Declining interspecific competition during character displacement: Summoning the ghost of competition past

Abstract

Prevailing theories of biotic diversification incorporate resource competition as a leading cause of divergence between new species. In support of this, many cases of divergent character displacement between close relatives (congeners) are known. Yet, experimental tests of underlying mechanisms are uncommon. In a pond experiment with threespine sticklebacks (Gasterosteus spp.), we tested the prediction that competition between species should decline as character divergence proceeds, yielding descendants whose present-day interaction is a ‘ghost’ of its former strength. Competition’s impact on the marine threespine stickleback (G. aculeatus) was contrasted between two treatments simulating early and late stages of a hypothesized character displacement series that began at the end of the last ice age when marine sticklebacks colonized lakes containing an earlier descendant. Growth rate and niche specialization of marine sticklebacks were higher in the ‘post-displacement’ treatment than in the ‘pre-displacement’ treatment, suggesting a decline in competition strength through time. The result supports the idea that interspecific competition favoured divergence between sympatric sticklebacks, with reduced competition the outcome. The influence of other interactions on divergence between sympatric species may be tested with analogous experimental designs.