Invasive Species: Unwelcome National Park Guests

Katherine RivardNPF Blog
Invasive lake trout at Yellowstone National Park
Invasive lake trout at Yellowstone National Park — Jay Fleming/NPS

Invasive species are organisms that don’t belong in the ecosystem in which they’re found. Usually a pest in their non-native environments, they are capable of wreaking havoc on native populations in numerous ways. In some cases, negative effects may go beyond environmental harm, potentially hurting the economy and even human health. They can even exacerbate the already bleak effects of other threats, like shifts in the environment and weather, as well as fragmentation from land use change.

What’s being done about the problem?

When it comes to unwelcomed invasive species in the parks, the battle never ends. The National Park Service works hard to mitigate their impact and educate visitors on how to prevent the spread of invasive species. With the help of grants from the National Park Foundation and other philanthropic foundations, as well as volunteer assistance, parks across the country have organized to fight against intruders.

A Cutthroat Existence

Native Cutthroat Trout swimming in Yellowstone National Park

The native cutthroat trout at Yellowstone National Park

Jay Fleming/NPS

In Yellowstone National Park, the cutthroat trout’s name may suggest this fish deserves no pity, but in a plot twist, the lake trout turns out to be the counterintuitive villain. With the illegal introduction of the non-native lake trout into the waters of Yellowstone, the smaller, native cutthroat trout population has decreased dramatically.

Grizzly bears in the park delight in snacking on cutthroat trout, especially when restoring body fat after a long winter. Unlike these natives, the lake trout do not spawn upstream for the bears to prey on, reducing the food supply for bears in the park. Lake trout instead remain in the park’s lake, breeding in the deeper waters, feeding on the cutthroat’s food supply, and sometimes even eating the cutthroat trout themselves. These dangerous interlopers can live up to 25 years and grow as large as 50 pounds!

The situation is dire for the cutthroat trout, but much has been done to research and reverse the effects of this new population in the park. The National Park Foundation has worked closely with Yellowstone Forever, the park’s philanthropic partner, to help preserve the natural function of the ecosystem and protect the native cutthroat trout. Researchers hope that by 2025, the threat to cutthroat trout will no longer be imminent.  

Leave Our Swamps Alone

Melaleuca trees in New South Wales

Geoff Derrin/Wikimedia

Everglades National Park suffers from the growth of several exotics, including the Melaleuca tree, also fittingly nicknamed the punk tree. Native to Australia, New Guinea, and the Solomon Islands, this lanky tree was originally brought to southern Florida in the early 1900s with the intent to dry up the swamp.

Melaleuca trees spread quickly and produce over a million seeds per year – a fact that makes them an extreme threat to the natural ecosystem. The native sawgrass marshes, wet prairies, and aquatic sloughs are transformed into dense thickets with the invasion of these punk trees. 

In order to restore the swamp, long-term plans have been implemented, using a variety of techniques. The techniques used at each site in the park is unique and based on a multitude of factors, including its proximity to wetlands, availability of volunteers and workers, and extent of infestation. Research is also being done by the U.S. Department of Agriculture to learn whether other biological controls, such as Australian snout beetles, may also help slow the tree’s ability to reproduce.

Boats Beware

Zebra mussel and quagga mussles

Zebra mussle on top, quagga mussle on the bottom


The quagga/zebra mussel is already infesting or at risk of infesting at least 52 parks across the National Park System, one of them being Lake Superior’s waters in Isle Royale National Park, where it was found for the first time in 2009. If these aggressive mussels enter the park’s inland lakes, they have the capacity to cover almost half the habitable surface of the inland lake floors within 4 years!

Originally from the Black and Caspian Seas of Eurasia, these mussels are able to reproduce within a year and attach to submerged surfaces using byssal threads. They cause costly damage to boats and outcompete native mussels, thus taking away a food supply for local fish. Furthermore, their spiky edges can be dangerous for humans hoping to swim in lakes or walk along the beach barefoot.

To stop the continued spread, prevention is key. The National Park Service has put out a wealth of information for boaters and visitors to ensure that these spiky clingers do not continue to travel throughout the parks.

Stage Six Clingers

sea squirt

Sea squirt

Dann Blackwood/USGS

Drakes Estero in Point Reyes National Seashore is an estuary complex comprised of 5 branching bays and 2,300 acres of underwater wilderness. It’s also the only West Coast marine wilderness south of Alaska and home to some amazing marine wildlife, including bat rays and leopard sharks. Unfortunately, the invasive sea squirt had taken up residence in the estuary, growing on hard surfaces, like the area’s former oyster racks and other debris.

The National Park Foundation teamed up with the National Park Service for the restoration of Drakes Estero, cleaning the debris from harbor seal pupping areas, removing wooden oyster racks, and conducting long-term research on non-native species, the local seal population, and water quality.

With the hard surfaces gone and the pests removed, eelgrass beds will be able to expand into these areas, allowing for further fish reproduction, revival of the ecosystem’s nurseries, and the greater health of the estuary. Initiatives such as the Beach Clean-Up Stations now allow visitors to pick up any trash they may see, preventing further debris from entering the waters and become the base for future sea squirts.

Island Mini Colonizers

Tim Hauf/NPS

Insects already have a bad rap, but non-native Argentine ants are particularly unlikable for their infestation of Channel Islands National Park. Since their introduction to Santa Cruz island around the 1960s, the ants have branched out across the park. Their presence can significantly affect the ecosystem and are particularly dangerous to pollinating bees, who they harass or kill. Native ants are displaced as these aggressive cousins colonize new areas of the island.

To counteract the negative effects of these Santa Cruz invaders, the park is working with researchers, government agencies, and nonprofits to develop new methods to eliminate the population while causing minimal effects to native populations.

Beatles Be Gone

Purple iridescent Emerald ash borer with wings open

Emerald ash borer


Shiny green beetles conjure up images of beautiful drawings on Egyptian tombs. In reality, the Emerald Ash Borer is an invasive species from Northeast Asia with very few predators in the U.S. Larvae feed off the trees’ inner bark, leaving ash trees unable to receive crucial nutrients and water.

The beetles have contaminated the trees at Sleeping Bear Dunes National Lakeshore and numerous other parks throughout the U.S., a continued threat to park ash tree populations. Insects like the Emerald Ash Borers are capable of lurking within dead or dried wood, even if you don’t see them. To prevent further contamination, parks request that anyone planning to build a campfire use wood from specified vendors and that no lumber be moved in or out of the park.

What Can We Do When We Visit?

Even though most invasive species introductions are done unintentionally, awareness if key. Visitors play a critical role in preventing the further spread of invasive species into the parks. In fact, many visitors do not even recognize that they are vectors for these organisms!

To help preserve the natural balance of our national parks, follow these rules:

1. Be judicious when choosing a pet!

Sounds fictional, but many invasive species were once pets. When choosing a pet to bring home, make sure that you are able to properly care for your animal and can ensure that they will not escape. Your frog might seem like a cool pet, but know that 80% of the nonnative reptiles and amphibians in Florida were escapees or unintentional releases!

2. Be a thoughtful gardener.

Show off your green thumb in your garden. However, when planting in your backyard, remember that your choice of plants can affect the spread of invasive species and attract pollinators. Breezes often transfer seeds from your garden to other areas nearby. Some nonnative plants even have the capacity to out-compete local plant pollinators needed to survive, threatening the bees, butterflies, and birds that keep the ecosystem running smoothly.

3.  Keep your contaminants at home.

The parks are meant for recreational activities, but if you’re an angler, boater, scuba diver, snorkeler, or performing any number of other activities using equipment, make sure to clean off your gear before bringing it back into the park. The National Park Service offers a wealth of information on proper protocol for preventing the spread of invasive species, and each risk of an invasive species may require specific precautions.

4. Be a researcher for a day.

You may or may not be into the sciences, but spend the day researching like a biologist. A quick check on your park’s website to learn more about any listed invasive species will help you become a more mindful visitor to the park. Each park’s ecosystem is unique, so learn about the threats to your specific destination’s wildlife.

We love to see thriving flora and fauna, so long as they are in their native habitats and do not harm the environment in the process. Ongoing efforts are being made by the National Park Service and its partners to ensure the preservation of our parks for years to come. However, this requires cooperation from visitors and your support to help prevent the spread of invasive species.

Next time you #FindYourPark/#EncuentraTuParque, remember to keep an eye out for invasive species and report any you might come across!

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