Invasions are not just military (Part 2)


Butterfly Bush

Butterfly bushes displace native vegetation and in spite of the name, negatively affect native butterflies. We didn’t have to go far to take this photo.

In a previous post about invasive species, we learned what invasion meant and who the invaders look like. Now let’s discuss how invasive species get a foot hold in the first place and what can be done about it.

How Invasions Happen

Invasive species can be introduced intentionally or unintentionally. They may be introduced intentionally to benefit the ecosystem by restoring habitat, increasing fish stock, or controlling pests. Unintentionally, they:

  • Are released in ship ballast;
  • Escape from fish farms;
  • Are used in recreational activities;
  • Are used as live bait;
  • Arrive through canals;
  • Are released/escape from aquariums;
  • Are used in unauthorized fish stocking; and
  • Can be introduced by many other means.

In order to successfully invade a new environment, certain biological characteristics are necessary. Many invasive species have high reproduction rates, short generations, long life, high dispersal rates, broad native range, and broad diet. However, not all species immediately survive in new environments. They can fail multiple times before flourishing. Invasion success is context dependent.

Controlling the Invasion

Strategies to control invasive species include (1) keeping potential invaders out, (2) eradicating potential invaders soon after invasion, (3) biological control, (4) chemical control, and (5) mechanical control.

Keeping potential invaders out

Keeping potentially damaging invaders out in the first place is the most cost-effective method. The danger can be reduced by monitoring the common invasion pathways such as ship ballast water, wooden packing material, and horticultural plants.

 Eradicating after Invasion

It is easier to eradicate invasive species if they are discovered quickly and population levels remain low. Even if it proves impossible to totally eliminate an invader, early intervention can keep the population at acceptably low levels. For example, Giant African Snails were effectively eliminated from Florida. Currently researchers in California are attempting to eradicate the marine green alga Caulerpa, a recent invader.

Biological Control

Biological control involves introducing an enemy of an invasive plant (for example, a disease, parasite, predator, or competitor) in an attempt to lower invader population size.

Sometimes introducing a natural enemy from the native range of the introduced pest can be effective. For example, prickly pear cactus, which invaded Australia from the Americas, has been effectively controlled by introducing a moth from South America whose caterpillar feeds on the cactus. In other cases finding an enemy from a different area (a novel association) works because the invader may not have evolved defenses to a species with which it has never been in contact. For example, a virus from South America has been used to control European Rabbits in Australia.

A disadvantage of biological control is that some agents attack nontarget species, becoming noxious invaders themselves, and it is very difficult to remove a troublesome introduced natural enemy once it is established.

Chemical Control

Although chemical pesticides can effectively control some species (for example, water hyacinth in Florida), it can have problems. Pesticides may affect non target species, can be expensive, and may only be effective for a limited time if pests evolve resistance.

Mechanical Control

Mechanical control involves using machinery or human effort to remove invaders, often manually. Mechanical control has been an effective control strategy for invasive Tamarix (arid climate adapted shrub) in the Southwestern United States. Volunteer convict labor has been used in Florida to cut paperbark trees and in Kentucky to rip out Eurasian musk thistle.

Ecosystem Management

The newest technology for managing invaders is ecosystem management, in which the entire ecosystem is subject to a regular treatment (such as a simulated natural fire regime) that tends to favor adapted native species over most exotic invaders. Because it is so new, the specific ways in which ecosystem management can be employed must be determined in each type of habitat.

Want to learn more?

Invasive species are everyone’s problem. Learn more about what you can do to help prevent them:

Washington Invasive Species Council

Washington Department of Fish and Wildlife Aquatic Invasive Species

US Department of Agriculture, Invasive Species State by State