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Invasive Species 101, Part 1: Learning the Lingo

by | January 27, 2023

Join us over the next few weeks as we go back to basics with Invasive Species 101, a four-part series all about the ecology and biology of invasive species.

When it comes to the topic of invasive species, you’re likely to see and hear several terms tossed around: alien, exotic, non-native, weed, pest, etc. This goes for books and other publications, outreach materials, policy, and more. Many of them seem to be used interchangeably (even within a single source) and, while some are more-or-less synonymous, others have distinct definitions.

One of the foundations for learning the basics of invasive species and invasion ecology is understanding the lingo.

Yellow flowers in grass
It’s important to have a standard and common language when talking about the ecology of invasive species. The plethora of terms, and their often-incorrect usage, can cause confusion and even hinder understanding of the threats of invasive species. For example, common dandelions are a non-native species in North America. Many people consider these common garden weeds a pest or nusiance. Photo: Jen Chauvet/HSWCD

What’s in a name?

In the big picture, species can be broadly grouped into three categories: 

  • Non-native, alien, exotic, non-indigenous, foreign: These all refer to a species for which there’s no basis to believe the species occurred in a particular location prior to colonization. While you may see/hear these terms used frequently, some can be confusing and/or offensive. We stick with “non-native.”
  • Native, indigenous: These terms refer to a species that, according to historical records, naturally occurs in a particular ecosystem or place. The preference here is “native.”
  • Endemic: Refers to a species that is both native and restricted to a particular region – that is, the species is naturally found in that region and nowhere else.
A brown spotted fish swims in murky water and a small brown rodent stands on a moss-covered rock
In Alaska, northern pike are both a native and non-native species – they are naturally found in much of Northern and Western Alaska but are not native to Southcentral, where they were illegally introduced. Singing voles (Microtus abbreviatus) are endemic to two islands in the Bering Sea: St. Matthew Island and Hall Island. Photos: USFWS

What does it mean to be a native species?

So, how do we know if a species is native to an area?

This is typically a question of timing. In North America, we can consult traditional Indigenous knowledge or consider if the first non-Indigenous naturalists to a region documented the species. In other words, was the species found in the area before the arrival of European settlers? If so, that’s probably an indication that the species is native. Is it a 2,000-year-old tree? Must be native.

What does it mean to be a non-native species?

In contrast to native species, non-native species have crossed geographic barriers (e.g., a mountain range or the ocean), they wouldn’t have been able to cross without human assistance. In other words, people moved them.

For example, caribou or fox native to mainland Alaska appear on a remote island in the Aleutians; northern pike, native to parts of Interior Alaska, turn up in lakes and streams in Southcentral Alaska; European bird cherry trees, jump continents and grow along a stream bank in Anchorage. These are all examples of human-mediated introductions that have taken place in Alaska.

Words of introduction

Sometimes the introduction is intentional. People brought caribou to the Aleutians for hunting, and foxes for trapping. Northern pike were released into waterways of Southcentral Alaska for sport fishing (note: it’s illegal). People imported plants, like European bird cherry trees and numerous species of flowers, grasses, and shrubs to Alaska for ornamental planting, soil stabilization, forage, and food crops.

Unintentional introductions occur when species hitch a ride on our vehicles, equipment, boats, planes, fishing gear, boots, firewood, topsoil, animal feed, nursery stock, and more.

A brown-colored fox stands on a coastal rock
Intentionally released on several Aleutian Islands in the 1800s for trapping, non-native foxes are a threat to native seabirds and their eggs. Removing foxes from the islands helps save seabird populations. Photo: USFWS

Are all non-native species bad news?

The good news is, not all non-native species have negative impacts. All introduced species are non-native, but not all non-native species are invasive.

Think about the crops we grow for food. They require amended soil, irrigation, and regular fertilization – they wouldn’t survive without our help. If you take a carrot seed and throw it into the woods, it’s not going to turn the forest into a carrot patch. Similarly, the peonies and delphiniums in your garden aren’t going to overrun a meadow of native wildflowers. Typically speaking, for every 100 species introduced to a new area, only one has the potential to become invasive.

A garden full of vegetables
Most non-native species will never become invasive, as they cannot survive without the help of humans. Photo: Local Food Initiative, CC BY 2.0, via Flickr

Harmful words

A few other words you’re likely to encounter and that we’ll be using in future installments of the Invasives Ecology 101 series:

  • Invasive, injurious, nuisance: A non-native species that causes harm to the economy, environment, or human health.
  • Noxious: A legal designation indicating a plant causes harm to public health, agriculture, recreation, wildlife, or property. Noxious species come with legal mandates for quarantines and other actions to contain or destroy the species. The Alaska Department of Natural Resources maintains the Alaska noxious weed list, but it needs updating.
  • Weed, pest: A species unwanted in a particular place at a particular time. This can be a non-native or native species. For example, spruce bark beetles are native to Alaska, but under certain weather regimes, their population can explode with undesirable impacts on forests. Another example: while native, Pushki (cow parsnip) is often considered a weed by some people who don’t want it in their yards or parks because contact with the plant can cause skin irritation.

Somewhere in the middle

Of course, not everything is clear cut, and there are a few additional terms to capture species that fall between introduced and invasive.

  • Casual, transient: A species that may reproduce occasionally but doesn’t form self-replacing populations. Pheasants, for example, are found in some neighborhoods around Homer, but they’re not expanding their populations.
  • Naturalized: An introduced species that maintains self-sustaining populations but doesn’t cause harm. An example is rhubarb. The plants may persist for a long time after a garden is abandoned, but they don’t cause harm to the economy, environment, or human health.
A multi-colored bird stands in grass
While casual or naturalized species are not invasive, they may become invasive if environmental conditions shift in their favor over time. Photo: Gary Noon, CC BY-SA 2.0, via Wikimedia Commons

Up next

Now that you’ve got some of the lingo under your belt, you’ve built a solid foundation to learn more. Stay tuned for the next installment of Invasive Species 101, “Sleeper Populations, Lag Time, and the Invasion Curve,” where we’ll dive deeper into invasive species ecology.

Ready to take a deeper dive or can’t wait for Part 2? Visit the resources page of our website to peruse the full publication that inspired this series.


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