GED Science: Invasive Species
Some populations of plants and animals in ecosystems are native, meaning that they naturally occur in a particular ecosystem or geographic region. For example, in Washington state, the evergreen huckleberry is a native species. Native species are uniquely suited to the environmental conditions present in the ecosystem, such as temperature, rainfall, and soil conditions. While this might make you think that native species are particularly strong in terms of their ability to survive, native species have often specialized in terms of a particular food source or habitat within their ecosystems. Because of this, native species often do not respond well to fluctuations in their ecosystems.
Non-native, or introduced species are those plants and animals that have been introduced into an ecosystem by humans. Sometimes humans introduce species on purpose, and sometimes the introduction of the species into an ecosystem is accidental. Introduced species have not specialized in a particular food source or habitat within the ecosystem it is introduced. For this reason, introduced species are typically considered generalists—they feed generally on a number of food sources within the ecosystem or can grow quickly in many different locations. While it may seem beneficial to have a plant that can grow quickly in many environments, or to have animals that have the flexibility to consume a number of food sources, introduced species often become invasive species, outcompeting native species for food and shelter.
The far-reaching and drastic effects of invasive species may be more apparent if you consider their effects from an ecological perspective. Consider the introduction of a invasive species of plant. If that plant outcompetes, and causes a decrease in the populations of two to three other native plant populations, the first-order consumers that typically feed on those native plants may not be able to obtain enough food to survive, and experience a decrease in population numbers. Remember that those native consumer species have specialized in a particular food source; they cannot simply make use of the invasive species as an alternative food source. Next, the second-order consumers that consume the first-order consumers can experience decreases in population for the same reason. The invasive plant species also can affect abiotic components in the ecosystem, such as the soil pH or other soil characteristics. The total effects to an ecosystem can reach far beyond biology or ecology, affecting tourism, financial gains from native species as food sources for humans, and/or the desirability of a location as a place to live. Kudzu is an example of an extremely invasive species in the Southwest. The following picture shows the aggressive growth of this vine. One can easily imagine how such a vine could outcompete native plants in that ecosystem:
Specific examples of the effects described above can be found in almost all ecosystems. The feral pig is an example of an invasive species in Hawaii. These pigs are a cross between small pigs introduced by Polynesians, and larger pigs introduced by Captain James Cook. These pigs trample existing vegetation in the Hawaiian ecosystem, and change the landscape by leaving depressions that fill with water after eating ferns. These water-filled depressions are breeding grounds for mosquitos that carry avian (bird) malaria. Additionally, because the feral pigs are large, they cover huge amounts of land and play a role in spreading the seeds of other invasive plant species. Because Hawaii is so isolated, introduced and invasive species can quickly alter this tropical ecosystem. The following shows the water-filled depressions that are caused by feral pigs:
While some systems are in place to prevent the accidental introduction of invasive species, such as the agricultural checkpoint at some entrances into the state of California, the introduction of plant and animal species sometimes happens on purpose. Species can be purposefully introduced because they are a desired food source, or species can be introduced as a way to control another species considered to be a pest. These introductions can be successful, and not all introduced species become invasive species. One such example is of the introduction of a beetle and a fly species from Australia to control a scale that attacked citrus trees in California. However, these decisions are based on careful predictions and modeling by ecologists. It takes a thorough understanding of the intricacies of ecological relationships to predict which species will or won’t become invasive. Ecologists never simply “experiment” by adding species to an ecosystem. This would be unethical. Great care should be taken with the introduction of any non-native species into an ecosystem.