The introduction of non-native species into new environments can alter ecological balance and threaten existing populations. An invasive species is a non-native organism whose introduction causes or is likely to cause environmental, economic, or human health harm. These species are introduced through various pathways, both intentional (e.g., agriculture, ornamental use) and unintentional (e.g., global trade, travel). Shipping can transport marine organisms in ballast water or on vessel hulls, while goods moved across land or air can carry insects or plant materials. This movement sets the stage for ecological disruptions.
Direct Threats to Native Species
Invasive species directly impact native populations through several mechanisms, including competition, predation, disease transmission, and habitat alteration.
Competition arises when invasive species vie with native species for essential resources like food, water, light, and nesting sites. Invasive plants, for instance, often grow faster and produce more biomass, allowing them to outcompete native flora for sunlight. This resource monopolization can lead to declines in native species less efficient at acquiring these elements.
Predation by invasive species introduces a new pressure on native wildlife. Native species may lack evolved defenses against these new predators, making them vulnerable to population reductions. For example, the brown tree snake has impacted native bird populations on Guam through predation, leading to the extirpation of several bird species. Feral cats also prey on native birds and mammals, contributing to widespread declines and even extinctions, particularly on islands.
Invasive species can transmit pathogens or parasites to native populations that may have no immunity. The Asian tiger mosquito acts as a vector for diseases like West Nile Virus and Dengue fever, spreading these illnesses to new areas. Invasive plants can also facilitate the spread of ticks and associated pathogens by altering microhabitats. These diseases can lead to widespread mortality and population collapse in susceptible native species.
Invasive species can modify the physical or chemical characteristics of habitats, rendering them unsuitable for native life. Some invasive plants, like saltcedar, deposit salt into the soil, making it inhospitable for native flora. Invasive cattails in wetlands can alter nutrient levels and accumulate dense mats of dead stems, causing native wetland plants to struggle. These alterations reshape the environment, forcing native species to adapt or face decline.
Disruption of Ecosystem Dynamics
Beyond direct threats to individual species, invasive organisms can disrupt the broader dynamics of entire ecosystems. A significant impact involves the alteration of food webs. Invasive species can eliminate key prey species, introduce new predators, or change food source availability, leading to cascading effects throughout the ecosystem. For example, zebra mussels, efficient filter feeders, consume plankton, causing declines in fish populations and other filter feeders. This reduction in the base of the food web can have consequences for higher trophic levels.
Invasive species can alter nutrient cycling and hydrology within ecosystems. Invasive plants can change the rates at which nutrients like carbon and nitrogen move through the soil. They can also modify leaf litter decomposition. Invasive plants can modify water flow and availability by consuming more water than native species or by altering the physical environment, such as trapping sediments. These changes can affect water tables, soil moisture, and the overall water balance of a region.
Ultimately, the cumulative effects of direct impacts and ecosystem disruptions contribute to biodiversity loss. Invasive species are a major driver of global biodiversity loss, ranking among the top threats after habitat destruction. By outcompeting, preying upon, introducing diseases, or altering habitats and food webs, invasive species can lead to the decline and extinction of native plants and animals. This reduction diminishes the overall variety of life within an ecosystem, impacting its resilience and function.
Illustrative Case Studies
The widespread impact of invasive species is evident through numerous real-world examples.
The Burmese python in Florida’s Everglades represents a clear case of invasive predation and competition. Introduced through the pet trade and accidental releases, these large constrictors have established a breeding population. They prey on a wide range of native birds, mammals, and even alligators, causing declines in mammal populations. This predation disrupts the Everglades’ food web and reduces prey availability for native predators.
Kudzu, often called “the vine that ate the South,” illustrates the impact of invasive plants through competition and habitat alteration. Introduced from Asia, Kudzu spread rapidly across the southeastern United States. It grows at an exceptional rate, forming dense mats that smother native vegetation by blocking sunlight. This transforms diverse native habitats into monocultures, reducing biodiversity and altering the physical structure of forests. Kudzu also affects nutrient cycling as a nitrogen-fixing species.
Zebra mussels in the Great Lakes exemplify how invasive species can disrupt food webs and alter aquatic ecosystems. Originating from Eurasia, these small bivalves were introduced via ship ballast water in the 1980s. As filter feeders, they consume phytoplankton, leading to a decrease in this food source for native species. This reduction in plankton has cascading effects throughout the aquatic food web, impacting native fish and mussel populations. Zebra mussels also attach to hard surfaces, impacting infrastructure and altering the physical habitat by increasing water clarity, which can promote nuisance algae growth.