Invasive, exotic plants threaten biodiversity

During the late fall it is easy to spot invasive, exotic plants on the landscape. Many are the plants that still have green foliage. (Not counting evergreen natives.) Actively growing late into the season is one way these opportunistic species are able to outcompete their neighbors. Invasive plants also produce a lot of seed, usually at an early age, and this seed stays viable in the soil for longer periods of time than their native counterparts. This seed production strategy helps ensure that invasives can quickly spread when circumstances are favorable, or bide their time in the soil until conditions turn in their favor.

Invasive plants tend to have compact growth habits, forming very dense thickets that strangle nearby vegetation and make it difficult for other plant seed to germinate and grow. They also have many ways of spreading their seed great distances by the wind, hitchhiking on the fur of an animal, or having a seed that can work its way through a bird’s digestive system and be deposited under a convenient perch.

Some invasive plants, like the ailanthus tree, can spread from their root system. When this tree is damaged, such as being cut down, it sends chemical messages to the roots to send up sprouts. That’s why cutting down one Tree of Heaven may result in 50 more sprouts coming up from its root system. Since these sprouts are connected to a well- developed root system, they tend to grow much faster than a small seedling. Other plants, such as Canada thistle, spread by rhizomes, an underground or ground-level stem. As the stem lengthens, more seed producing thistle head’s appear; thus the plant continues to spread by both seeds and rhizomes. Other plants, such as the wineberry, spread vegetatively by having one of its stolons touch the ground, develop roots, and then produce a new individual.

Invasive plants can dominate the landscape by creating a nonviable environment for native plants. For example, the garlic mustard and ailanthus tree have allopathic properties, producing a natural herbicide that will kill or weaken neighboring plants, thus gaining a competitive advantage. The garlic mustard also produces chemicals that kill beneficial soil microorganisms that are involved in breaking down organic matter into its chemical components or obtaining nitrogen gas from the atmosphere and depositing it into the soil. The net effect of this chemical warfare is that the soil is less fertile, favoring those plants that can tolerate infertile conditions, such as most invasive plants.

Many of our invasive plants came from that part of the globe that has a temperate climate like Maryland such as Japan, England and China. These plants did not evolve over many thousands of years in our region like the native plants, so that they do not have many natural controls such as insects, disease, or animals that eat them, to keep their populations in check. For example, deer don’t like barberry or stiltgrass, so they grow unchecked. Without such natural controls, these plants gain an advantage.

The aggressive nature of invasive plants negatively impacts the environment because they affect biodiversity, creating plant communities that consist of only a few plants, many of which are exotic invasive. Invasive plants also tend to develop very dense thickets which may be desirable for some animals, but overall are not, often obstructing access to an area. Invasives can change soil chemistry, which can decrease the fertility of the soil or cause erosion and the problems associated with this condition. In addition, when invasives overtake a native community, they can disrupt the food web, displacing native varieties that are more nutritious to wildlife. The spread of invasives also has an economic impact in agricultural areas or other natural communities where control strategies must be employed.

Nature Notes for 12/13/2015