Why Are Invasive Species a Threat to Native Species?

Animal and plant species that are not endemic to a certain area will need to have been introduced. This can be due to various causes, including being introduced to tackle a problem. This has been carried out successfully by conservationists who want to redress imbalances in an ecosystem. It can also be caused by accidental introduction, such as when animals are introduced during international transport.

Some species are even introduced by willful negligence. A common example is buying an exotic pet and illegally depositing them in an ecosystem. This is one of the many reasons the illegal pet trade is so potentially damaging.

Not all introduced species will cause harm to the ecosystem, but those that do are known as invasive species. These are species of remote origin that establish themselves and spread rapidly upon arrival in a new environment. They modify the structure and functioning of the ecosystem in the process.

As a common factor, all invasive species have characteristics that explain their success. These include high growth and reproduction rates. Biological invasions can also be favored by the similarity of climatic conditions with those of the region of origin and by the degree of vulnerability of the native biota.

Discover more about how animals are distributed around the planet with our article explaining cosmopolitan animal species distribution.

While there are various routes by which a species can become invasive, they need to go through the following basic stages:

1. Transport stage: where a species is transported from its place of origin to a new territory. From that moment on, the species is classified as exotic. How invasive species reach new territories is a complex issue. Although most species are transported deliberately, some specimens are transported accidentally, such as weeds whose seeds are accidentally harvested with commercial seeds or animals which infest commercial transport ships.
2. Settlement stage: occurs when the exotic species finds favorable conditions to survive and reproduce in the new ecosystem, allowing it to form large populations. At this stage, the species has become naturalized.
3. Propagation stage: occurs when the naturalized species has a greater capacity to spread than the ecosystem's native species, leading to environmental disruption. The species is now considered invasive.

When considered as a whole, invasive species threaten entire ecosystems. The man consequence of a biological invasion is the loss of biological diversity. With such a loss, the ecosystem may no longer be able to function as usual and the result is serious environmental degradation.

The ways in which invasive species threaten native species are diverse. These invasive species can be very similar or completely different to native species, but can threaten them regardless. This is due to the disruption to trophic systems and other biotic factors in an ecosystem. The ways in which invasive species threaten native species include:

• Predation: some native species may provide the right nutrition for an invasive species to survive. This results in high levels of predation. The native species may not have evolved successful methods to defend against the invasive species because of their introduction to the ecosystem being relatively quick.
• Resource depletion: an invasive species can deplete the natural resources of other plant and animal species. This means native species can no longer feed, even if they are not directly predated upon by the invasive species.
• Disrupt trophic networks: invasive species can compete with native species for pollinators or dispersers, resulting in disruptions to food networks which can have knock-on consequences. Such consequences can be very hard to predict.
• Bring disease or pests: whether the invasive species is a pest itself or is a carrier of parasites, these can have detrimental consequences for native species. The same applies if they bring diseases to which they have immunity, but to which native species do not.

Learn more about the three types of biodiversity with our related article.

As we have already explained, both plants and animals can be invasive species. Some of these have managed to travel across the world from their own native ecosystems and cause widespread disruption. Some common examples include the following:

Invasive plant species

• Kudzu (Pueraria montana): this is a plant which was introduced to the Southeastern U.S. as a means of preventing soil erosion, but resulted in it becoming known as the vine that ate the South. It smothers native vegetation by blocking sunlight and outcompeting native plants for nutrients and space.
• Japanese knotweed (Fallopia japonica): this plant is so strong that it is able to grow through concrete and becomes incredibly difficult to remove. It is invasive to Europe and North America where it displaces plants and damages infrastructure.
• Water hyacinth (Eichhornia crassipes): a tropical invasive plant, it covers bodies of water. In the process, it blocks sunlight and reduces oxygen level, killing many types of aquatic organisms.
• Giant hogweed (Heracleum mantegazzianum): this is a toxic plant which is invasive to Europe and North America. Various restrictions have been put in place to restrict its distribution, being illegal in many places to grow in the wild.
• Lantana (Lantana camara): this is an invasive plant species in Australia, Africa and Asia. Not only does it crowd out native species, but it is poisonous to grazing animals and can cause their death.

Invasive animal species

• European starling (Sturnus vulgaris): while it was first introduced to North America for aesthetic reasons, it is now one of this continents most abundant birds. It competes aggressively with native cavity-nesting birds for space.
• Brown tree snake (Boiga irregularis): it has caused the extinction of multiple bird and reptile species native to Guam, devastating its ecosystems after accidental introduction post-WWII.
• European rabbit (Oryctolagus cuniculus): as these mammals are such prolific breeders, they have become a very destructive invasive species in Australia. They erode soil and damage crops, leading to the degradation of habitats belonging to other animals
• Cane toad (Rhinella marina): also invasive to Australia, it is poisonous to predators. These animals do not have the evolutionary information to stay away from them. Ironically, they were originally introduced to control pests before becoming a pest themselves.
• Red imported fire ant (Solenopsis invicta): aggressive and venomous, this ant damages crops, invades homes and reduces populations of ground-dwelling animals.
• Common carp (Cyprinus carpio): this is a bottom-feeding fish which has an almost worldwide distribution thanks to being invasive on a global level. They uproot aquatic plants, increase turbidity and alters entire freshwater ecosystems.

Learn more about a specific aquatic species which threatens ecosystems with our article on the lionfish as an invasive species.

Possible solutions to the problem of invasive species are not easy to find. Once a species has established itself, it is difficult to eradicate. There are some management strategies which may be implemented:

• Ecosystems are constantly monitored to achieve early detection.
• Reducing the abundance of the species and limiting its distribution.
• Mitigation is implemented in the form of strategies that focus not on managing the invasive species, but rather on the resource to be conserved.

The best strategy to deal with invasive species is prevention. Avoiding the introduction of exotic species which can later become invasive is less costly than other management strategies. Learn more about the threats to our world's ecosystems with our article on what causes mass extinctions?

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