Variation in basic life history across a predator’s geographic range

Background/Question/Methods

Individuals of a single species are generally held to share a common set of fundamental life history traits.  This belief may be strengthened by the tendency of individual studies to be conducted at sets of adjacent or nearby sites.  However, a species may be confronted with very different sets of ecological challenges across its geographic distribution, creating the potential for a single species to exhibit large variation in its basic life history.  The predatory dogwhelk Nucella ostrina must cope with differing thermal regimes and prey recruitment patterns across its range; in Washington state, low tides occur later in the day and barnacle recruitment occurs almost exclusively in the late spring and early summer, whereas Oregon has earlier low tides and more continuous recruitment of barnacles throughout the year.   I recorded survival, feeding, growth, and reproduction rates of both caged whelks and closed, free-living populations of N. ostrina in order to determine how whelks respond to the disparate environmental conditions present across their range.

Results/Conclusions

Whelk survival was consistently higher in Oregon than Washington; more than half of adult whelks survived a full year in Oregon, while fewer than 20% of Washington whelks were still alive after 12 months.  Feeding and per capita reproduction rates were not significantly different between states, but were nominally higher in Washington than in Oregon. This suggests that whelks employ different life history strategies in the two regions, and that the differences in mortality are not simply driven by differences in habitat quality between the regions.  Whelks in cages that provided shelter from sun and waves did not outperform whelks in other treatments. While rates of mortality did not vary significantly during the course of a given year, the greatest number of Washington whelks died in the summer months, following the peak period of egg laying and prey recruitment.  This suggests that continuous versus pulsed recruitment of prey may drive differences in the basic survival and reproductive strategies of predators.