| Abiotic stress, particularly temperature, is often invoked as the cause of species' geographic range limits. Yet, the processes that determine the limits to species' geographic distributions are poorly understood. I used a combination of descriptive and experimental approaches to examine the northern range boundary of the intertidal limpet Collisella scabra (also Macclintockia scabra).; A survey of populations across 700 km of the California and Oregon coast identified the location of northern range boundary as Cape Mendocino, CA (40.4072°N, 124.3984°W). Detailed comparisons of density, size distributions, recruitment, and fecundity revealed that individual performance remained high up to the range boundary, even as density declined. Transplant experiments disclosed striking differences in survival, growth, and fecundity; however, performance was often as great or greater at sites more than 100km north of the range boundary than at control sites within the range boundary. Performance was also reduced at warm temperatures, but there was little relationship between latitude and temperature. Thus, although temperature affects individual performance, it doesn't control the poleward range boundary.; A separate study of growth, mortality, recruitment, and fecundity in five populations spread across the northern 300 km of Collisella scabra's distribution found strong effects of temperature on growth and survival. However, as in the transplant experiments, populations closer to the range margin did not show lower survival or growth. Instead, both recruitment per unit area and per capita declined with increasing latitude, indicating recruitment limitation in edge populations. The most likely cause of recruitment failure is the increase in habitat patchiness at the range margin, reducing the chances that dispersing larvae find suitable habitat for settlement. |
