Invasive species are widely accepted as plants or animals introduced from other parts of the world that cause ecological or economic harm. These non-native organisms are often portrayed as the primary threat to local ecosystems, aggressively outcompeting local flora and fauna. This common understanding creates a paradox when a plant that has grown in a region for millennia suddenly begins to dominate a landscape. The question is whether an indigenous plant can be classified using the same terminology reserved for foreign invaders.
Defining Invasive Versus Native Species
A native species is defined as a plant or animal that occurs naturally in a specific ecosystem without intentional or accidental human introduction. These species have co-evolved with the local climate, soils, and other organisms for thousands of years, forming complex, balanced relationships.
The standardized definition of an invasive species requires two conditions: the species must be non-native to the ecosystem, and its introduction must cause or be likely to cause economic or environmental harm, or harm to human health.
This non-native requirement is a fundamental ecological distinction, making it impossible for a truly native plant to be invasive by definition. Non-native species that do not cause harm are simply classified as “introduced” or “naturalized.” The term “native invasive” is considered inaccurate by many ecologists because it conflates the global problem of biological invasions by foreign species with the local issue of a native species becoming problematic.
The Direct Answer: When Native Plants Exhibit Aggressive Behavior
Native plants cannot be invasive, but they can be aggressive or overly dominant. Ecologists often use terms like “aggressive native,” “weedy native,” or “nuisance species” to describe this phenomenon. This behavior means a native plant out-competes other native species, leading to a reduction in local biodiversity and the simplification of the plant community.
A native plant might form a dense monoculture, suppressing the growth and diversity of dozens of other species in the same habitat. This results in a loss of varied habitat structure crucial for supporting native insects, birds, and other animals. While the ecological outcome resembles the effects of a non-native invasion, the underlying causes relate to a disturbed balance rather than a foreign introduction. Aggressive native plants, such as certain species of goldenrod or native cattails, still provide ecological services and food sources for local wildlife.
Environmental Factors Driving Native Dominance
A native species usually becomes overly dominant when a major change or disturbance disrupts the historical ecological controls that previously kept its population in check.
Habitat Disturbance
Significant habitat disturbance, such as construction, logging, or agricultural conversion, removes slow-growing, established competitors. This creates a resource gap that fast-growing, “ruderal” native species are well-adapted to exploit, allowing them to rapidly colonize and dominate the newly opened space.
Nutrient Enrichment
Changes in nutrient availability are a powerful driver of native dominance, especially the addition of excess nitrogen or phosphorus from agricultural runoff or atmospheric deposition. Many native ecosystems are adapted to low-nutrient conditions. The sudden influx of nutrients favors native species with competitive traits for rapid growth and resource uptake. This enrichment can lead to a few native species increasing their biomass and cover, displacing slower-growing local flora.
Loss of Natural Controls
The loss of natural controls, such as specialized herbivores or grazers, is also a significant factor. In grasslands, the removal of large herbivores can lead to the unchecked growth of native plants that are naturally less palatable or more resistant to grazing. When these species are no longer consumed, their populations surge, increasing their dominance and reducing the overall diversity of the plant community.
Altered Hydrology
Altered hydrology is particularly important in aquatic and wetland systems, where changes to water flow or depth shift the competitive balance. Development that alters drainage patterns or water levels can create ideal conditions for certain native aquatic plants, such as some yellow waterlily species or aggressive eelgrass. These indigenous plants thrive under the new abiotic conditions and cause similar disruptions to water flow and recreational use as non-native weeds.
Managing Overly Dominant Native Species
Managing an overly dominant native species requires addressing the root ecological imbalance rather than simply removing the plant itself. The most effective long-term strategy involves restoring natural controls or reversing the environmental factors that enabled the species to take over. This might include reducing external nutrient inputs, such as fertilizer runoff, to shift the competitive advantage back to slower-growing, less demanding native species.
In restoration settings, manual control methods, such as targeted weeding or cutting, can reduce the biomass of the aggressive plant and temporarily open up space. This intervention must be paired with reintroducing competition by planting a diverse mix of other native species. A dense, diverse community of native plants provides biotic resistance, making it harder for any single species to achieve overwhelming dominance.