Salvinia molesta

Aquatic Fern (South America)

Floating aquatic fern that thrives in slow-moving, nutrient-rich, warm, freshwater. It may form dense vegetation mats that reduce water-flow and lower the light and oxygen levels in the water.


Photo: Scott Bauer, USDA ARS,


Alternanthera philoxeroides

Aquatic plant (South America)

Perennial stoloniferous herb that can be found in many parts of the world, infesting rivers, lakes, ponds and irrigation canals, as well as many terrestrial habitats.


Photo: USDA ARS,


Elodea canadensis

Aquatic plant (North America)

Submerged, rather densely bushy, aquatic perennial that grows and multiplies fairly rapidly in diverse habitats and conditions and has spread rapidly and easily throughout the world. It alters habitat, competes with native species, and threatens biodiversity.


Photo: Kristian Peters,


Heracleum mantegazzianum

Herb (Asia)

Enormous herb that may reach 6 meters in height. It is most common on roadsides, along streams and rivers, and in vacant lots. If forms a dense canopy, outcompeting native riparian species and increasing soil erosion along the stream banks where it occurs.



Photo: Terry English, USDA APHIS PPQ,


Ailanthus altissima

Tree (Asia)

Small to medium-sized tree of the mostly tropical Quassia family. Very prolific seed producer that grows rapidly and can overrun native vegetation. It produces toxins that prevent the establishment of other plant species.



Photo: James H. Miller, USDA Forest Service,


Bursaphelenchus xylophilus

Nematode (North America)

The pinewood nematode, causes pine wilt disease and harms pine trees and other conifers. Vectors of this nematode are long-horned beetles, especially those in the genus Monochamus.


Photo : L.D. Dwinell, USDA Forest Service,


Corbicula fluminea

Seashell (Asia)

Freshwater clam that spreads when it is attached to boats or carried in ballast water, used as bait, sold through the aquarium trade, and carried with water currents. It outcompetes native clams for food and space, and it damages intake pipes used by power, water, and other industries.


Photo: Noel M. Burkhead, U.S. Geological Survey


Dikerogammarus villosus

Crustacean (Europe)

Voracious predator that kills a range of invertebrates, particularly native shrimps and young fish, sometimes causing their extirpation. It tends to dominate its habitat, killing and maiming unselectively.



Pterois volitans

Fish (Oceania)

Beautiful but dangerous tropical fish that has spread to new marine environments through the aquarium trade. It has the potential to displace commercially important species such as groupers and to reduce recruitment of juvenile fishes, which in turn disrupts marine ecosystem processes.


Photo: Forest & Kim Starr


Python molurus bivittatus

Reptile (Asia)

Nocturnal predator that kills its prey by constriction. It is the largest and most water-dependent of the Python molurus, though it lives on land or on trees when juvenile. It threatens native species of amphibians, birds, lizards, snakes, bats and mammals.