Invasive species are non-native organisms that cause ecological or economic harm when introduced to a new environment, typically by human activities. They pose a significant global threat to natural ecosystems and the biodiversity they support. Biodiversity encompasses the vast variety of life on Earth, from genes within species to entire ecosystems, maintaining ecological balance and providing essential services. Invasive species disrupt this balance, directly impacting native populations and altering ecosystems.
Direct Impacts on Native Species
Invasive species directly harm native flora and fauna through several mechanisms, threatening the survival of indigenous populations. They compete for resources like food, water, light, and space. Invasive plants, for instance, often grow rapidly, shading out native vegetation and monopolizing vital nutrients, leading to a decline in native plant diversity. Invasive animals can outcompete native species for food, as seen with zebra mussels in the Great Lakes, which deplete plankton supplies for native fish and mussels.
Some invasive species act as novel predators in their new environments, where native prey species have not evolved defenses against them. The brown tree snake on Guam, for example, decimated native bird and lizard populations. Invasive species can also transmit diseases to native populations lacking immunity. Hybridization can occur when invasive species interbreed with closely related native species, diluting native gene pools and potentially leading to the loss of unique genetic traits.
Altering Ecosystem Structure and Function
Beyond direct interactions, invasive organisms can fundamentally change ecosystems. Invasive plants can significantly modify habitats by altering soil chemistry through their root exudates or by changing litter decomposition rates. For example, some invasive plants increase soil nitrogen availability, favoring their own growth over native species adapted to lower nutrient conditions.
Invasive species also disrupt food webs, impacting energy and nutrient flow. They can introduce new trophic levels or remove existing ones, leading to cascading effects. Invasive filter feeders, for instance, can reduce phytoplankton populations, affecting organisms that rely on them.
Some invasive plants can alter fire regimes by increasing fuel loads, leading to more frequent and intense wildfires. These fires can create conditions that favor the invasive species, forming a destructive feedback loop. Invasive species can also impact water flow and availability within ecosystems, with some plants consuming more water than native counterparts, potentially lowering water tables and affecting aquatic habitats.
Case Studies of Biodiversity Loss
Real-world examples powerfully illustrate the impact of invasive species on biodiversity. The brown tree snake ( _Boiga irregularis_ ) on Guam is a well-documented case of catastrophic biodiversity loss. Accidentally introduced, this highly adaptable snake encountered no natural predators and a naive prey base. Its predatory nature led to the extinction of at least 10 of Guam’s 12 native forest bird species and significant declines in lizard populations. This loss of avian seed dispersers and pollinators further disrupted the island’s plant communities, demonstrating a cascading effect.
Another impactful example is the introduction of zebra mussels ( _Dreissena polymorpha_ ) to the Great Lakes of North America in the 1980s, likely via ballast water. These efficient filter feeders consume vast quantities of phytoplankton, a primary food source for many native aquatic organisms. This competition has severely impacted native mussel populations and reduced food for various fish species, leading to declines throughout the aquatic food web. Zebra mussels also attach to native mussels, smothering them, and increase water clarity, which can promote nuisance algae growth.
Kudzu ( _Pueraria montana_ ), a fast-growing vine native to Asia, is an invasive species across the southeastern United States. Initially introduced for erosion control and as an ornamental plant, kudzu grows aggressively, smothering native vegetation by blocking sunlight and physically crushing plants under its weight. This leads to a dramatic reduction in native plant diversity, transforming diverse forest ecosystems into monocultures. Kudzu’s ability to fix nitrogen also alters soil chemistry, potentially favoring its continued spread while disadvantaging native species.