INVASIVE PLANTS DEFINED
- native plant: in North America, a plant that was found in a particular area prior to European settlement or has arrived since then through natural means of dispersal.
- non-native plant: in North America, a plant that was introduced to a particular area after European settlement via human-assisted means, either purposefully or accidentally. While many non-native plants cause little harm to native ecosystems, some become highly invasive.
- weedy plant: may be a native or non-native plant; often annual or biennial and adapted to disturbance. May threaten newly-establishing native plants in the early phases of ecological restoration, but are typically outcompeted or easily controlled through stewardship (maintenance) once the planting has matured.
- invasive plant: undesirable, non-native plant that outcompetes native plants for light, space, water, food and nutrients. An invasive plant, left unchecked, will destroy the integrity of an ecosystem and will often become the only plant inhabiting a particular landscape (monoculture)
- aggressive native plant: undesirable, native plant that outcompetes other native plants and may form large monocultures. Many ecologists reserve the term “invasive” for non-native plants, but some native plants may act in a similar manner as invasive plants. Many of our remaining, intact natural areas are very small and may be damaged when aggressive natives outcompete more conservative native species.
WHERE DID INVASIVE SPECIES COME FROM?
Invasive species were brought to North America, typically from Europe and Asia, normally with good intent. Often, upon arriving in North America, settlers yearned for plants and animals they grew up with in their home countries. Many plants that have become highly invasive were brought here for landscaping, medicinal properties, or use as culinary ingredients. But sometimes, invasive species were introduced by accident, such as through ballast water, hitchhiking on boats or other vehicles, or unintentionally as pathogens on other purposefully introduced species.
WHAT MAKES A PLANT INVASIVE?
Invasive plants typically have many of the following traits:
- Thrive in disturbed habitats
- Germinate quickly
- Grow and reproduce in a short time frame
- Produce many seeds per individual
- Disperse seeds widely (on fur, by birds, by wind)
- Reproduce vegetatively or self-fertilize
- Resistant to herbivory or disease
- Alter growth form to suit a variety of soil, moisture, or other conditions
- Alter soil conditions to prevent growth of other plants
WHAT MAKES INVASIVE SPECIES BAD?
Invasive species threaten to destroy local biodiversity and native ecosystems by forming dense stands where nothing else can grow, or by otherwise outcompeting or destroying local flora and fauna. Some non-native plants may create habitat that indirectly harms animals by acting as ecological traps – increasing nest predation or parasitization.
About 90% of plant-eating insects are considered specialists because they feed on plants from only a few plant families. This adaptation inhibits insects from eating other plants that differ in timing of development, leaf chemistry, or physical defenses. That means that native herbivores that co-evolved with our native plants are unable to eat many invasive plants. For example, in its native range in Europe, common reed (Phragmites australis) was found to support over 170 species of herbivores – in North America where it is invasive, it only supports 5 species of our native herbivores.
Native plants provide appropriate food and habitat for insects, cycling up the food chain to birds and other animals. In studies conducted by Dr. Douglas Tallamy, North American native plants produced over 4x the herbivore biomass, 35x the butterfly and moth biomass, and over 3x as many herbivore species as non-native plants. That’s a lot more food for growing baby birds!
In addition to their ecological threats, the cost of invasive species damage and control in the United States may run upwards of $130 billion each year. Just a few of the negative impacts to humans include reduction in property values, damage to stormwater systems, loss of income from tourist/recreation industry, impacts to water quality, damage to agriculture, and even physical harm.
BUCKTHORN BY THE NUMBERS
|¼||Diameter in inches of buckthorn’s small, but numerous fruits. Common buckthorn fruits are green when unripe whereas glossy buckthorn fruits are reddish to brown. Both have black fruits when ripe.|
|1||Rank held by European buckthorn as the most common tree/shrub found in the Chicago region.|
|2||Number of common, invasive buckthorn species in our region (northeastern Illinois) – 1) common/European buckthorn (Rhamnus cathartica) and 2) glossy buckthorn (Frangula alnus). Less common are Japanese buckthorn (Rhamnus japonica), Dahurian buckthorn (Rhamnus davurica), and Chinese buckthorn (Rhamnus utilis).|
|6||Years buckthorn seeds may remain viable in the soil.|
|28.9||European buckthorn trees per acre in suburban Cook County.|
|30||Approximate maximum height of this understory shrub/tree.|
|45.8||European buckthorn trees per acre in Lake County.|
|58||Average number of days buckthorn’s leaves remain on the tree longer than native counterparts, shading out other shrubs and native understory plants throughout the growing season.|
|200||Approximate years ago European buckthorn is thought to have been introduced to North America.|
|226||Stewardship groups in the Chicago region working to restore and transform native habitat by removing invasive species like buckthorn.|
|620||Average seeds per square meter found in the top few inches of soil underneath a mature buckthorn.|
|700||Times more leaf material consumed by herbivores on native oak trees than on European buckthorn – buckthorn does a poor job feeding our local fauna.|