Resources are more important than predation in driving the size at maturation of freshwater threespine stickleback (Gasterosteus aculeatus)

Background: Resource availability and predation are thought to be the most important ecological factors shaping age at maturation. They are expected to have a similar effect in causing populations to mature earlier, but a contrasting effect on size at maturation. Greater resource supplies have a positive effect on size at maturation by increasing growth rate, whereas heavier predation has a negative effect since it induces populations to mature earlier (and thus at smaller size). Question: Is resource availability or predation pressure more important in shaping the size at maturation when resources and predation covary in the environment? Organism: Freshwater threespine stickleback (Gasterosteus aculeatus). Field site: Natural lochs on the Scottish island of North Uist. Methods: We estimated size at maturation using two different approaches: probabilistic maturation reaction norm (PMRN) and Stearns' size at maturation. Results: Size at maturation was strongly influenced by resource supply but not by predation. We obtained this result with both approaches: PMRN and Stearns' size at maturation. Populations in higher resource environments, even with higher predation, tend to mature as larger and older individuals. The large increase in fecundity achieved by delaying maturation could provide an explanation for this. Populations living in resource-rich environments also clearly have a faster growth rate, as indicated by a larger average size of individuals at age one year. On the other hand, populations with higher predation have a significantly lower life span, as indicated by a lower proportion of fish older than one year.