Why Are Invasive Plants Invasive?

Photo © Katie Ashdown

Contrary to how it may seem, invasive plants are not inherently evil. All plants possess a suite of traits that allow them to compete against the myriad forces of nature around them to survive, flourish, and successfully reproduce. For millennia, plants have developed the traits to battle it out for dominance, or at least inclusion, in the landscape, competing for resources while surviving predation, weather-events, environmental pressures, fungi, pathogens, and disease.

Northern spicebush

Photo © NatureServe

Amidst this silent assault, only a small proportion of attempted procreation will actually succeed. Left unfettered of the competitive conditions in which they evolved, many plants would demonstrate the invasive qualities we find malicious; but, among these conditions populations remain in balance.


Photo © Barbara

It is when a plant species is transported to such a distance, where the familiar forces of competition and resource availability are out of sync with the plant’s traits, that invasive properties can emerge. The plant’s growth habits, in this new location, will either be insufficient for survival (and it will perish), or will be adequate (allowing it to survive amid the competition), or those traits will go unchecked (enabling prolific growth and reproduction). The Japanese stiltgrass (Microstegium vimineum) , growing unchecked in the above photo, is an invasive plant introduced into Tennessee as a result of its use as a packing material for shipping porcelain from Japanopen_in_new.


Photo © Derek Markham

This tree-of-heaven (Ailanthus altissima) is native to a region extending across Northeast China. In its native range, there are a reported 46 arthropods, 16 fungi, and one known virus that attack tree-of-heaven, some causing significant damageopen_in_new. Without this onslaught of natural predators or disease in North America, it has the ability to grow vigorously in a wide variety of places–this tree is only two years old.


Photo © Kjeannette

Plants with an invasive nature outside of their historical range tend to have a common set of traits which help them tolerate a broader range of environmental conditionsopen_in_new. They are usually early successional plants (meaning they are plants who thrive in the sometimes harsh conditions of recently disturbed landscapes), well-equipped for rapid growth and efficient reproduction to effectively colonize an area. They tend to be prolific seeders–see the barberry (Berberis) above–and can reproduce vegetatively in multiple ways.


Photo © Buddha Dog

The Chinese Privet (Ligustrum sinense) above will produce hundreds of fruits and thousands of seeds per plant every year. They can sprout new stems from the roots and can regrow from broken pieces of stem laying on the ground.


Photo © Katja Schulz

Often differences in seasonal timing will give nonnative plants an advantage over native plants. Earlier leaf-out and a later fall leaf drop help Norway maple (Acer platanoides) trees shade-out competing species during critical times of the year, reducing competition and providing the tree a longer growing season than native trees. Their seeds will be the first to sprout in the spring and, like many nonnative seedlings, are generally not preferred by local herbivoresopen_in_new, and are more likely to survive than native tree seedlings in the same areas.


Photo © Virens (Latin for greening)

Some plants are able to affect the soil conditions around them by emitting toxins into the soil that either inhibit the growth or germination of other species or that can disrupt mutualisms with the soil biotaopen_in_new. If the fungi, bacteria and other organisms that native plants have relationships with are not supported by the nonnative plants, those mutualistic relationships degrade, creating soils that are more difficult for native the plants, which are dependant on those relationships, to become established. Garlic mustard (Alliaria petiolata), shown above, is a good example of this. It does not use the mycorrhizal fungi in the soils it tends to invade and may even inhibit the fungi’s productivityopen_in_new. Restoration efforts after garlic mustard invasion require time as well as other inputs into the soil to rebuild these necessary mutualisms.


Photo © liz west

The combinations of these traits become a problem in the landscape when the nonnative plants invade a natural environment by outcompeting native plants for resources, often creating a monoculture, decreasing species diversity, and reducing ecosystem services like food and shelter for wildlifeopen_in_new. The purple loosestrife (Lythrum salicaria), blooming in the photo above, has replaced the unique diversity of plant species normally present in this wetland area. It has also filled in the open water, reducing habitat for aquatic and avian species. The original marsh vegetation here likely supported pollinators with seasonally unique selections of pollen, nectar, and host plants that produced caterpillars and other larvae.


Photo © Darin House

Usually missing from the environment where invasion of nonnative species occurs are control mechanisms. Whether it be predators, disease, or healthy competition by other plants, in native ecosystems there is usually some form of regulation on growth and/ or check on reproduction of individual species. Without this control invasive plants continue to prosper unabatedly, while native plants are at the mercy of herbivores, fungus, and stricter environmental requirements they’ve co-evolved with. Too many herbivores with a preference for native plants, like the deer above, can play a large role in limiting native plant populations, giving invasive plants yet another edge. To keep invasive species from becoming the dominant–or only–species in a landscape, some form of control must be applied to put pressure on the growth and reproduction of the nonnatives. This is where you can come in.

North American Natives Acting as Invasive Plants Elsewhere

Plants native to North America can become invasive when they are introduced to other ecosystems. These might even be plants known and loved in their home ranges, but causing problems elsewhere. A few examples:
Robinia pseudoacacia -Europe
Amorpha fruticosa – Europe
Opuntia (prickly pear) – Australia

invasives map

Photo © USDA

Currently, many invasive species are far too widespread and abundant to have any practical goals of complete eradication. Above we flip through maps showing the locations of major invasions by 7 plant species in the U.S.. Local and specific preservation of plant communities is still possible, however, with efficient and concentrated management practices. In the debate about whether invasive species are taking over an area and what to do about it, we need to look closely at whether the invaders are driving or the ecosystem changes or whether they are just passengers of preexisting environmental changes. The type and amount of effort put into control of a given species depend on the factors that create or allow the invasion in the first place. It is also important to weigh the effects of that invasion, and the effects of the management practices, afterwards.


Photo © Mark Kirchner

Many plants labeled as invasive have qualities that allow them to easily colonize disturbed areas. Is an increase in the proportion of disturbed areas in our landscape part of the cause, then, of the spread of invasive plants? Perhaps local growing conditions have changed and native species can no longer thrive in an area, leaving it otherwise barren. An invasive species can work its way into even a pristine environment, slowly gaining ground and eventually pushing out and replacing the original plant community. But, is it the strength of the invader or the weakening of the environment from added pressures like climate change, or from soil, water, or air quality degradation from nearby industrial pollution making an ecosystem vulnerable? These distinctions are important for determining effective control options, if any.


Photo © Ian Crowther

When looking at the photo above we see a number of invasive species taking advantage of the difficult growing conditions in this vacant lot. They did not create the lot, nor was it created for them, but since no other vegetation will compete successfully there, these invasives have found their niche. In many inner-city vacant lots, exotic species have increased the diversity and ecosystem services to urban areasopen_in_new. Perhaps the difficult growing conditions will control these invasive species and allow a new kind of diversity to emerge, one suited to these specific conditions.


Photo © Jason Janelle

Before investing energy and resources into invasive species management, consider how you can best apply control to the forces that are creating, allowing, or encouraging the invasion to take place. Blanket goals of complete eradication are not always practical, nor are they necessary. Remember diversity and ecosystem services are key to successful habitat. Native plants make that equation possible but it that does not mean it cannot include nonnative plants, especially if, in the environment and conditions present, the growth and reproduction habits of those nonnatives are regulated to maintain diversity and ecosystem services.

Gaining Control of Invasive Plants
Control Measures for Invasive Plants