin.

The pickerel frog is a notable indicator because it needs especially cold, clear, flowing waters—the kind found in the southeastern part of Minnesota.

“They need the coolness of those streams, and they also need the quality of those streams to be high,” LeClere explains.

The waters where pickerel frogs spend much of their lives are also preferred by native brook trout, which have very similar habitat needs.
The pickerel frog looks similar to the more common northern leopard frog, but it’s much pickier about where it lives. While the northern leopard frog is found all over the state, you’ll lay eyes on a pickerel frog only in and around those southeastern trout streams.
“They are tied to those habitats,” says LeClere.

Rusty Snaketail
Aquatic insects that are sensitive to pollutants and sediments play a significant role as indicators in the work of Holly Masui, a macroinvertebrate biologist for streams and rivers at the Minnesota Pollution Control Agency. One of Masui’s primary interests is dragonflies, and in particular, the snaketail genus.

“The snaketail dragonflies tend to be found more in our cleaner, clearer, more natural and pristine kinds of rivers and streams throughout the state,” she says. “It’s one of those critters where you can look and say, OK, this has probably got enough dissolved oxygen. It’s got cool water. It’s not overly silty.”

Among dragonfly watchers, “snaketails are kind of a prized genus to see in the field,” she says, and the rusty is the most common snaketail in the region. It’s a bit more tolerant to silty conditions than other snaketails, Masui says, “So it’s not the strictest indicator, but that’s probably why we see it more places.”

Most snaketail records come from trout streams in the southeastern part of the state and other cold, clear streams along the North Shore and in northern forests, Masui says.

“When people think of these pristine coldwater streams, they often think of trout,” she says, “but these guys are often seen in trout streams as well.”

Mucket and Higgins Eye Mussels
Minnesota’s native mussels excel as indicator species for a host of reasons, says DNR malacologist Bernard Sietman.
Some can live up to a century, “So they have long-term presence,” Sietman says. Also, after their larval stage, “They are largely sedentary,” he says, “so they can’t escape whatever’s coming at them.”

They can be sensitive to low oxygen levels, which often occur in impaired waters. They have ample soft tissue for chemical analysis, he says, and their durable shells leave a record long after their lives have ended. “We’ve gotten a ton of information about the historical distribution of mussels from the presence of their shells,” says Sietman.

Those shell finds have often revealed that native mussel populations were extirpated from waterways around the state by pollution and degraded habitat. Now Sietman and his colleagues at the DNR’s Center for Aquatic Mollusk Programs, or CAMP, are working to restore threatened and endangered mussels.

Two species that have served as indicators in this work are the mucket and the Higgins eye. Once widely distributed across the state, the mucket disappeared from many rivers and in 1996 was listed as threatened in Minnesota. Still thriving in the St. Croix River, which is protected as a National Wild and Scenic River, the mucket’s shrunken range indicates “how stream fragmentation and general habitat degradation influence mussel assemblages,” says Sietman.

The Higgins eye is among more than 40 mussel species that historically lived in the Mississippi River between St. Anthony Falls and Lake Pepin but were largely wiped out by pollution. Now, thanks to improvements in water quality and a reintroduction effort by the DNR and federal and Wisconsin partners, the Higgins eye has reappeared in this stretch—an indication that we’re doing something right.

Red-Headed Woodpecker
Red-headed woodpeckers have a well-known predilection for oak savanna, where scattered oak trees grow above open grasslands. And they are definitely found in these now-rare native habitats—but also in places that mimic savanna characteristics.

“Many people consider red-headed woodpeckers to be an indicator of oak savanna,” says DNR ornithologist Steve Stucker. “More accurately, though, they’re an indicator of oak savanna and structurally similar habitats, that is, relatively open areas with scattered dead and dying trees.”

These woodpeckers, he says, can be found in semi-open floodplain forest where flooding or disease has killed off trees and in tree groves on isolated farmsteads in southwestern Minnesota, where Stucker often found pairs in recent surveys.

The red-headed woodpecker is designated as a species of greatest conservation need in the state, as its numbers have declined precipitously—by more than 95 percent since 1967, according to the Breeding Bird Survey.

Brook Trout
Minnesota’s only native stream trout, brook trout are real beauties, with colorful dots peppering their sides and, in fall, a wash of reddish orange along their bellies. While they’re wonderful to behold, brook trout also tell us something about their environment.

“They require cold, well-oxygenated, clean water,” says DNR native fish biologist Brett Nagle. “They’re vulnerable to siltation in a stream or any land-use changes that can increase water temperature. In general, the warmer the water is, the less dissolved oxygen it has.”

Found in groundwater-fed streams in southeastern Minnesota, brook trout also inhabit streams along the North Shore of Lake Superior and other small pockets of pristine water including some tributaries of the lower St. Croix River. In the southeast especially, the current healthy brook trout population indicates a turn for the better in habitat management.

“There were large-scale and rapid land use changes at the turn of the last century, and by the early 1900s brook trout were very nearly wiped out of southeastern Minnesota,” Nagle explains.

Today, many of those streams have been repopulated with the help of better land management and DNR brook trout stocking. While those stocked trout came from outside Minnesota, DNR Fisheries biologists have found a promising new indication of native fish persistence: They’ve recently identified brook trout populations in the southeast (see “Brook Trout Stronghold,” March–April 2018) that are believed to be a remnant native strain.

Mudpuppy
Minnesota’s largest salamander, the mudpuppy is particularly attuned to water quality issues, says Jeff LeClere, amphibian and reptile zoologist for the DNR’s Minnesota Biological Survey.

“Mudpuppies are good bioindicators because if their water quality is not great, they will not be able to persist,” LeClere says. “They go through all cycles of their life in the water, living in it year around every second of every day, so they’re the ones that often tell us first if there’s an issue with water quality.”

Mudpuppies can be found in major rivers and their larger tributaries throughout the state, except for the Arrowhead region, where they are uncommon. They also can inhabit colder, clearer streams, and biologists have noticed differences in appearance between mudpuppies from different waters.

Looking at a mudpuppy from the Minnesota River, for example, which carries a lot of sediment, “You’ll see that their gills are a lot larger, they’re fluffier, because they have to do more work in those types of situations,” he says—whereas a mudpuppy from the much clearer St. Croix will have notably smaller gills.

While the population is stable statewide, some die-offs of mudpuppies have occurred in western Minnesota in recent years, and while biologists don’t know the cause of the die-offs, LeClere says they’ve noticed lesions and discoloration in mudpuppies living in low-quality waters—another way that these amphibians indicate what’s happening.

Goblin Fern
You’re unlikely to simply happen across a goblin fern, says DNR botanist Welby Smith. In fact, even a sharp-eyed expert like Smith can have a hard time finding it.

“It’s very rare,” he says. “It often can’t be found even in forests that look right. And even if it is there, it is hard to find—it’s very small.”

In fact, the matchstick-size species, which looks more like a succulent than a leafy fern, may not even reach high enough to peek out from leaf litter on the forest floor.

However, this puny plant, when it does show itself, signifies a very specific—and disappearing—type of forest in the state.

“Goblin fern is a strong indicator of mature hardwood forests in northern Minnesota that have not been invaded by earthworms,” says Smith, author of the guidebook Ferns and Lycophytes of Minnesota.

Nonnative earthworms introduced from Europe can remove humus from the soil and render it unsuitable for this fern, which is listed as threatened in the state due to habitat loss.

If you are ever fortunate enough to see a goblin fern, take it as an indication: You are in a very special place.