Invasive species

An invasive species is a non-native organism that causes harm to its new environment. These organisms can be plants, animals, fungi, or microbes. They cause damage to habitats, bioregions, and the economy. They may also threaten human health. While species moving to new areas is a natural phenomenon, humans have changed the process. Since the 20th century, human activity has increased the rate and scale of these invasions. People act as dispersal agents through both accidental and deliberate movements. This acceleration has become a serious global threat to biodiversity and social stability.

For a species to become invasive, it must first successfully establish itself. Usually, an introduced species must persist at low population densities first. They often face challenges when trying to reproduce or sustain a viable population. Sometimes, multiple introductions are required before they can truly establish. A key factor is high propagule pressure. This occurs when there are repeated opportunities for species to arrive. Examples include ship traffic between ports or vehicles traveling on major highways.

Once established, invasive species often change the balance of an ecosystem. Stable ecosystems maintain a resource equilibrium. Invasive species can disrupt this balance by outcompeting native species for food. This is especially true when resources like nitrogen and phosphorus are limited. Every species occupies an ecological niche, which is a specific role in its environment. Invaders may occupy unused niches or create entirely new ones. For example, edge effects occur when land is cleared for agriculture. This creates a boundary habitat that can host species that would not thrive elsewhere.

Scientists study these events through invasion biology or invasion ecology. Much of this research was influenced by Charles Elton. In 1958, he published "The Ecology of Invasion by Animals and Plants." Elton suggested that high species diversity makes an ecosystem harder to invade. He believed that more diverse areas had fewer unoccupied niches. However, later studies showed that large-scale ecosystems might actually be more susceptible. This debate often depends on the spatial scale of the study. Research in this field grew significantly during the 1990s. It remains an interdisciplinary field involving zoology, agriculture, and pathology.

Different systems respond to invasions in unique ways. Island ecosystems are often very prone to invasion. This is because island species frequently face few competitors or predators. This leaves many "open" niches for newcomers to fill. For instance, the brown tree snake has decimated native bird populations on Guam.

In contrast, the ocean can act as a barrier. Island nations like Australia and the Galapagos have many endemic species. These are species found nowhere else, protected by geographical isolation.

Invasive species also display specific biological traits. Many invaders are extremely resilient and grow or reproduce very quickly. Some plants use vegetative reproduction to spread without seeds. One study found that 86% of invasive species could be identified by these traits. Other invaders change the physical landscape through processes like bioturbation or bioconstruction. The Chinese mitten crab, for example, increases bioturbation and bioerosion rates.

Even native species can become invasive if humans alter the food web. The purple sea urchin harmed California kelp forests after humans overharvested sea otters.

There are many notable examples of these biological shifts. The kudzu vine is a famous example in the southeastern United States. It was originally introduced to control soil erosion, but it grew uncontrollably.

In aquatic environments, the water hyacinth forms dense mats. These mats limit light penetration and harm other organisms.

Animals like the domestic cat can also become invasive predators. In places like the Florida Keys, feral cats negatively impact local wildlife.

These examples show how deeply invasive species can reshape the natural world.