Nonnative species, also known as alien, exotic, or introduced species, are organisms brought to a new location either intentionally or accidentally by human activities. Biodiversity refers to the variety of life in a specific place or across the entire Earth, encompassing the diversity of species, genetic variations, and ecosystems. Nonnative species can disrupt the natural balance of these ecosystems, posing a significant threat to global biodiversity.
Direct Impacts on Native Species
Nonnative species directly harm native flora and fauna through competition for resources, predation, disease transmission, and hybridization.
Competition arises when nonnative species vie with native organisms for essential resources like food, water, light, and space. For instance, invasive zebra mussels in the Great Lakes filter significant plankton, depleting the food supply for many native species. This intense competition can lead to declines in native populations.
Predation by nonnative species can decimate native prey populations that have not evolved defenses against these new threats. Invasive predators often lack natural controls, allowing their populations to grow unchecked and significantly impact local wildlife. For example, the brown tree snake (Boiga irregularis) introduced to Guam led to the extinction of several bird species.
Nonnative species can introduce novel pathogens or parasites to native populations, which may lack immunity. White-Nose Syndrome, a fungal pathogen affecting bats, was introduced by a non-native fungus, leading to significant declines in native bat populations. The chytrid fungus, spread through global trade, has also caused drastic declines in amphibian populations worldwide.
Hybridization occurs when nonnative species interbreed with native species, potentially leading to a loss of genetic distinctiveness. This interbreeding can dilute native gene pools. For example, hybridization with introduced cordgrass threatens California cordgrass (Spartina foliosa) in San Francisco Bay.
Ecosystem-Level Disruptions
Beyond direct impacts on individual species, nonnative species can fundamentally alter the structure and function of entire ecosystems. These disruptions have far-reaching consequences for environmental health.
Nonnative species can physically change habitats, altering conditions native species depend on. This includes modifying soil composition, changing water flow, or altering fire regimes. For instance, invasive plants like cheatgrass can increase the frequency and intensity of wildfires, which native ecosystems are not adapted to withstand.
Food webs can be significantly disrupted when nonnative species break critical links within feeding relationships. They might remove key food sources for native organisms or become a new, unmanageable food source. The lionfish, for example, preys on herbivorous fish in coral reefs, leading to increased algae growth that harms the reef ecosystem.
Some nonnative species can alter nutrient cycling, impacting the availability of essential elements. They may change nutrient availability or their cycling within the soil, affecting plant growth and overall ecosystem health. Invasive plants can affect soil nitrification rates, altering the microbial community and nutrient forms available.
Certain nonnative plants consume excessive water, leading to drought conditions for native species. This disruption of hydrological cycles can impact native species that rely on specific water availability patterns. For example, invasive saltcedar (Tamarix) consumes large quantities of water, reducing water availability for native riparian vegetation.
Traits of Successful Invasive Species
Certain characteristics enable some nonnative species to become invasive and pose significant threats to new environments. These traits allow them to thrive and outcompete native organisms.
Many successful invasive species exhibit rapid reproduction rates and short generation times. This allows them to establish large populations quickly, colonizing new areas. For example, a single purple loosestrife plant can produce millions of seeds, contributing to its rapid spread.
Broad environmental tolerances allow invasive species to adapt to a wide range of conditions, including variations in temperature, moisture, and soil types. This adaptability makes them resilient in diverse environments, unlike many native species with narrower environmental requirements. The zebra mussel, for instance, tolerates a wide range of water temperatures and salinity levels.
Invasive species often possess a generalist diet or broad resource use, meaning they can utilize various food sources or resources. This flexibility provides a competitive advantage, as they are not limited by specific resource availability. Their ability to exploit diverse resources allows them to outcompete native species.
A lack of natural predators or diseases in the new environment is a significant factor in the success of invasive species. In their native habitats, populations are kept in check by specific predators, parasites, or diseases. Without these biological controls, their populations can grow unchecked, leading to rapid expansion.
High dispersal capacity enables nonnative species to spread quickly across landscapes. This can involve efficient seed dispersal, rapid movement, or human-mediated transport. Many invasive species are transported accidentally through global trade and travel, allowing them to reach distant habitats.