Evolutionary and physiological adaptations of aquatic invasive animals:: r selection versus resistance

Invasive species have been characterized as tolerant of environmental extremes. This hypothesis was evaluated for invasive aquatic species in North America, particularly Asian clams, Corbicula fluminea, and zebra mussels, Dreissena polymorpha. Both species have rapid growth, early maturity, short life spans, and elevated fecundity, allowing rapid population recovery after reductions by rarefractive, environmental extremes. Extensive resistance capacities offer little adaptive value to invasive, r-selected species, because population reductions occur in their unstable habitats regardless of degree of stress tolerance. Thus, both species have relatively poor physiologic resistance, depending instead on elevated growth and fecundity for rapid population recovery. In contrast, native North American bivalve species are often adapted to stable habitats where perturbation is infrequent (i.e., freshwater unionoidean bivalves). They are characterized by slow growth, extended life spans, and low effective fecundities, slowing population recoveries (K-selected), and have evolved extensive resistance adaptations to avoid extirpation during environmental extremes. Review of resistance adaptations in other North American aquatic invaders revealed poorer or equivalent physiological tolerance relative to taxonomically related native species, suggesting that extensive physiological tolerance is not required for invasive success.