How a Flower and Fish Changed the Niobrara River

People planted yellow iris and stocked northern pike in the river for beauty and sport. The iris altered the river channel, and the pike ate the native fish.

By Lusha Tronstad

Man holds a Hess sampler as he stands in the Niobrara River surrounded by green vegetation
Bryan Tronstad, Wyoming Natural Diversity Database, University of Wyoming, prepares to use a Hess sampler in the Niobrara River at Agate Fossil Beds National Monument. We use the sampler to collect aquatic invertebrates to estimate how invasive species are changing the ecosystem.

Image courtesy of Lusha Tronstad

Anine Rosse is an aquatic ecologist in the National Park Service’s Northern Great Plains Inventory and Monitoring Network.

Rosse’s office monitors natural resources and facilitates information sharing for Agate Fossil Beds National Monument and other national parks in the Dakotas, Nebraska, and Wyoming. Rosse calls Agate Fossil Beds “a little park in a big landscape.” Eleven miles of the Niobrara River run through the park, and the river, which occupies the lowest points in its watershed, is affected by land practices in the larger area. Excess sediment and nutrient runoff from agriculture, for example, can change the water’s chemistry and damage its quality. Despite this, Rosse says the Niobrara River at Agate Fossil Beds is an “untouched gem” compared to most of the highly altered streams in the Midwest. But like other rivers throughout the nation, it has a problem with invasive plants and animals.

Historically, native plants like cattails, willows and bull rush grew in the river and the adjacent flood plain. And abundant native fish like brassy minnow and creek chub swam in the river. But two invasive species, yellow iris and northern pike, have changed the Niobrara. To determine these changes, Agate Fossil Beds and University of Wyoming scientists measured the river’s physical characteristics and inventoried its animals from 2010 to 2019. Our unpublished data show that stream depth, width, and flow increased during the last decade. Aquatic invertebrates became more diverse, but we collected fewer sensitive species, suggesting a decrease in water quality. Other studies show that native fish declined. These changes have profound consequences for the river’s other inhabitants. We also investigated a new sampling method for aquatic invertebrates, which are good indicators of river health.

Beautiful but Destructive

Yellow iris is native to Europe, Asia, and northern Africa. It was introduced as a decorative plant to the Niobrara in 1906 in an artificial, stream-fed pond at Agate Springs Ranch, upstream of the park. This was long before Congress designated the national park in 1965. Yellow iris spreads to new areas through dispersing its seeds and rhizomes in the river, moving them downstream. The iris has spread along the stream channel in the park over time, and more plants are now found in the eastern portion of the park than elsewhere. Yellow iris competes with native plants for space and nutrients, and resists insect predators by producing noxious chemicals in its leaves.

Yellow flower with green stalk amid many green stalks.
Yellow iris (scientific name: Iris pseudacorus) was planted near the Agate Springs Ranch house in 1906. Over the years, the plant spread downstream. It is not native and spread aggressively, changing the structure of the Niobrara river channel.

Image credit: NPS

The oxygen in river water where iris grows abundantly becomes dangerously low after the plants die each winter.

Living aquatic plants increase the amount of oxygen in the water, but dead, decomposing, aquatic plants use oxygen. The amount of decomposition is much higher where yellow iris plants dominate because they produce so much plant material. This means that the oxygen in river water where iris grows abundantly becomes dangerously low after the plants die each winter. This extreme change in oxygen concentration poses a risk to other plants and animals in the river.

The river channel has also changed. Yellow iris plants are stabilizing its banks because of their large root systems, making the channel deeper and steeper. This causes the river channel to become free of vegetation and the river to be less connected to its floodplain. We also found that over the last decade, the maximum water depth increased, the width of the river channel that is free of plants became wider, and the volume of water flowing downstream increased. The floodplain retains water and cleans water, both of which are critical functions in the western U.S. It is also home to many plants and animals, so these changes have significant impacts on them.

Native cattails dominate the riparian vegetation in the central portion of the park where iris plants are rarer. In these areas, the cattails slow the water by growing in the river channel. In fact, some lengths of the river lack a channel. Instead, the river flows through dense stands of cattails.

A Voracious Predator

Introducing a new predator to an ecosystem can induce a trophic cascade—a change in the food web structure. Historically, native fish were at the top of the food web in the Niobrara. They were abundant and mainly fed on aquatic invertebrates. Nearly 400,000 northern pike were introduced to a reservoir downstream of the park between 1949 and 1985 for sport fishing. The northern pike swam upstream to the park and reduced the number of native fish species from nine to two between 1979 and 2011. The introduction of northern pike to the Niobrara altered the population and community dynamics of organisms living in the river.

Large, dark fish with light spots and belly, swimming.
Northern pike (scientific name: Esox lucius) is not native to the Niobrara River. It was introduced decades ago to provide sport fishing opportunities, but it decimated the native fish.

Image credit: NPS

Eighty percent of the native fish species that once lived in the river are now absent due to predation by northern pike.

Eighty percent of the native fish species that once lived in the river are now absent due to predation by northern pike. Aquatic invertebrates are now more abundant because there are fewer native fish eating them. The northern pike changed the balance of the ecosystem, resulting in more winged insects emerging from the stream, including biting blackflies. But more winged insects also means more food for birds and frogs, so this change had mixed results.

Northern pike are even more difficult to manage than yellow iris. The fish can escape harsh conditions in one part of the stream, such as during a drought, by swimming to another location. This means that any removal of northern pike must include the larger watershed of which Agate Fossil Beds is only a small part.

Sentinels of River Health

Aquatic invertebrates like mayflies, mussels, and snails are widely used by state and federal agencies to estimate the quality of stream ecosystems. Invertebrates are excellent indicators of river quality because they live in the stream year-round, are abundant, move very short distances, and are easy to collect. There are also hundreds of species of them, so they give us many different ways to study the ecosystem.

Brown insect on green leaf
An adult mayfly (species unknown) on a leaf. Mayfly larvae are good indicators of stream quality. They are very sensitive to pollution, so their presence shows the water is relatively pristine.

Image credit: NPS

What makes aquatic invertebrates so useful for measuring river health is that different species have different tolerance to disturbance, pollution, and invasive species. Mayflies, caddisflies, and stoneflies are very sensitive to pollution, low dissolved oxygen, and other indications of poor water quality so they are absent in streams with those conditions. But aquatic invertebrates such as fly larvae and snails can tolerate disturbances and live in streams with both good and poor water quality. By collecting invertebrates, we can learn a lot about the river.

Hester-Dendy plates in the river with green vegetation around it.
Hester-Dendy plates have been used to sample aquatic invertebrates in the Niobrara River for 25 years. But they collect fewer invertebrate species than Hess samplers. They also collect debris and filamentous algae as shown here, which make them harder to use.

Image courtesy of Lusha Tronstad

Two Collection Methods Face Off

Agate Fossil Beds first collected aquatic invertebrates in 1989 using Hester-Dendy plates. These samplers consist of nine Masonite square plates separated by plastic washers that are hung in the river channel for about a month in July and August each year. Algae grows on the plates and then invertebrates take up residence on them

Hester-Dendy plates are not ideal for collecting invertebrates, because not all invertebrates can live on them. Invertebrates that live in the sediment on the bottom of the river cannot live on the plates, because there is no place to burrow. Craneflies that shred and eat leaves do not live on the Hester-Dendy plates, because these samplers do not have leaves to feed on.

But for some invertebrates, Hester-Dendy plates provide ideal conditions for them to live or feed. Filter-feeding invertebrates that eat by removing particles from the river water are abundant on Hester-Dendy plates. Mayflies, caddisflies and stoneflies also find the samplers a great place to live. These same invertebrates are sensitive to changes in water quality and can bias our results. The calculated water quality of the river is thus higher when estimated with Hester-Dendy plates.

The Hester-Dendy plates always produce values that indicate better conditions compared to the Hess samples.

Hester-Dendy plates are also more difficult to maintain. This is because they collect a lot of debris, which can push them out of the water. And filamentous algae that grows in long, hair-like strands can become very dense on the plates. Both factors cause the data we collect from Hester-Dendy plates to vary widely. Because of the challenges with these plates, the National Park Service is seeking better techniques to estimate the river’s water quality. We hope to find a new standardized method that is less biased toward certain invertebrates and will better represent river conditions.

Man stands in a vegetation-covered river with a Hess sampler
A Hess sampler is a good alternative method to collect aquatic invertebrates. It collects more taxa and at densities that are natural in the river.

Image courtesy of Lusha Tronstad

I began working in the Niobrara River in 2010 to investigate sampling methods other than Hester-Dendy plates. After some experimenting, Hess samplers emerged as a better option. A Hess sampler is a metal tube with a net on each side. The net on the upstream side allows water to flow into the sampler, but the mesh is small enough to keep invertebrates from getting in. The downstream net is long and tapered, with fine mesh that retains invertebrates.

We place the Hess sampler over plants at the edge of the stream to collect invertebrates, which become trapped in the cylinder. We scrub the plants and agitate the sediment using the downstream flow of water to capture invertebrates in the net. Then we strain the sample and take the invertebrates back to the laboratory to see what we captured.

We collect about a third more species of invertebrates using Hess samplers than with Hester-Dendy plates. The values calculated to assess water quality differ between sampling methods. The Hester-Dendy plates always produce values that indicate better conditions compared to the Hess samples. This is because we collect more invertebrates on Hester-Dendy plates that are sensitive to changes in ecosystem quality. Hess samplers collect a much wider array of invertebrates.

Maintaining Data Integrity and Continuity

Switching to a new collection method took careful consideration to maintain the continuity of the park’s historical dataset. We collected data using both methods for a decade to ensure that switching to Hess samplers would not hamper our ability to detect changes. We found that we can detect changes in invertebrates using both methods, but Hess samplers appear to detect more changes than Hester-Dendy plates. We plan to eventually switch to monitoring annually with a Hess sampler while collecting samples with Hester-Dendy plates every three to five years to maintain the legacy dataset and ensure continuity.

“I like to think of it as holding a flashlight on a problem instead of a floodlight.”

Long-term monitoring in the Niobrara River has helped us understand how invasive species continue to alter the river. The data separate the impacts of invasive species from impacts due to changes to the watershed or impacts from climate change. “What has been unique…about Agate Fossil Beds,” says Rosse, “is the power of the long-term dataset and…the smaller site scale.”

Rosse thinks a site-specific approach allows us to clearly see changes in the invertebrate community while other large-scale environmental factors are still at play, something that “isn’t obvious to the casual observer.” She adds, “We are into our third decade of monitoring. It is only this long-term monitoring that reveals this change and later helps…scientists to understand it. I like to think of it as holding a flashlight on a problem instead of a floodlight.”

Niobrara River in Agate Fossil Beds National Monument with lush green vegetation, cattails and bullrush.
In the absence of invasive yellow iris, the Niobrara River at Agate Fossil Beds is lush with native cattails (Typha species) and bullrush (Scirpoides holoschoenus). The banks are gently sloped, connecting the river to the floodplain.

Image credit: Lusha Tronstad

An Ounce of Prevention

An ounce of prevention is worth a pound of cure when managing invasive species. They are typically extremely difficult and costly to remove once they invade an ecosystem. The best way to manage them is to reduce the risk of invasion. But invasive species were sometimes introduced to ecosystems for seemingly beneficial reasons. Yellow iris was introduced because it has beautiful flowers. Northern pike was introduced to provide sport fishing opportunities.

“Problems caused by invasive species aren’t readily apparent or even anticipated by those who introduced them with good intentions.”

The effects of invasive species are often not seen immediately after the invasion. “Unfortunately, problems caused by invasive species aren’t readily apparent or even anticipated by those who introduced them with good intentions,” says Rosse. In the Niobrara River, the impacts of yellow iris and northern pike were not apparent until scientists had collected decades of data.

Preventing the spread of invasive species is especially difficult in small parks like Agate Fossil Beds. This is because the park is highly influenced by the larger landscape beyond the park boundaries. But it’s a top management priority for the park. Arthur Stiles is the Natural Resource Program manager at Agate Fossil Beds. He says long-term, the park’s vision is to “have a better functioning and structured riparian ecosystem dominated by native plants and animals on this stretch of the Niobrara River.” Stiles is experimenting to find the most effective method for using herbicide to reduce invasive plants, including yellow iris.

Same Story, Different River

The changes occurring in the Niobrara River are not unique to Agate Fossil Beds. The same story with different actors occurs in national parks across the country. Aquatic invasive species are one of the most challenging management issues that parks are facing today. These species have the potential to destroy our natural resources, which national parks were created to protect. Preventing their spread is the most effective strategy. One way to do that is to share our insights with the public about the damaging effects invasive species can have on ecosystems.

But sometimes, prevention is not an option. That is why developing good methods to assess how non-native species affect native ecosystems and organisms is crucial. Lessons learned from long-term datasets, such as at Agate Fossil Beds, help us understand how invasive species alter ecosystems and which strategies can reduce their impact.

Lusha Tronstad

About the author
Lusha Tronstad is the lead invertebrate zoologist for the Wyoming Natural Diversity Database at the University of Wyoming. Image courtesy of Lusha Tronstad.

Agate Fossil Beds National Monument

Last updated: January 6, 2023