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 100 km 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.