Invasive species are non-native organisms that cause harm to the environment, economy, or human health in their new habitats. Introduced intentionally or unintentionally, they establish populations outside their natural range. Their ability to thrive and spread aggressively poses a problem for native ecosystems, impacting biodiversity and ecological processes.
Release from Natural Controls
A primary reason for invasive species’ success is their escape from natural controls in their native environments. This is known as the “Enemy Release Hypothesis”: species often arrive in new territories without the predators, parasites, and pathogens that regulated their populations back home. This absence allows them to allocate resources differently, redirecting energy towards growth and reproduction instead of defenses.
This reallocation enables rapid population expansion. For example, the cane toad produces toxic ooze that native predators are not adapted to, leading to unchecked growth. European rabbits proliferated in Australia due to a lack of natural predators, impacting local vegetation. The brown tree snake on Guam decimated native bird populations because the birds had no evolved defenses.
Inherent Biological Advantages
Many invasive species possess intrinsic biological characteristics that provide a competitive edge. They often exhibit faster growth rates, quickly dominating resources like light, water, and nutrients. Kudzu, for instance, is an aggressive vine capable of growing up to 26 centimeters (1 foot) a day, smothering native vegetation.
Some invasive plants employ “novel weapons,” producing biochemicals (allelochemicals) that inhibit native species’ growth or reproduction. Garlic mustard, for example, excretes sinigrin, which interferes with beneficial mycorrhizal fungi associations of native tree roots. The Tree of Heaven and spotted knapweed also demonstrate this chemical warfare, creating an unfavorable environment for native flora.
Invasive species often show high fecundity, producing many offspring. They frequently exhibit rapid maturation and frequent breeding cycles, accelerating population growth. Effective dispersal mechanisms aid their spread; some plants use wind (e.g., Giant Hogweed) or water (e.g., giant ragweed), while animals and human activities spread seeds (e.g., common burdock) or aquatic organisms (e.g., ship ballast water). Many invasive species are generalists, thriving across a wide range of food sources and environmental conditions.
Exploiting New Environmental Niches
Invasive species capitalize on opportunities in new surroundings, often thriving in disturbed habitats. Areas altered by human activities (development, agriculture) or natural disasters (fires, floods) become susceptible. Native species struggle to recover, creating openings invasive species exploit. Their ability to tolerate or benefit from disturbances gives them an advantage over native flora and fauna.
Some invasive species fill “empty” ecological niches when native species are absent, establishing themselves without direct competition. More commonly, they outcompete native organisms for resources, including food, water, and habitat. This aggressive competition can lead to declines or extinctions of native populations. Their success is linked to a broader range of environmental tolerance, allowing them to adapt to varied conditions like temperature, salinity, or soil types. For example, the European green crab tolerates a wide range of salinities and temperatures, aiding its spread into new marine environments.