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.
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
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.