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

Prevailing theories of biotic diversification incorporate resource competit ion as a leading cause of divergence between new species. In support of thi s, 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 s pp.), we tested the prediction that competition between species should decl ine as character divergence proceeds, yielding descendants whose present-da y 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 di splacement series that began at the end of the last ice age when marine sti cklebacks colonized lakes containing an earlier descendant. Growth rate and niche specialization of marine stickle-backs were higher in the 'post-disp lacement' 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 stickl ebacks, with reduced competition the outcome. The influence of other intera ctions on divergence between sympatric species may be tested with analogous experimental designs.