Experimental test of predation's effect on divergent selection during character displacement in sticklebacks

Abstract

Ecological character displacement is common in nature but the mechanisms causing divergence are not well understood. The contributions of ecological interactions other than competition have received little attention. We conducted a pond experiment to explore the contribution of both competition and predation to character divergence in threespine stickleback species. We estimated the strength of divergent selection on a morphologically intermediate target population between competition treatments under two alternate predation treatments. Divergent selection on the target population tended to be stronger in the predator-addition treatment than in the predator-reduction treatment, a difference that approached significance (P = 0.09). This trend occurred even though competition was strongest in the predator-reduction treatment. Overall, the strength of divergent selection was best predicted by stickleback mortality (P = 0.025) being strongest where mortality was highest. These results indicate that predation and other agents of mortality can enhance the rate of change in competition per unit of phenotypic divergence and, thereby, divergent selection, even as they lower the overall strength of competition. In this way, predation and other agents of mortality may facilitate, rather than hinder, character displacement.

Fig. 1.

Growth of target individuals as a function of gill raker number when in the presence of benthic (•, solid line) or marine (+, dashed line) sticklebacks. Individuals with higher gill raker counts are more similar to limnetic and marine sticklebacks in their trophic morphology, whereas individuals with lower counts are more benthic-like. For clarity, gill raker counts of individuals in the two competition treatments are displaced slightly.

Fig. 2.

Strength of divergent selection in relation to stickleback mortality, calculated as one minus the survival of all sticklebacks (number retrieved/number introduced) from a whole pond. Number labels identify individual ponds. □, predator-reduction treatments; ▪, predator-addition treatments.

Fig. 3.

Strength of divergent selection in each pond in relation to the overall strength of resource competition, which was calculated as the inverse of the mean final standard length (mm) of all sticklebacks retrieved from a pond. Greater positive values indicate stronger competition. Pond labels are as in Fig. 2.