Explaining variation in life-history traits: growth rate, size, and fecundity in a marine snail across an environmental gradient lacking predators

摘要

Intertidal invertebrates are model organisms for examining the environmental control of life-history traits because of the sharp gradients in biological and physical factors found in their habitat, including wave action, temperature, food availability, and predation. However, the effect of a particular factor on ecological and evolutionary processes is difficult to separate when multiple factors vary simultaneously. We examined shifts in life-history traits in the intertidal gastropod Littorina sax-atilis Olivi across an intertidal shoreline where predation appears to have little influence. We determined the density, size, fecundity, and size at maturity of snails in 4 distinct zones across the shoreline and estimated growth rates in each zone. Size, fecundity, median size at maturity, and growth rate all exhibited pronounced clines across the shoreline with higher values in the lower zones, where food abundance was highest. Snail density, however, peaked in the mid-zones. The clines in life-history traits corresponded with food availability and environmental stress (wave forces and desiccation stress), and these factors were the likely determinants of the patterns observed. We also conducted a reciprocal transplant experiment between the extreme zones to assess if any differences in growth rate might have a genetic basis. Snails transplanted from the high zone to the low zone grew 7 times faster than snails that remained in the high zone and, more unexpectedly, 1.6 times greater than local snails from the low zone. Snails transplanted from the low zone to the high zone did not grow at all while local snails from the high zone had slow, although measurable, growth. These divergent responses of transplanted snails (i.e. counter-gradient variation) suggest that there may be differential size-dependent selection between the extreme zones with the greater wave forces of the low shore selecting against fast growth which would otherwise be supported by the abundance of food found there.