Introduction

Invasive Species Defined

What's Being Done

 National Invasive Species Information Center

Submodule 5: Invasive Animals

General Characteristics

Invasive animals can be divided into two major groups – vertebrates, or those animals with backbones like mammals, and invertebrates, or those without backbones, like snails. In addition, animal invaders can be divided into those invading terrestrial habitats and those invading aquatic habitats. In general, invasive animals, like other invasive organisms, have these characteristics in common:

• Reproduce at high rates
• Are highly competitive for resources
• Are able to grow and flourish in diverse habitats

Habitats that are invaded by non-indigenous vertebrates generally have low biodiversity, meaning that the variety of life currently present in the area is low. Observations of past animal invasions throughout the world have revealed that small, isolated habitats, for example Hawaii, seem to be more susceptible to animal invasion than larger landmasses, for example the continental U.S.

Invading animals, particularly vertebrates, have been very instrumental in altering native habitats and contributing to the extinction of native species by predation on native species and introducing pathogens into the habitat. An example of this is the introduction of rats and other ground-dwelling rodents in Hawaii and the resulting decimation of many native bird populations. Alteration of habitat by animals can facilitate the invasion of other non-indigenous species; for example, the rooting of feral pigs in Hawaii caused disruption of native vegetation allowing for the establishment of weeds.

Invaders seem to be able to fill certain niches that have been previously unoccupied. Historical data has shown that when an animal invades a particular habitat, animals similar to the invader were absent in this particular habitat. Also, it has been observed that animals generally invade habitats that are similar to their native environment, although there are exceptions to this rule. It seems that many vertebrates are able to tolerate variable environmental conditions. It also seems that many invading vertebrates exhibit a higher tolerance for human-altered environments, such as non-native fish that are able to thrive in artificially created reservoirs.

Invading animals can sometimes cause a decrease in population or even extinction of native animals by out-competing them for resources such as food and habitat, or by introducing new diseases. For example, the New Zealand mud snail (Potamopyrgus antipodarum), first discovered in the Snake River of Montana and Idaho in 1987, has been shown to out-compete native mollusks for resources in addition to being a poor source of food for fish. These characteristics can decrease or eliminate populations of both native mollusks and fish. Extinction can also occur through the hybridization of genes from native animals with genes from exotic species, which can lead to the creation of a new species. The Pecos pupfish (Cyprinodon pecosensis) is seriously endangered, and may have become extinct as a species, due to its genetic hybridization with sheepshead minnow (Cyprinodon variegatus). Both fish are found in the Pecos River of New Mexico and Texas, where the minnow was introduced as a baitfish. Primarily in the western U.S., native bull trout (Salvelinus confluentus) and non-native brook trout have hybridized producing sterile offspring, which has greatly contributed to the displacement of the native fish.