Quick question: All plants are equally valuable for a Metropark, yes or no? If you said yes, you’ve never met an invasive species before.
Invasive species? What’s that? We’re glad you asked, because this is one of the most easily misunderstood aspects of managing the Metroparks.
An invasive species is one that A) does not naturally occur in a specific area AND that B) can cause harm if/when it gets introduced to that area.
An invasive species can crowd out (and kill) existing native species; it can consume resources that would otherwise go to those existing species; it can cause very expensive damage to the ecosystem it has invaded. There are many ways invasive species can cause harm.
Keep in mind, invasive species can include both plant and animal species. They are often introduced to the area by humans – sometimes unintentionally, other times on purpose. But they’re not inherently “bad” species. They may just be in the wrong place. What typically happens is an invasive species enters a habitat where it doesn’t have natural controls. This allows that species to spread rapidly and outcompete native species for resources like sunlight and water.
Invasive species represent some of the most serious threats to biodiversity and ecological health, in the Metroparks’ natural areas as well as the world beyond. In fact, after habitat destruction, invasive species are the second-leading cause of global species extinction.
Facts About Invasive Species
- They can form dense thickets that shade out native plants and prevent them from growing.
- They can prevent native plants from growing by releasing chemicals that poison them or disrupt their nutrient uptake.
- They can completely replace native vegetation, changing the ecosystem and causing wildlife to migrate away in search of a more favorable habitat.
- They can cause changes to the soil microbial community, disrupting decomposition and nitrogen fixation, which is essential for plant growth.
- They can fix too much nitrogen into the soil, which can lead to increased leaching of nitrogen into aquatic ecosystems.
- They can change soil pH, which affects the ability of native plants to take up nutrients.
- They can change soil nitrogen content, causing the litter layer on the forest floor to decompose faster and exposing bare soil that can be eroded away.
- Invasive vines can kill mature trees by girdling (strangling) them, preventing light from reaching their leaves or bringing them down under the weight of the vine.
- Some invasive plants hybridize with native plants, threatening the genetic survival of the native plant.
- In densely invaded areas, wildlife may browse more heavily on native plants, reducing their supply of food as well as the native plant population.
- Some invasive plants use chemical signals to “trick” insects into laying eggs on them instead of on native species, which leads to greater mortality of the young insects.
What can we do about invasive species? That depends.
Some invasive plants can be controlled by physical removal – pulling, cutting, mowing or even prescribed burning. For instance, invasive shrubs that colonize an old field can be controlled by periodic mowing with a brush hog. And garlic mustard, an invasive woodland forb (“forb” means it’s a herbaceous plant other than grass), is controlled by hand pulling it during spring and early summer.
Herbicides can also be used to control invasive plants that do not respond well to physical removal. Or they can be used in combination with those methods.
Localized applications involve applying a small amount of herbicide to a specific part of the plant. One type of localized application, cut stump treatment, is frequently used for controlling invasive shrubs such as buckthorn, honeysuckle and autumn olive. Foliar applications with a backpack or boom sprayer are more appropriate for controlling large or dense patches of invasive vegetation, such as extensive stands of oriental bittersweet, spotted knapweed or Phragmites.
It’s important to know that chemical control always must be implemented in such a way as to minimize potential impact to native species and, of course, human health.