Fabio Pupin / Red swamp crayfish (Procambarus clarcki), invasive species, captive, Italy, July.

Nick Hawkins / European green crab (Carcinus maenas), an invasive species in North America, Kejimkujik Seaside National Park, Nova Scotia, Canada. July.

Shane Gross / Invasive Lionfish (Pterois volitans) which have taken over and are wiping out native fish in the Atlantic ocean. The highest densities are in the northern gulf of Mexico. Destin, Florida, USA.

Shane Gross / Invasive Lionfish (Pterois volitans) which have taken over and are wiping out native fish in the Atlantic ocean. The highest densities are in the northern gulf of Mexico. Destin, Florida, USA.

David Shale / Pram bug (Phronima) swimming, Mid-Atlantic Ridge. Deep sea species.

John MacPherson / Reindeer (Rangifer tarandus) fighting, Cairngorms National Park, Scotland, UK, March.

Franco Banfi / Freshwater jellyfish (Craspedacusta sowerbii) Lake Lugano, Ticino, Switzerland. August. Invasive species.

Franco Banfi / Freshwater jellyfish (Craspedacusta sowerbii) Lake Lugano, Ticino, Switzerland. August. Invasive species.

Jordi Chias / Atolla jellyfish (Atolla wyvillei) with Gentoo penguin (Pygoscelis papua) in the background near the surface. Antarctic Peninsula. Finalist in the Underwater category of the Wildlife Photographer of t...

Sven Zacek / American mink (Mustela vison) on ice with Common frog (Rana temporaria) prey, Tartumaa, Estonia, January. Introduced species.

Sven Zacek / American mink (Mustela vison) shaking off fur on ice with Common frog (Rana temporaria) prey, Tartumaa, Estonia, January. Introduced species.

Sven Zacek / American mink (Mustela vison) running on ice with Common frog (Rana temporaria) prey, Tartumaa, Estonia, January. Introduced species.