Threats to Biodiversity: Invasive Species
Not all species that arrive in a new location become invasive—in fact, most do not. Lots of garden plants are imported from other places, and even if they sometimes grow wild, they do not achieve big populations and do not make a huge splash on native species survival. These are just called non-native or introduced species. The introduced species that do become invasive are the ones that cause big problems.
Some places are especially vulnerable to invasive species. Islands usually have lots of endemic species and few large grazers or predators; this makes island species more at risk when non-native species are introduced.
Other places have very low rates of species invasion, usually because the climate is especially harsh, like in the Arctic, or organisms need special adaptations to live in that particular habitat, like mangrove trees that can tolerate living in saltwater. As humans move around the globe, more non-native species are turning up in these harsh habitats, some from similar habitats in other parts of the world. These species are more likely to become invasive.
Here are some ways invasive species impact native ecosystems:
- Habitat modification
- Compete with native species for resources
- Predation of native species
- Herbivory on native plants
- Bring in pathogens
- Hybridize with natives, leading to loss of genetic diversity
In another instance of habitat modification, North American beavers (Castor canadensis) became invasive in Tierra del Fuego, Argentina. In the 1940s, 50 beavers were introduced to start a fur industry. Now, there are between 50,000 and 100,000 beavers wreaking havoc on the native trees. Unlike many North American trees that grow back from their roots when beavers chew them down, South American trees such as the southern beech die when a beaver gnaws down their trunk. Beavers have turned 40 million acres of forest into meadows and converted streams into bogs.
Competition for resources: Invasive species can compete with natives for food and space. Zebra mussels (Dreissena polymorpha) have spread throughout the Great Lakes region very quickly since the 1980s. Mussels need hard surfaces to live on, and zebra mussels take up space that native mussels could use, or sometimes just settle on top of the native mussels, killing them in the process.
Zebra mussels (black) took up residence on this endangered species, the Higgins eye pearly mussel (yellow). Image from here.
Predation: Guam, an island in the Pacific Ocean, has lost most of its birds to an invasive species. The brown tree snake was introduced to the island and wiped out three-quarters of the native bird species and two of the eleven native lizard species. Brown tree snakes have been introduced to other Pacific islands and had similar impacts in those places by eating eggs, young and adult birds, and reptiles.
The brown tree snake can climb trees and catch birds.
Herbivory: On the eastern side of the United States, two insects have been wiping out dominant trees. The Asian balsam woolly adelgid (Adelges piceae) has killed 90-99% of Fraser fir trees in some areas in the Appalachians by sucking the sap out of the trees. This adelgid also affects fir trees in Washington state and Oregon. Its relative, the hemlock woolly adelgid (Adelges tsugae), has killed most hemlock trees in New England.
Pathogens: Invasive species can be pathogens. One famous example is chestnut blight, a fungus that killed off pretty much all the chestnut trees in the eastern US by the 1940s. Chestnut trees, which were dominant throughout much of the eastern US, were replaced by oak trees. The effect of this change in dominant species was not fully studied at the time, but scientists believe that several species of moths went extinct when their chestnut tree hosts disappeared.
Hybridization: Sometimes species' genetic compositions can change drastically by mating with closely related species, making the less common species extinct by hybridization. Endemic species—those that live only in one place and nowhere else—are particularly vulnerable to hybridization if closely related species are introduced. North American mallards (Anas platyrhynchos) have been introduced in many places around the world and mate with other ducks in those places. Endemic ducks in Florida, Hawaii, New Zealand and Africa are in danger of extinction because of hybridization with the introduced North American mallard.
The Hawaiian duck is threatened by hybridization with the North American mallard. Image from here.
Since invasive species have such a big impact on biodiversity, many groups try to control or eradicate them. Efforts include manual removal, pesticides, and biological controls. Biological controls, or "biocontrols," work by finding a predator or pathogen from the native range of the invasive species and introducing it where the species has become invasive. Sometimes this is successful but other times it has awful unforeseen consequences. Cane toads were introduced to Australia to control the native cane beetle, which was harming sugar cane crops. The toads did not control the cane beetle, and reproduced so quickly that they spread all over Australia. Cane toads produce toxic secretions, which harm native predators and makes eradicating the toads difficult.