The spread of non-native species is an increasing challenge to ecological stability. An invasive species is defined as an organism non-native to a specific ecosystem whose introduction causes or is likely to cause environmental, economic, or human health harm. While a non-native species may live in a new area without causing issues, an invasive species actively disrupts the local biological community. These biological invasions are recognized as a top driver of global biodiversity loss, inflicting severe damage on ecosystems worldwide.
Disrupting the Food Web and Resource Availability
Invasive species often utilize resources more aggressively or efficiently than native organisms, frequently leading to competitive exclusion. Invasive plants, for instance, may germinate earlier or grow faster, enabling them to monopolize resources like light, water, and soil nutrients before native flora can establish. This competitive advantage can cause native populations to decline or be displaced entirely.
Predation by introduced animals is another direct mechanism that destabilizes established food webs. The Brown Tree Snake, accidentally introduced to Guam, caused the extinction or severe reduction of most of the island’s native bird species. Island ecosystems are particularly vulnerable because native animals often lack evolved defense mechanisms against new predators. Feral hogs also act as invasive omnivores, consuming the eggs and young of ground-nesting birds, reptiles, and amphibians, disrupting the reproductive success of native wildlife.
Invasive species can also outcompete native organisms for physical space and nesting sites. The European Starling, for example, is highly aggressive and often forces native cavity-nesting birds, such as woodpeckers and bluebirds, out of their nests. This displacement directly reduces the reproductive success of native species, leading to population decline. In aquatic environments, filter feeders like Zebra Mussels can attach to and suffocate native mussels while also depleting plankton, the base of the food web for other aquatic life.
Altering Ecosystem Structure and Function
Beyond direct competition, invasive species fundamentally change the physical and chemical characteristics of an environment, altering how the ecosystem operates. Certain invasive plants can drastically impact a region’s water cycle; Saltcedar (Tamarix species), for example, has deep taproots that consume vast amounts of groundwater, potentially lowering the water table. This excessive water use can desiccate riparian areas, making them unsuitable for native plant communities that rely on higher soil moisture.
Invasive plants can also modify soil chemistry by altering nutrient cycling, specifically nitrogen. Species with nitrogen-fixing capabilities, such as certain Acacia or Prosopis species, introduce excess nitrogen into nutrient-poor soils. This enrichment favors the growth of other non-native species that thrive on high nitrogen levels, facilitating further invasion and disrupting the nutrient balance native flora is adapted to. Additionally, the litter produced by invasive plants often decomposes differently than native material, affecting the soil’s carbon and nutrient pools.
A major functional change involves the alteration of fire regimes, leading to a destructive cycle known as the grass-fire cycle. Invasive grasses like Cheatgrass (Bromus tectorum) grow quickly and dry out much earlier than native perennial grasses, creating a continuous, highly flammable layer of fine fuel. Cheatgrass invasion has been shown to shorten the fire return interval in some regions from 60–100 years to as little as five years. Native plants, such as Sagebrush, are not adapted to survive such frequent fires, but the invasive grass regenerates quickly from seeds, establishing a monoculture. This process permanently converts diverse shrublands into fire-prone grasslands, fundamentally changing the habitat structure.
Introducing Novel Biological Threats
Invasive species pose a threat by carrying diseases and parasites that native species have never encountered, leading to population crashes. This phenomenon, known as pathogen spillover, occurs when an introduced host transmits a novel infectious agent to a susceptible native population. For example, the chytrid fungus, which causes chytridiomycosis in amphibians, has been spread globally by invasive species like the American Bullfrog.
Another biological threat is “genetic pollution,” the unintended hybridization between an invasive species and a closely related native species. When the invasive species interbreeds with the native one, the unique genetic identity of the native population is diluted or replaced by hybrid genes. This process can lead to the extinction of the genetically distinct native species, as seen with the hybridization of native cordgrass and an introduced cordgrass in San Francisco Bay.
Invasive species can also introduce novel toxins into the food web, proving lethal to native predators that attempt to consume them. The Cane Toad, introduced to Australia, secretes a poison called bufotoxin from glands behind its eyes. Native predators like the Northern Quoll, freshwater crocodiles, and certain snakes have suffered population declines because they were not adapted to recognize the toad as toxic prey.