A paradox of trout invasions in North America

A paradox of invasion biology is that even though native species are locally adapted to environmental conditions, nonnative species without this advantage often invade. Ecologists have advanced four main theories to explain why invaders are successful in some places and not others: biotic resistance, environmental resistance, human disturbance, and natural enemies. However, none of these theories alone can account for invasions by two trout species outside their native ranges in North America. Brook trout (Salvelinus fontinalis) are able to displace native cutthroat trout (Oncorhynchus clarkii) in the inland western US, but are themselves displaced by nonnative rainbow trout (O. mykiss) in the southeastern US. An alternative hypothesis is that an interaction among zoogeography, evolutionary history, and environmental resistance from the natural flow regimes can account for this paradox. The nonnative species invade successfully at the southern edge of the ranges of the native species, which are farthest from their ancestral origins. Due to their evolutionary history, the native species are poorly adapted to the natural disturbance regime at the southern limit of their ranges, but the nonnative species are preadapted by chance due to theirs. This alternative hypothesis about the interaction between the historical contingency of evolution and environmental resistance should be more widely tested, to inform both invasion biology and the conservation of native trout.