A nonnative species is any plant, animal, or organism living in an area where it does not naturally occur. It got there because humans moved it, whether on purpose or by accident. The term is purely geographical: a species that’s perfectly normal in one ecosystem becomes “nonnative” the moment it shows up in another where it has no evolutionary history.
Most nonnative species blend quietly into their new surroundings. Only a fraction cause serious problems. But that fraction has been responsible for 60% of all recorded plant and animal extinctions worldwide and costs the global economy at least $423 billion every year.
What Makes a Species “Nonnative”
The classification comes down to two things: where a species evolved and how it arrived. A species is native to the ecosystems where it developed over thousands of years through natural processes like migration, wind dispersal, or ocean currents. It becomes nonnative when human activity carries it beyond that natural range into an ecosystem where it has never existed before.
This applies within a single country, not just across continents. A fish species native to rivers in the American Southeast is considered nonnative if it’s released into a lake in the Pacific Northwest. The boundary isn’t political; it’s ecological.
Deciding where “native” ends requires picking a historical baseline, and that’s trickier than it sounds. Many scientists use the year 1500, before European colonial expansion reshuffled species across the globe. Others use 1750, the start of the industrial era. For specific populations, the baseline can shift dramatically. The house sparrow, for instance, is nonnative to England relative to a baseline of 6,000 years ago but is considered a familiar native bird by any modern standard. These cutoff dates are still debated, because even baselines set several hundred years back can underestimate impacts for species and regions that were affected earlier.
How Species End Up in New Places
Almost every nonnative species arrived through one of two broad pathways: intentional introduction or accidental transport.
Intentional introductions happen when people deliberately bring a species to a new location. This includes plants imported for gardens and landscaping, fish stocked in lakes for recreational fishing, and animals brought in as pets or for agricultural use. Some introductions are even meant to solve existing problems, like releasing one insect species to control another pest.
Unintentional introductions are subtler and often harder to prevent. Ships take on ballast water in one port and discharge it in another, releasing microscopic organisms thousands of miles from their origin. Insects and plant diseases hitchhike in imported firewood, agricultural products, and food packaging. Zebra mussels spread from lake to lake on the hulls of recreational boats. Even hiking boots and camping gear can carry seeds and tiny organisms into new territory.
The sheer variety of pathways is staggering. Species move through the aquarium trade, classroom science experiments, internet plant sales, fishing bait, marine debris, and even the luggage of international travelers. Every point of contact between human activity and the natural world is a potential doorway for a nonnative species.
Nonnative vs. Invasive: A Key Distinction
Every invasive species is nonnative, but most nonnative species are not invasive. The difference is damage. A nonnative species earns the “invasive” label only when it causes economic harm, environmental harm, or harm to human health. Under the U.S. federal definition established by Executive Order 13112, both criteria must be met: the species must be nonnative to the ecosystem in question, and its presence must cause or be likely to cause harm.
Many nonnative species are harmless or even useful. Honeybees, originally from Europe and Africa, are nonnative across much of North America but are essential pollinators for agriculture. Numerous crop plants that people grow and eat every day are technically nonnative. The EPA notes that while invasive traits are relatively common among nonnative species, they’re rare in native ones, but being nonnative doesn’t automatically make a species a threat.
Some nonnative plants even provide ecological benefits in their new homes. Autumn olive, honeysuckle, and oriental bittersweet produce fruit that several bird species depend on. Bees readily forage on nonnative plants like Japanese knotweed and spotted knapweed for pollen and nectar. The ecological picture is rarely black and white.
When Nonnative Species Cause Harm
The nonnative species that do become invasive can reshape entire ecosystems. A well-documented example is the spiny water flea, a predatory zooplankton native to Eurasia that invaded the Great Lakes in the 1980s and spread to inland lakes. After reaching Lake Mendota in Wisconsin in 2009, this tiny creature consumed massive quantities of the native zooplankton that had been keeping the lake’s water clear by eating algae. Within a few years, the grazer population dropped by 60%, and water clarity declined by nearly a meter. A single small predator triggered a chain reaction that amplified existing pollution problems and degraded the lake for everyone who used it.
Zebra mussels tell a similar story on a larger scale. After arriving in the Great Lakes, they collapsed the amphipod populations that native fish rely on for food, severely affecting fish health across the region. Their spread has been linked to avian botulism outbreaks that killed tens of thousands of birds. Managing mussels at power plants, water systems, industrial facilities, and boats costs over $500 million per year in the Great Lakes alone.
Globally, the numbers are striking. A four-year assessment by 86 researchers from 49 countries identified roughly 3,500 harmful invasive species and found they collectively cost the world economy at least $423 billion annually. Those costs come from agricultural losses, infrastructure damage, water treatment, and the enormous effort required to control species that have already established themselves.
How Nonnative Species Are Managed
Prevention is the most effective and least expensive strategy. Inspecting imported goods, regulating the pet and plant trades, treating ballast water before discharge, and cleaning boats between waterways all reduce the chance of new introductions. Once a nonnative species establishes a breeding population, removing it becomes exponentially harder.
For species already present, management follows an approach called Integrated Pest Management, which combines biological, cultural, physical, and chemical tools rather than relying on any single method. This might mean introducing a natural predator from the species’ home range (biological control), physically removing plants before they set seed, or carefully targeted herbicide or pesticide use. The goal is to minimize not just the pest, but also the risks to people, property, and the surrounding environment.
Complete eradication is sometimes possible on islands or in small, isolated water bodies, but in large, connected ecosystems it’s rarely achievable. Most management programs aim to keep populations below the threshold where they cause serious damage.
Climate Change and Shifting Ranges
Rising temperatures are adding a new layer of complexity. As climate zones shift, species naturally move toward cooler areas, blurring the line between a native species expanding its range and a nonnative one spreading into new territory. Research from the U.S. Geological Survey shows that nonnative species are expanding their ranges orders of magnitude faster than native species. This reflects both traits that enable rapid spread and the ongoing boost from human-mediated introduction.
Nonnative species also tend to have broader climatic tolerances, which means they’re better equipped to persist or expand as conditions change. With faster spread rates and larger potential ranges, nonnative populations have a distinct advantage in a warming world. For ecosystems already stressed by habitat loss and pollution, the accelerating arrival of nonnative species adds pressure that native communities may struggle to absorb.