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Invasive Species 101, Part 2: Sleeper Populations, Lag Time, and the Invasion Curve

by | February 13, 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.

If you’ve read Part 1 (if you haven’t go back and do that now), you’ll recall that all invasive species are introduced, but not all introduced species are invasive. That is, not all introduced species become harmful. However, it’s possible for an introduced species to start out harmless and, after a few years – or decades – begin to have harmful impacts on the environment and/or economy in the place it has been introduced.

How does this happen?

A plant with red stems and large green leaves
The majority of introduced species never become invasive. For example, the rhubarb many of us grow in our gardens persists year after year but doesn’t cause harm to the environment, economy, or human health. Photo: Creative Commons Zero – CC0, via Hippopx.com

Becoming invasive: the European bird cherry example

European bird cherry trees in Alaska are an excellent example.

Selected for their attractive spring flowers and tolerance to Alaskan growing conditions, European bird cherry trees were planted along roadsides and trails throughout Anchorage. For several years, they grew where they were planted, doing their job – being ornamental trees. Eventually, rogue saplings began to sprout up along creeks and bike trails, and it became clear the trees were outcompeting native vegetation.

Later research showed that the trees were also impacting salmon habitat. Anchorage has since banned the sale of European bird cherry trees, and they are activity being removed from communities throughout the Kenai Peninsula. It took time for European bird cherry trees to become invasive. It didn’t happen overnight, but they were right there in front of us the whole time.

A man in an orange safety vest and hardhat removes a tree in a forest
European bird cherry trees escape cultivation and invade our native forests, where they outcompete native plants and alter habitat for moose and salmon. Photo: Patrick Houlihan/HSWCD

Hidden in plain sight

A sleeper population is defined as “an established (i.e., reproducing and self-sustaining), non-native population that persists at low abundance and has innocuous or undetectable impacts but that has the potential to become invasive when triggered by an environmental factor” (Spear et al. 2021).

This period of relative inactivity, also referred to as lag time, can last from years to decades.

Similarly, inherent lag refers to the time required to develop enough seeds, propagules, or offspring to cause a population irruption. Several environmental factors can cause a lag time to end, including an unusually warm or cold summer, an unusually mild winter, unusually dry or wet conditions, or changes in the food web.

A large blanket of vines overgrows a tree
As our global climate changes, some currently harmless introduced species may become invasive. Other species, like invasive kudzu that has overtaken much of the southern U.S. are projected to expand their range. Photo: Carol M. Highsmith, public domain, via Library of Congress

It’s a process

As you dig deeper into the ecology of invasive species, you’re bound to come across the invasion curve. The invasion curve graphically depicts how a species transitions from its initial introduction to widespread abundance, and associated management strategies across these phases.

A colorful graph that depicts a curve
If not addressed in the early stages of introduction, the population of an introduced species grows, management strategies change, and the cost of management increases. Image: U.S. Government Accountability Office, Washington, DC, United States, public domain, via Wikimedia Commons

At the early stage of introduction, eradication of a species is most feasible, but detection is least likely because the population is so small – that is, there are few individuals of the species. Community scientists can be a great asset at this phase. If they notice and report something new or seemingly out of place, invasive species managers can take quick action to prevent further spread.

As a species increases in distribution and abundance, land managers shift their goal from species eradication to containment and resource protection. At mid to late stages of invasion, a species may only be managed in sensitive habitats, where invasion has the most detrimental impacts. An example of this is reed canarygrass management in the Homer area. Though common throughout Homer, particularly along roadsides, reed canarygrass is typically only managed where it threatens salmon streams and at outlying populations where eradication is feasible.

Green grass growing in a ditch
Common along roadsides of the Homer area, reed canarygrass is targeted for management primarily in areas where it threatens salmon habitat. Photo: Jen Chauvet/HSWCD

Up next

Invasive species infestations don’t just happen – it’s a process.  Given the many factors that determine how the process plays out, managing invasive species can be complex and requires an integrated and systematic approach. Tools like the invasion curve help managers make informed decisions about how to move forward.

Stay tuned for Part 3 of the Invasive Species 101 series, “The Biology of the 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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