Not every species introduced to a new place becomes a problem. Most non-native species that arrive in a new ecosystem — intentionally or by accident — either fail to establish or settle in without causing significant harm. An invasive species is different: it's a non-native species that spreads aggressively in its new environment and causes measurable ecological or economic damage, usually because it arrives without the predators, parasites, or competitors that kept it in check in its original range.
The scale of the problem is larger than most people realize. In 2023, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) released the first global assessment dedicated to invasive alien species, approved by representatives of its 143 member states. Its findings put hard numbers on an issue that had mostly been studied region by region before.
The global cost of biological invasions
The IPBES assessment estimated the global economic cost of biological invasions at more than $423 billion a year, based on 2019 figures — covering impacts like crop losses, infrastructure damage, disease spread, and the cost of control efforts. The report also found that these costs have at least quadrupled every decade since 1970, a pace that reflects both the growing number of invasive species and the accelerating movement of people, goods, and shipping that helps them spread.
Invasive species and extinction
Beyond economic cost, the ecological toll is severe. The IPBES assessment found that invasive alien species are a major contributing factor in 60% of all recorded global animal and plant extinctions, and the sole documented driver in 16% of them. The report tracked 1,215 local extinctions of native species caused specifically by 218 invasive species — concrete, documented cases rather than broad estimates. Islands are especially vulnerable: alien plant species now outnumber native plants on more than a quarter of all islands worldwide, because island species often evolved without mainland-level competition or predation pressure.
How invasive species disrupt an ecosystem
| Mechanism | Effect on the native ecosystem |
|---|---|
| Predation | An invasive predator targets native prey that never evolved defenses against it. |
| Competition | An invasive species outcompetes natives for food, light, water, or nesting space. |
| Habitat alteration | An invasive plant or animal physically changes the habitat — for example, altering fire frequency or water chemistry — in ways natives can't tolerate. |
| Disease introduction | An invasive species carries a pathogen or parasite that native species have no resistance to. |
| Hybridization | An invasive species interbreeds with a closely related native species, eroding the native's distinct genetic identity. |
Scale of the problem
IPBES counted more than 3,500 invasive alien species currently causing documented harm worldwide, with roughly 200 new alien species recorded globally each year as global trade and travel continue to accelerate introductions. Not all of these 3,500 are invasive in every location they've reached — status depends on the specific ecosystem a species has entered — but the trend line of new introductions is clearly upward.
A documented case: the brown tree snake on Guam
One of the most thoroughly studied invasions on record is the brown tree snake (Boiga irregularis) on the Pacific island of Guam. The snake is believed to have arrived as a stowaway in military cargo shortly after World War II, reaching an island whose native forest birds had evolved with no snake predators at all. Within a few decades, USGS and U.S. Fish and Wildlife Service surveys documented the loss or near-loss of most of Guam's native forest bird species — several found nowhere else on Earth — along with cascading effects on the forest itself, since many of those birds had been the island's primary seed dispersers and pollinators. Researchers have since linked the bird losses to measurable declines in forest regeneration, since fewer birds meant fewer seeds carried away from parent trees. Guam's experience is now a standard case study in invasion biology precisely because it shows how a single introduced predator can unravel an entire island food web, not just the species it directly eats.
The case also shaped prevention policy elsewhere: fear of the brown tree snake reaching Hawaii — another isolated island ecosystem with no native snake predators — has driven specific cargo-inspection protocols for flights and shipments originating in Guam, illustrating the "prevention over cure" principle that invasion biologists consistently recommend.
What actually works: prevention over cure
- Border biosecurity and import inspection — stopping species before they arrive is consistently cited as the most cost-effective tool.
- Early detection and rapid response — acting within the first few years of an introduction, before a species becomes established, dramatically improves eradication odds.
- Public reporting — citizen observations through platforms like iNaturalist often provide the earliest warning of a new invasive species; see our citizen science article.
- Ongoing control — once a species is fully established, management shifts from eradication to long-term, often costly, suppression.
Common mistakes
- Using "invasive" and "non-native" interchangeably. Most non-native species don't cause significant harm; "invasive" specifically implies documented ecological or economic damage.
- Assuming invasive species are always animals. Plants, fungi, and microbes are invasive species too, and plants dominate many island invasions.
- Assuming the problem is shrinking. IPBES data shows costs and new introductions both trending upward, not down.
Frequently asked questions
What is the difference between a non-native species and an invasive species?
A non-native (or alien) species simply lives outside its original range, often introduced by human activity, but causes little harm. An invasive species is a non-native species that spreads aggressively and causes measurable ecological or economic harm in its new environment.
How much do invasive species cost the global economy?
The 2023 IPBES Invasive Alien Species Assessment put the global economic cost at more than $423 billion a year, based on 2019 figures, with costs reported to have at least quadrupled every decade since 1970.
How often are invasive species linked to extinctions?
The IPBES assessment found invasive alien species were a major factor in 60% of recorded global animal and plant extinctions, and the sole driver in 16% of them, with 1,215 documented local extinctions linked to 218 invasive species.
What's the most effective way to manage invasive species?
According to IPBES, prevention — strong border biosecurity and import controls that stop new invasive species from arriving in the first place — is consistently the most cost-effective approach, far cheaper than controlling or eradicating a species after it has established.
Sources
- IPBES — Invasive Alien Species Assessment (2023), accessed November 4, 2026.
- UNEP — summary coverage of the IPBES Invasive Alien Species Report, accessed November 4, 2026.
- USGS — Brown Treesnake research and Guam forest bird decline documentation, accessed November 4, 2026.