Nonnative species, often referred to as introduced or alien species, are organisms that establish populations in areas outside their historical geographic range. These species are transported by human activities, either intentionally or unintentionally, to new environments. Their presence can significantly influence the ecological dynamics of the ecosystems they enter. Understanding their origins and spread is a first step in recognizing their broader ecological impact.
The Concept of Natural Ecosystem Variability
Natural ecosystem variability refers to regular fluctuations in ecosystems. This includes shifts in species populations, such as cyclical predator-prey numbers or seasonal plant abundance changes. Environmental conditions also vary, with temperature, rainfall, and nutrient availability fluctuating based on weather, climate, and geological processes. These dynamic changes allow resilient ecosystems to adapt and persist through disturbances. A variable ecosystem maintains diverse species and ecological processes, supporting long-term stability and function.
How Nonnative Species Arrive and Take Hold
Nonnative species are introduced into new environments through human-mediated pathways. Intentional introductions often occur for economic or aesthetic purposes, like agricultural crops or ornamental plants. Some species were introduced for biological control, sometimes with negative consequences.
Unintentional introductions are frequent, with global trade and travel as primary vectors. Ballast water can transport marine organisms across oceans, releasing them in new ports. Contaminated goods (e.g., timber, agricultural products) can harbor insects, seeds, or pathogens. Species can hitchhike on vehicles, equipment, or even clothing, inadvertently spreading to new locations. For a nonnative species to “take hold,” it must establish a self-sustaining population, able to reproduce and survive in its new environment.
Ways Nonnative Species Alter Ecosystems
Nonnative species disrupt the variability of ecosystems through several mechanisms. One way is through competition, where introduced species outcompete native organisms for resources like food, light, or space. For example, zebra mussels ( Dreissena polymorpha ) in North American freshwater systems compete with native mussels for food, leading to native population declines. Another mechanism is direct predation, where nonnative predators consume native species lacking defenses. The brown tree snake (Boiga irregularis) on Guam, for instance, decimated native bird populations lacking natural predators.
Nonnative species can also physically alter habitats, changing ecosystem structure and chemistry. Some plants, like saltcedar (Tamarix spp.), extract much water, lowering water tables and increasing soil salinity, impacting native vegetation. Others alter fire regimes; buffelgrass (Cenchrus ciliaris) creates dense fuel loads that increase fire frequency and intensity, harming native plants. Nonnative species can also disrupt food webs by introducing new trophic levels or changing prey availability, causing cascading effects. Novel diseases, to which native species have no immunity, can cause widespread mortality and alter population dynamics.
Widespread Consequences of Ecosystem Alteration
Nonnative species cause widespread consequences. A primary outcome is native biodiversity loss, as species are displaced or driven to extinction due to competition, predation, or habitat changes. This diversity reduction diminishes the genetic pool and ecological resilience. Such disruptions also impact ecosystem services, benefits humans derive from nature. For example, nonnative species changing plant communities can impair water purification or reduce natural pollination, affecting agricultural productivity.
Economically, impacts can be substantial, including direct costs for managing invasive populations, agricultural losses from pest species, and damage to infrastructure like water pipes or power lines. The estimated annual cost of invasive species in the United States alone is over $120 billion. Beyond these costs, nonnative species contribute to ecosystem homogenization, making distinct biological communities more similar. This reduces unique regional characteristics and can diminish ecological complexity and aesthetic value.
Strategies for Managing Nonnative Species
Managing nonnative species begins with prevention, the most effective approach. This involves stricter trade and travel regulations to minimize new introductions, and public awareness campaigns about responsible practices like cleaning outdoor gear or avoiding pet release. When prevention fails, early detection and rapid response are important. This involves monitoring for new introductions and quickly implementing control measures to eradicate or contain populations before widespread establishment.
Control methods vary by species and ecosystem. Mechanical methods, like hand-pulling plants or trapping animals, can be effective for small-scale infestations. Chemical control involves targeted herbicides or pesticides, applied carefully to minimize harm to native species. In some cases, biological control agents (natural enemies from the nonnative species’ native range) are introduced after extensive testing to ensure they do not harm native organisms. These efforts, combined with ecological restoration, aim to mitigate disruptions and restore natural variability.