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 the first three parts of the series, you’ll recall that all invasive species are introduced, but not all introduced species become invasive. Some introduced species have biological traits that make them more successful invaders than other species.
In Part 4, we look at how ecosystems become invaded and the role of people in the invasion process.
Invasible Habitats and the Role of Disturbance
On the flipside of the invasiveness of a species, is the invasibility of a landscape.
Intact ecosystems generally provide few opportunities for non-native species to establish. That’s because all the habitat (the place that provides all the necessities for an organism to live) and niches (the role an organism plays in its habitat) are filled, and there’s no room for an interloper. On the other hand, a disturbed ecosystem has gaps in its habitats and niches that can readily be filled by strongly competitive non-natives.
Ecosystem disturbance can be caused by people or natural events. Examples of human disturbance are roads, trails, campgrounds, and utility corridors. Any activity that leads to areas of bare ground favor species that establish easily, which are quite often non-native species like hempnettle, but can also include native species such as horsetails. Also, some species are well-adapted to frequent mowing of lawns and sports fields, like orange hawkweed, which will grow sideways along the ground great distances rather than putting up tall flower stalks, thereby expanding its footprint through the help of human disturbance.
Natural events that cause disturbance, and thus increase the invasibility of a place include blowdown of trees in strong winds, forest fires, and flooding. For example, streams and rivers frequently change course with high water events. Gravel bars are a great example of a naturally disturbed habitat, and invasive white sweetclover loves to invade them.
Invasive species versus climate-induced range shifts
As a reminder, an introduced species is one that crosses geographic boundaries with human assistance. The phrases natural biological invasion or range shift are used to describe a species moving into a new region, but not through human-mediated means, and not crossing an insurmountable geographic boundary.
In the case of natural species migration, individual species tend to gradually move northward and to higher elevations from their previous and adjacent range. The move north and uphill is a response to warming temperatures in their previous home zone; going north and up typically leads to cooler temperatures. Individuals that don’t adapt this way may lose their historical habitat to changing weather and climate conditions. Sometimes these two concepts – species introduction and range shifts – get confused and even used as a case against managing new species in an area.
What’s the argument against managing invasives?
A common argument against managing invasive species (i.e., intentionally doing something to contain them or prevent their spread) claims that assemblages of introduced species should be classified as novel ecosystems and valued as such. For example, an abandoned lot within a city may thrive with lush vegetation, all introduced from other countries, but nonetheless filtering runoff, absorbing carbon dioxide from the air, and supporting pollinator insects. In other words, providing the same functions and services as a native ecosystem, so it should be valued as such. And because most native species aren’t hardy enough to thrive in a relatively harsh urban environment, this argument concludes that we might as well let the invasive species have at it!
Another argument against management is that humans, too, are animals that operate within the ecosystem. So, if we move seeds around the world, that’s no different from a squirrel moving seeds around the forest.
Still, another argument is that it’s too difficult to know what species were where before humans came along.
And finally, some scientists want to call all non-native species “bad” and all native species “good.” However, in this author’s experience, this isn’t true and is instead a false oversimplification.
Integrated pest management
When it comes to managing invasive species there’s no “right” answer. Every situation is different, and each requires a unique approach.
That’s where Integrated Pest Management (IPM) comes in. IPM is a strategy of controlling unwanted species that considers pest biology, management goals, and using a combination of control methods including – but not limited to – mechanical control (e.g., hand weeding), biological control (e.g., using ladybugs to control unwanted insects), and chemical control. IPM gives invasive species managers a plan for moving forward with the best management approach, tailored to a particular place and/or species.
Ultimately, the goal of invasive species management is about stewardship – protecting and preserving our native ecosystems, wildlife, livelihoods, and well-being from harmful invasive species.
We hope you enjoyed the Invasive Species 101 series. Want more? Visit the resources page of our website to peruse the full publication that inspired this series.