Invasive species are organisms not native to a particular environment whose introduction causes or is likely to cause harm to the environment, economy, or human health. Their presence disrupts natural habitats, leading to ecological changes. Human activities, whether intentional or accidental, often facilitate their movement across geographical barriers into new regions.
Outcompeting Native Species
Invasive species frequently outcompete native flora and fauna for vital resources like food, water, sunlight, and space. They often exhibit faster growth rates and higher reproductive capacities. A key advantage is the absence of their natural predators, diseases, or competitors in the new environment, allowing their populations to increase rapidly. This enables them to monopolize limited resources, leaving insufficient amounts for native organisms to survive and thrive.
When an invasive plant grows quickly and forms dense stands, it can shade out native plants, depriving them of sunlight. Invasive animals might consume resources native species rely on, leading to their decline. For example, the invasive carp in the Great Lakes can consume plankton, a food source crucial for many native fish. This intense competition places pressure on native populations, often leading to reductions in their numbers or even localized extinctions.
Altering Habitats and Ecosystems
Invasive species can change the physical, chemical, and structural characteristics of an ecosystem, making it less suitable for native species. Some invasive plants, like kudzu, grow extensively, smothering native vegetation and altering soil composition. This physical alteration can block light for understory plants and modify nutrient cycling. Invasive animals, such as feral pigs, can disturb soil through rooting and digging, leading to increased erosion and changes in water flow patterns.
Invasive species can also chemically modify their environment. Some invasive plants release compounds into the soil that inhibit native plant growth, a process known as allelopathy. Beyond physical and chemical changes, invasive species can alter fire regimes, such as invasive grasses increasing wildfire frequency and intensity. These alterations degrade the ecosystem’s ability to support its original biodiversity, favoring invaders while disadvantaging native life.
Disrupting Food Webs
Invasive species can interfere with an ecosystem’s food web. Invasive predators, like the lionfish, can directly consume native prey species that have not evolved defenses, leading to population declines. A single lionfish, for example, can reduce juvenile fish populations on a reef. Invasive herbivores can overgraze native plants, diminishing a food source for native animals.
The lack of natural predators allows invasive populations to expand, intensifying their impact on food webs. This can have cascading effects; if an invasive species outcompetes a native species for food, it can indirectly affect higher trophic levels. Such disruptions can lead to the decline or extinction of multiple species, destabilizing the ecosystem’s balance.
Spreading Diseases
Invasive species can introduce novel pathogens, such as viruses, bacteria, fungi, or parasites, to native populations. Native species often lack natural immunity or resistance to these diseases, making them vulnerable. For example, white pine blister rust threatens native white pines in North America that have no innate resistance.
Invasive species can also serve as vectors, transmitting diseases to which they might be resistant but which are devastating to native wildlife or humans. The Asian tiger mosquito, for instance, transmits West Nile virus and Dengue fever. The introduction of new diseases can quickly decimate native populations, causing ecological imbalance and weakening an ecosystem’s resilience.