Invasive and Exotic Species in Maryland

What is an invasive species? Are all non-native species invasive?

This is a terrific question, because many people get confused by the terms exotic, native, and invasive. In fact, they refer to two different and distinct characteristics of species.

“Native” and “exotic” refer to whether or not a species was present in a given ecosystem before a given time: usually the date of European settlement. For instance, everyone can agree that white oaks are native to Maryland because they have been here for thousands of years. Similarly we can all agree that English boxwoods are exotic because they were brought to North America by English colonists to grace the grounds of America’s plantations.

“Invasive” describes a species that, when introduced into an ecosystem aggressively establishes itself at the expense of native plants or animals. Mile-a-minute vine, with its amazing growth speed, nasty thorns, and ability to entirely smother native plant species, is an example of a species that is both exotic and invasive. Kudzu, gypsy moths, and English sparrows are commonly-known exotic invasive species.

“Invasive” is also commonly applied to species that cause economic difficulties in cultivated landscapes. The commonest examples of these species are the plants we refer to as “weeds”, many of which are in fact native species that aggressively invade our painstakingly cultivated gardens of exotic species.

So the original home of a species (its nativity) and its invasiveness are not necessarily related terms. Yet the majority of the species that our civilization relies upon are exotic. Wheat, barley and rice are all exotic, but they are not invasive. Cattle are exotic, but also scenic and delicious and not usually invasive. And those English boxwood planted around the great colonial mansions have demurely stayed where they belong for hundreds of years. In fact, most exotic species do not become invasive in either natural ecosystems or agricultural settings. While a subject of on-going research, it remains very difficult to predict whether an introduced exotic will become invasive. Because any exotic could become invasive or be detrimental to the ecosystem, the introduction of any exotic is a bad idea.

Photo collage of 5 invasive species

Non-native Invasive Species

They are all non-native, invasive species in Maryland.

This means that they evolved somewhere other than Maryland or the Mid-Atlantic states of the U.S., AND they have caused, or have the potential to cause, ecological or economic harm, or they endanger human health.

Humans have always carried plants and animals from their countries of origin to new homes in far-away places. For North America, this transport sped up during the 17th century with trans-oceanic commerce, and it continues today. In most instances, those plants and animals introduced to new habitats provided food, clothing, medicine and a touch of beauty from home for colonists, explorers and merchants. These introduced species survived because they were tended and cared for in their new surroundings. Many more recent introductions were accidental, hitching a ride in packing material, bilge water or on international cargo. In the last fifty years, we have recognized that some of these introduced species can become problematic in their new surroundings.

Of those, 10%, or 1 in 10, of exotic, established species become invasive. Roughly 10% of introduced plant species escape cultivation or city docks or pastures into natural areas and survive on their own. Of those, about 10% thrive and become established. Of those, 10%, or 1 in 10, of exotic, established species become invasive.Of those, 10%, or 1 in 10, of introduced species do so well that they begin to compete with the native species already present. For introduced animals, the percentages are higher. These introduced species, released from the pressure of the predators and diseases that might have kept them in check within their home ranges, can spread out of control -- Zebra mussels provide one startling example of this. Sometimes this release from predation allows introduced species to focus energy on reproduction and spread rather than on defending themselves; kudzu displays this behavior. They can undergo genetic changes in response to their new environments. They outgrow and outspread and outcompete native flora and fauna. In short, they became invasive.

Invasive species can cause damage that far outweighs their numbers. They can be mammals, birds, insects, reptiles, fish or shellfish, plants, viruses, fungi – any type of living organism. They can have major effects on natural habitats and native species. In the U.S., 45% of species listed as rare, threatened or endangered are listed as such in part due to invasive species. It often takes years before an introduced, escaped species begins to reproduce and spread to become invasive, an interval referred to as lag time. Once recognized, an invasive species can be present in such numbers that it is hugely expensive, or even impossible, to eradicate. Habitat managers must then settle for minimizing the invader’s negative impacts by reducing its numbers or containing its geographical range.

Department personnel are involved in invasive species management in many fronts. The department does tracking and control work for invasive animals and plants on public land. We work closely with Maryland Department of Agriculture on insect and disease invaders, especially those that affect state forests.