Why Do Some Species Become Problems?

No one can predict with absolute certainty which species will become invasive, or how long after entering a new area a species may become a problem. One important factor, however, is that when a species is transported to a new place, it often leaves behind a host of other species that competed with, preyed upon, or otherwise limited its size and extent. Freed from these constraints, exotics can often become much more widespread in their new home than they could in their original habitat.

Not every organism that is moved far from its own pests and predators becomes invasive, however, so other factors must be at work too. For instance, peculiarities of biology make some types of organisms more likely to be invasive.

  • Many invasive plants are fast growing, and can produce a large number of seeds as well.
  • Large litters and short turnover time between pregnancies are also common in invasive animals.
  • Many invasives can tolerate a wide variety of conditions, and organisms that can exploit their surroundings a little better than the other creatures in the area – for instance a plant with a deeper root system, or a fish that hatches earlier in the year than its competitors – are prime candidates to become invaders.

Even the best-adapted and fastest-growing species, however, would have a hard time gaining a foothold if all the available space was taken up by thriving native communities. Therefore, disturbance is often an invader’s best friend: cattle grazing that creates gaps in the local plant community, roadsides stripped of their native vegetation, and bird communities with vacant niches because of pesticides or destruction of wintering habitat. These examples all heighten the possibility that a potential invader will successfully establish itself.

Shifting Battlefields

Invasive species may enter and move around the country in any number of ways. Some are introduced deliberately because they were expected to be ornamental or economically useful – and were not expected to become invasive. Chinese tallow, for instance, was originally cultivated by Benjamin Franklin, who had learned that their oily seeds provided an alternative to candle wax. It is now a serious invader in Florida. Other invaders that were released deliberately include kudzu, starlings, purple loosestrife and flathead catfish. Nurseries selling exotic and interesting plants, transportation departments seeking groundcovers for erosion control, exotic pet enthusiasts and even researchers and weed control agencies have all contributed to the proliferation of alien species.

An even larger number of species arrive here unintentionally.

  • Ballast water, which ships take on for extra stability when they have little cargo and release elsewhere when they take on a full load, has introduced zebra mussels, crabs, jellyfish, and a range of microorganisms to areas where they were previously unknown.
  • Dry ballast, in the form of rocks and soil, was eventually outlawed because of the number of pests that were introduced along with it.
  • Lumber products and wood packing material harbor insects, fungi and diseases that can attack trees.
  • Shipments of grain, vegetables and live plants are sources of agricultural and other pests.
  • Travelers can unknowingly transport seeds, eggs, bacteria and fungi attached to their clothing or shoes while abroad.
  • Vehicle tires, hiking boots, planes and railroad cars regularly cause movement of species within the U.S. Movement also occurs when people dump bait buckets, release exotic pets, or take a cutting of the interesting-looking plant in Grandma’s backyard.

Even if a certain plant or animal has no history of being invasive, it could harbor unseen pests – and some species become invasive for unknown reasons after a long period of apparent innocence. The Asian Fig, for instance, seemed harmless for years – until one of its pollinators arrived in the country, allowing the fig to spread far and wide.