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Invasive Species 101, Part 3: The Biology of the Ecology

by | March 3, 2023

We’ve been going back to basics with Invasive Species 101, a four-part series about the ecology and biology of invasive species.

If you’ve read Parts 1 and 2 of the series, you’ll recall that all invasive species are introduced, but not all introduced species become invasive. Why do some introduced species become harmful and others don’t?

In Part 3, we look at the biological traits that make some species more successful invaders than others.

Trait #1: Specialists versus generalists

“Specialist” species rely on specific conditions to survive. They might, for example, require a particular type of food or nesting conditions. Plant specialists may need certain soil chemistry, structure, and/or water content. Because of their highly restricted needs, specialists sometimes become threatened or endangered.

On the other hand, “generalists” tolerate a wide variety of conditions and easily adapt to environmental change. As you might imagine, the ability to survive just about anywhere gives generalists an advantage over specialists.

Most invasive species are generalists.

Invasive in much of the world, rats survive in a wide range of landscapes and environmental conditions, from city life to marine environments, hot and cold climates, and thrive on a diet of just about anything. Photo: Zeynel Cebeci, CC BY-SA 4.0, via Wikimedia Commons

Trait #2: Type of Organism

If it seems like our Invasive Species 101 series has emphasized vegetation, that’s because we find far more non-native plants than non-native animals in Alaska.

The Alaska Center for Conservation Science currently tracks 414 plant species. The list includes those introduced in Alaska and adjacent parts of Canada and “watch list” species. The Alaska Department of Fish and Game lists only 17 species, including fish, birds, insects, and fungi.

While any organism can become invasive, far more plants become invasive than organisms belonging to other taxa. Photos: European green crab, Tammy Davis/ADF&G; Elodea, Christian Fischer, CC BY-SA 3.0, via Wikimedia Commons; creeping thistle, Jen Chauvet/HSWCD; Arctic fox/USFWS

Trait #3: Place in the food web

In a healthy ecosystem, thousands of years of evolution has led to a system of checks and balances. Native plants, animals, fungi, insects, and other organisms live together in relative harmony.

When a species gets introduced to an area outside its native range, it throws off this balance. Often, introduced species will experience a population explosion due to a lack of predators in their new environment.

The inverse can also be true, wherein native populations are quickly decimated by an introduced species. 

For example, Northern pike, native to Interior Alaska, are top predators in aquatic ecosystems and prey on other fish. In their native range, a balance remains between pike and the other fish because these species have evolved together. However, in Southcentral and Southeast Alaska, where pike have been introduced, they have quickly destroyed populations of native fish unaccustomed to the predation tactics of pike.

Complex and collaborative efforts initiated by KP-CISMA partners in 2021 led to the successful eradication of northern pike on the Kenai Peninsula. Removing these invasive fish from the Peninsula’s waterways helps protect native fish, like salmon. Photo: Kristine Dunker, ADF&G

Trait #4: modes of reproduction and seed longevity

Some plants, like dandelions, reproduce only by seed. Others, like strawberries or garden bulbs, reproduce vegetatively. And some plants reproduce both vegetatively and by seed.

Plants that use multiple modes of reproduction have an advantage. Vegetative spread increases the size of an existing population, and seeds can move great distances, spreading new populations far and wide.

Orange hawkweed is an example of this. It spreads both by rhizomes (horizontal belowground roots) and stolons (horizontal aboveground roots). Plus, it can produce 50-600 seeds per plant!

Multiple modes of reproduction make some plants, like orange hawkweed, more successful invaders than others. Photos: Bugwood.org

Trait #5: Allelopathy

Allelopathy means a plant produces chemicals that inhibit the growth and reproduction of other vegetation. This can be caused by different modes of action, and to varying degrees, and occurs in both native and introduced species. Orange hawkweed is one highly successful invasive plant thought to be allelopathic.

Trait #6: Extended growth season

Vegetation that greens up early in the spring has a head start in the growth process. Similarly, species that stay green longer into the fall have an advantage in that they can store up more energy, which helps them emerge earlier in the spring. And so, the cycle continues.

Reed canarygrass greens up earlier than many other plants and remains green long after native grasses have died back. Photo: Jen Chauvet/HSWCD

Putting it all together

This is certainly not a comprehensive list of species’ traits that can lead to invasiveness, but it’s a good starting place to understand some of the factors that drive invasions. The main theme is that invasive species have traits that allow them to outcompete native species, which in turn limits biodiversity and causes ecological and economic injury.

Up next

Stay tuned for Part 4, the final installment of Invasive Species 101, where we’ll talk about how humans fit into the picture.

Ready to take a deeper dive or can’t wait for Part 4? Visit the resources page of our website to peruse the full publication that inspired this series.


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