15.3 Invasive species and their ecological impacts

Invasive Species and Native Ecosystems

Invasive species are non-native organisms that harm ecosystems and economies in new environments. They spread rapidly due to a lack of natural controls, outcompeting native species and disrupting ecological balance. This topic covers how invasive species arrive, why they're so destructive, and what happens to ecosystems once they take hold.

Human-mediated pathways like global trade and travel are the primary drivers of invasive species spread. Understanding these pathways and the ecological consequences of invasions is crucial for developing effective conservation strategies.

Defining Invasive, Native, and Exotic Species

These three terms get mixed up a lot, but the distinctions matter.

• Native species are organisms that evolved and adapted within a particular ecosystem over long periods of time. They play essential roles in maintaining the balance and function of their habitats. Monarch butterflies, for example, are native pollinators in North American ecosystems, and saguaro cacti are keystone species in the Sonoran Desert.
• Exotic species (also called introduced or non-native species) are organisms transported to an environment outside their natural range. The key point: not all exotic species become invasive. European starlings were introduced to North America in the 1890s and became highly invasive, while many ornamental garden plants remain exotic without causing ecological harm.
• Invasive species are the subset of exotic species that cause ecological or economic harm. They tend to spread rapidly because they lack natural predators, parasites, or diseases in their new environment. Kudzu, a vine introduced to the southeastern U.S. for erosion control, now smothers native vegetation across millions of acres. Asian carp, introduced for aquaculture, now dominate portions of the Mississippi River basin.

So the relationship works like a funnel: all invasive species are exotic, but only a fraction of exotic species ever become invasive.

Ecological Niches and Competitive Exclusion

An ecological niche is the specific role an organism plays within its ecosystem, including its habitat, food sources, and interactions with other species. Every species occupies a niche shaped by thousands of years of co-evolution with its neighbors.

When an invasive species arrives, it often fills or disrupts established niches. Garlic mustard, for instance, invades forest understories and releases allelopathic chemicals that suppress native plant growth and disrupt the mycorrhizal fungi that native plants depend on for nutrient uptake.

Competitive exclusion occurs when two species compete for the same limited resources and one ultimately displaces the other. In natural systems, co-evolved species often partition resources to coexist (a process called resource partitioning). Invasive species haven't gone through that co-evolutionary process, so they tend to dominate aggressively:

• Red imported fire ants outcompete native ant species across the southern U.S., reducing native ant diversity by up to 90% in invaded areas
• Brown tree snakes, accidentally introduced to Guam after World War II, eliminated 10 of the island's 12 native forest bird species through direct predation

The brown tree snake example also shows that competitive exclusion isn't always about competition for resources. Predation by an invasive species with no local checks can wipe out native populations just as effectively.

Ecosystem Impacts

Biotic Homogenization and Biodiversity Loss

Biotic homogenization is the process by which invasive species replace diverse native communities across different regions, making ecosystems more similar to one another. As the same handful of aggressive invasive species spread globally, unique local communities lose their distinctiveness. Think of it as the ecological equivalent of every city getting the same chain restaurants.

Japanese knotweed, for example, forms dense monocultures along riverbanks across Europe and North America, crowding out dozens of native plant species wherever it establishes. The Nile perch, introduced to Lake Victoria in the 1950s, contributed to the extinction of over 200 endemic cichlid fish species. That's one of the most dramatic biodiversity losses in modern history, and it happened in a single lake.

Invasive species are the second-largest driver of biodiversity loss globally, behind only habitat destruction. Their effects go beyond simply replacing individual species:

• They alter ecosystem structure by changing vegetation layers or physical habitats
• They disrupt food webs by removing key species or adding new ones
• They modify nutrient cycles; feral pigs, for instance, root through soil and accelerate erosion, while water hyacinth chokes waterways and depletes dissolved oxygen

These three categories of impact often reinforce each other. When an invasive plant changes the vegetation structure, that shifts which animals can live there, which in turn alters decomposition rates and nutrient cycling.

Disruption of Predator-Prey Relationships and Disease Transmission

Invasive species can reshape predator-prey dynamics in two main ways: by preying on native species that haven't evolved defenses, or by competing with native predators for the same prey.

• Lionfish in the Caribbean and western Atlantic have no natural predators in those waters. A single lionfish on a reef can reduce native fish recruitment by up to 79%.
• Burmese pythons in the Florida Everglades have caused dramatic declines in mammal populations. Surveys show raccoon observations dropped 99.3%, opossum observations dropped 98.9%, and marsh rabbits effectively disappeared from areas where pythons are established.

These changes cascade through ecosystems. When pythons remove mid-level consumers like raccoons and opossums, it affects everything from seed dispersal to insect populations. This is a textbook example of a trophic cascade, where removing one level of a food web triggers ripple effects up and down the chain.

Invasive species also transmit diseases to native populations that lack evolved immunity:

• Chytrid fungus ($Batrachochytrium$ $dendrobatidis$), spread partly through the global amphibian trade, has caused population declines in over 500 amphibian species and driven at least 90 to extinction. It's considered one of the most destructive pathogens in recorded history.
• Avian malaria, carried by introduced mosquitoes in Hawaii, has devastated native honeycreeper populations, pushing many species to survive only at high elevations where mosquitoes historically couldn't reach. As climate change warms higher elevations, even these refuges are shrinking.

Pathways of Introduction

Human-Mediated Spread of Invasive Species

Most invasive species arrive in new environments through human activities, whether intentionally or by accident. Understanding these pathways is the first step toward preventing future introductions.

Intentional introductions happen when people deliberately bring species to new areas:

• The pet trade is a major source. Burmese pythons now established in Florida were originally imported as exotic pets and either escaped or were released by owners who couldn't manage a snake that can grow over 5 meters long.
• Horticulture and landscaping brought species like kudzu and Japanese knotweed, both originally planted as ornamentals or for erosion control.
• Agriculture and aquaculture introductions include Asian carp, brought to the U.S. in the 1970s to control algae in fish farms before escaping into the Mississippi River system during flooding events.

Unintentional introductions are often tied to global shipping and trade:

• Ballast water is one of the most significant pathways for aquatic invasives. Ships take on water in one port for stability and discharge it in another, transporting organisms across oceans. Zebra mussels arrived in the Great Lakes this way in the late 1980s, and they now cost the U.S. an estimated $1 billion per year in damages to infrastructure and ecosystems.
• Hitchhiking on cargo accounts for species like the Japanese beetle, which arrived in the U.S. in soil around imported plant roots.
• Accidental release includes red-eared slider turtles, the world's most common pet turtle, now invasive on every continent except Antarctica after being released into local waterways.

As global trade and travel continue to increase, so does the risk of new introductions. Regions with high biodiversity but limited biosecurity resources are especially vulnerable. Island ecosystems face the greatest risk because their native species evolved in isolation and often lack defenses against mainland predators and competitors.