Why Are Non-Native Species Dangerous to Ecosystems?

Species introduced to new environments, whether accidentally or intentionally, can have significant ecological consequences. While some integrate harmlessly, others become invasive, spreading rapidly and disrupting native ecosystems.

This disruption threatens biodiversity, ecosystem stability, and even human activities. Understanding these impacts is crucial for managing and mitigating the risks associated with non-native species.

Resource Competition

Non-native species often compete with indigenous organisms for limited resources such as food, water, and shelter. This competition is particularly intense when the introduced species has a higher reproductive rate, more efficient foraging strategies, or fewer natural predators, allowing it to outcompete native species. For example, zebra mussels (Dreissena polymorpha) in North American freshwater systems have led to severe declines in native mussel populations by filtering vast amounts of plankton, depriving native species of food and altering nutrient dynamics.

Invasive plants like kudzu (Pueraria montana) monopolize sunlight and soil nutrients, suppressing biodiversity by preventing native plants from establishing. This, in turn, affects herbivores reliant on those plants. Similarly, the European starling (Sturnus vulgaris), introduced to North America, aggressively competes with native birds for nesting sites, displacing species such as bluebirds and woodpeckers.

The impact of resource competition can accumulate over time, leading to long-term ecological shifts. Some native species may adapt by altering their feeding habits or habitat use, but these adjustments are not always sufficient. The introduction of the Argentine ant (Linepithema humile) has disrupted entire ecological networks by outcompeting native ants that play essential roles in seed dispersal and soil aeration. Their decline affects plant regeneration and soil health, demonstrating how competition for resources can reshape entire ecosystems.

Habitat Modification

Non-native species can dramatically alter the physical and biological characteristics of habitats, often disadvantaging native organisms. One striking example is the introduction of beavers (Castor canadensis) to Patagonia. These beavers have transformed vast tracts of forested landscapes by constructing dams that flood large areas, killing native trees and altering water flow. The resulting wetlands do not support the same ecological communities as the original forests, leading to declines in species dependent on intact woodlands.

Invasive plants also modify habitats by altering soil composition, hydrology, and fire regimes. Cheatgrass (Bromus tectorum), for instance, has spread aggressively across the western United States, replacing native grasses and increasing wildfire frequency. Unlike native vegetation, cheatgrass dries out early, creating highly flammable ground cover that fuels repeated fires. This cycle prevents native plants from reestablishing, reducing biodiversity and affecting animals reliant on native vegetation.

Aquatic environments are equally vulnerable. Water hyacinth (Eichhornia crassipes) forms dense mats that block sunlight from reaching submerged vegetation, reducing oxygen levels and leading to fish die-offs. These mats also impede water flow, affecting nutrient cycling and increasing sediment accumulation, making it difficult for native aquatic species to survive while providing breeding grounds for disease-carrying insects like mosquitoes.

Altered Food Webs

Non-native species disrupt food webs by altering predator-prey relationships, shifting energy flow, and displacing species at multiple trophic levels. When an invasive predator enters an ecosystem without natural checks, it can decimate native prey species. The brown tree snake (Boiga irregularis), introduced to Guam, has driven several bird species to extinction by preying on their eggs and nestlings. The loss of these birds has led to cascading effects, including declines in insect populations they once controlled and disruptions in seed dispersal.

Some invasive species disrupt food webs by competing for prey. The Nile perch (Lates niloticus), introduced into Lake Victoria, has devastated native fish populations by consuming vast numbers of cichlids, which played essential roles in controlling algae and detritus. This has contributed to algal blooms and oxygen depletion, affecting organisms throughout the ecosystem.

Others introduce novel feeding behaviors that native organisms cannot counteract. The European green crab (Carcinus maenas) preys aggressively on shellfish, including economically valuable species like soft-shell clams. Unlike native predators, which maintain a balance through co-evolved interactions, green crabs consume shellfish at unsustainable rates, leading to population crashes. This not only affects commercial fisheries but also impacts shorebirds and other marine organisms that rely on these shellfish as a food source.

Genetic Mixing

When non-native species interbreed with native counterparts, they introduce genetic material that alters local populations. This hybridization can lead to the loss of unique adaptations developed over thousands of years. One well-documented case is the hybridization between native cutthroat trout (Oncorhynchus clarkii) and introduced rainbow trout (Oncorhynchus mykiss). The resulting hybrids often lack the specialized traits that allow cutthroat trout to thrive in specific ecological conditions, threatening the survival of pure cutthroat populations.

Hybridization can also create organisms with traits that disrupt established ecological roles. Invasive mallards (Anas platyrhynchos) have interbred extensively with native duck species such as the Hawaiian duck (Anas wyvilliana), leading to the decline of genetically distinct populations. These hybrids may exhibit different migratory patterns, feeding behaviors, or habitat preferences, altering ecosystem dynamics. In some cases, hybrids outcompete purebred native species by inheriting aggressive or highly adaptable traits from their invasive parent, accelerating population declines.

Disease Transmission

Non-native species can introduce pathogens to ecosystems, leading to widespread disease outbreaks. Native species often lack resistance to these infections. The chytrid fungus (Batrachochytrium dendrobatidis), spread through the global pet trade and movement of invasive amphibians like the American bullfrog (Lithobates catesbeianus), has devastated amphibian populations worldwide. The fungus infects the skin of frogs and salamanders, disrupting their ability to absorb water and electrolytes, leading to mass die-offs.

Pathogen transmission is not limited to amphibians. The European rabbit (Oryctolagus cuniculus) in Australia facilitated the spread of myxoma virus, which was later deliberately released as a population control measure. While initially effective in reducing rabbit numbers, the virus led to the evolution of more resistant rabbit populations, demonstrating how disease dynamics can shift over time. Similarly, the spread of avian malaria (Plasmodium relictum) by introduced mosquitoes in Hawaii has driven several native bird species to the brink of extinction. These birds, having evolved without mosquito-borne diseases, lacked immune defenses, leading to catastrophic losses.

The introduction of non-native species can act as a vector for new diseases, creating long-term ecological imbalances that threaten biodiversity.