The ground beetle Merizodus soledadinus is a generalist predator originating Patagonia and accidentally introduced in 1913 in one of the most isolated places on Earth, the sub-Antarctic Kerguelen Islands. There, this flightless insect has invaded hypersaline strand lines and now inland habitats, causing local extinctions of invertebrates soon after its arrival in new sites. While its invasion has long been studied, little is known about its distribution and preferred habitats in the native range. It was first described from the Malvinas and Tierra del Fuego, but a field survey showed that populations can be found on mainland South America, further inland and northward than initially thought, where they most often thrive in the litter of Nothofagus sp. trees. This finding raised a number of questions on the factors and traits that facilitate the success of this non-cosmopolitan species in such contrasted conditions, from Patagonian forests to hypersaline habitats invaded in treeless islands. The good match between this voracious beetle and the endemic prey communities it encountered on Kerguelen is likely allowed by its broad opportunistic diet, high starvation resistance and high physiological plasticity. Its survival in inter-tidal areas is permitted by a high salinity tolerance, which is hard to reconcile with forest habitats discovered in Patagonia, questioning whether this insect first transitioned from forests to coastlines in native Patagonia or the opposite. Spatial sorting is now shaping eco-evolutionary dynamics on Kerguelen, ultimately accelerating the spread with climate change, and its persistence in areas where native prey are already impacted is secured by other invasions (invasional meltdown).