Fitness-related traits are maximized in recently introduced, slow-growing populations of an invasive clam: is this a response to strong r -selection?

ABSTRACT Many species are shifting their ranges being forced to rapidly respond to novel stressful environmental conditions. Colonizing individuals experience a complex mixture of selective forces that favor rapid evolution of life history traits notably affecting dispersal and reproductive rates in newly invaded habitats. Limited information is currently available on trait variation within the invasive range despite being critical for understanding ecological and evolutionary factors that drive the process of range expansion of invasive species. Here we evaluated life history shifts of the widely introduced Asian clam Corbicula within its invaded range. Through an exhaustive literature search, we obtained data for 17 invasive Corbicula populations from different ecosystems worldwide to test the relationship between population and individual parameters relevant to the process of range expansion. Our main results show that (i) recently introduced Corbicula populations are characterized by low conspecific density and low population growth rate, (ii) clams reproduce earlier in slow-growing populations, and (iii) density had no effect on population increase. Our findings support the standpoint that individuals from recently established populations of Corbicula face strong r-selective forces that favor traits associated with high reproductive rates, which could be interpreted as an effective mechanism to overcome difficulties associated with low densities and low population growth in newly colonized areas.