The ever-advancing world we live in facilitates the rapid movement of people and objects. With those travels sometime come introduced flora and fauna. Many introduced species serve as menaces to native plants and animals, invading their usual habitats and overtaking them.

Invasive species are a widespread and serious ecological problem. They are so prevalent, you probably have an invasive species of some sort in your backyard! Although a natural phenomenon, species migration to non-native areas has occurred faster than natural due to humans. 

Defining an “invasive” species

Though the exact meaning of an “invasive species” is debated, there are a few characteristics of them that are generally agreed upon. All invasive species are introduced species, for one. Among the traits of animals and plants that become invasive in nature are fast growth and reproduction, tolerance of many environmental conditions (ecological competence), and phenotype plasticity, which is the ability to change behaviors and physiological processes in response to environmental changes.

Some organisms that are common in an area do not belong in the places they reside, even despite their prevalence. For instance, the supposedly wild mustangs of the western United States are not truly wild; instead, they are descendants of domesticated horses (feral horses) brought to the country by Spanish explorers. Feral animals are sometimes considered a type of invasive species. 

However, the definition of “invasive” is further convoluted by the fact that some species are not thought of as being invasive because they do not cause considerable damage to the environment. Typically, an introduced species will only be labeled as invasive if it threatens biological diversity, a definition that the European Union follows

The term “invasive species” is sometimes extended to include those that have overpopulated their native and surrounding regions. Nevertheless, deciding what specifically constitutes an invasive species is often subjective. 

lionfish underwater

How species are introduced

Organisms can be transferred between locales through many mechanisms, and though some are natural, most are anthropogenic (caused by humans). The way in which an organism is moved is its vector. A honeybee is a vector for pollen, and a mosquito is a vector for West Nile Virus. 

But even natural vectors for introducing species such as wind and wildfires are exacerbated by human interactions with the environment. Wildfires commonly sterilize soil, and firefighters coming in from outside the area can bring non-native seeds and microbes along.

A 2008 study found that maritime trade was the dominant factor in species transfer  in the ocean. Biofouling, or the attachment of organisms to a submerged surface, is a frequent occurrence. While it can result in the spread of invasive species, checking ship hulls for biofouling is not heavily regulated. Ballast water, which is used to stabilize ships, is usually uptaken by cruises and cargo ships at one port and discharged at another; this process can also spread non-native species. A 2004 international maritime treaty known colloquially as the Ballast Water Management Convention established standards for reducing the number of harmful organisms discharged with ballast water.

Animal and plant trafficking is an intentional, anthropogenic way of introducing organisms to a new place; it comprises a multi-billion dollar business worldwide. Although the trafficking of some species, such as elephants and rhinoceroses, is on the decline, the opposite is true of other species. According to the United Nations’ 2020 World Wildlife Crime Report, the trade of pangolin scales has drastically increased, new trafficking markets are emerging, and organized crime groups are beginning to take control of international wildlife trafficking schemes. Trafficked animals and plants are sold for food, traditional medicine, clothing and accessories, and as pets.

Climate change is another (largely anthropogenic) contributor to the rise of invasive species. Shifts in ocean temperature will cause changes in dominant species, which can disrupt ecosystems. With altered temperatures and the resulting new environmental conditions, certain species can expand their range to previously non-native areas. 

brown rabbit in field

The impact of invasive species

Invasive species can have positive and negative effects on an ecosystem. At best, an invasive species can facilitate faster biodiversity restoration or restore the role of an absent native species. They have a significant economic cost and can also cause the loss of biodiversity. Additionally, invasive species have an impact on forest ecology, tourism and recreation, and human and animal health (since they can spread diseases). Invasive species can gain significant territory, as is seen with many feral animal populations such as that of cats, horses, and pigs.

A famous instance of invasive species are the European rabbits of Australia. Brought to the island by English settlers in the late 18th century, rabbits are now a serious pest there. The problem became so severe that around a hundred years after their introduction, a “rabbit-proof fence” was constructed in the eastern part of the country in an attempt to contain feral rabbits. The fence’s importance decreased when the Australian government introduced the Myxoma virus to the rabbit population in 1950. Myxoma virus causes a disease known as myxomatosis that is lethal to European rabbits. Yet still, European rabbits remain a problem in the nation.

Though a number of invasive species inhabit Everglades National Park in Florida, Burmese pythons are one of the most concerning. A breeding population of Burmese pythons has been associated with the decline in populations of a number of mammal species in the Everglades. As determined in a 2012 study, populations of cottontail rabbits and foxes virtually disappeared in the national park, while raccoon and opossum populations decreased by about 99%. The Florida Wildlife Commission has established a program that people can apply to so that they can get paid to hunt Burmese pythons  in the state.

Download a free PDF of a worksheet to go with this lesson by clicking the button below.

invasive species worksheet

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What other examples of invasive species do you know of?

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