Ecological communities often contain genetically diverse species carrying various functional roles. One major cause of such divergence is competition between species for resources. The ecological character displacement theory suggests that when species require similar niches, competition may drive disruptive selection, causing species to diverge to utilize other available niches in the habitat. In contrast, competitors can also reduce the population size of a focal species, slowing down the emergence of novel mutations, promoting the efficiency of stabilizing selection, and resulting in convergence in the population. Mechanisms that determine the directionality of competitive effects on species divergence still remain unclear. We considered that the effect of competition on species divergence might differ at the genotypic and phenotypic levels.
To test this possibility, we performed a growth chamber experiment with aquatic plants. Microcosms were 240 ml open Mason jars with 200 ml duckweed growth medium. Our experiment focused on populations of the duckweed Spirodela polyrhiza with three genetically identified strains. We manipulated the competitive environment: S. polyrhiza grew alone, in competition with one other species (either Lemna minor, Myriophyllum verticillatum, or Salvinia sp.), or against all three. S. polyrhiza was allowed to evolve over 42 days (~15 generations).
Results/Conclusions
At the genotypic level, our results showed that competition reduced the genetic diversity of S. polyrhiza populations. Both genotypic richness and evenness of S. polyrhiza declined as the number of competitors increased. At the phenotypic level, however, we found competition promoted the functional trait diversity of S. polyrhiza clusters. We observed greater variations in both discrete (frond and root number per S. polyrhiza cluster) and continuous traits (leaf area and maximum root length per cluster) in the treatment with competitors than that without competitors. Trait variations did not differ between the treatments with one and three competitors. Competition can exert positive and negative effects on species divergence, through influencing focal species’ niche availability and population size, respectively. We, therefore, suggested that genetic diversity might respond more strongly to changes in population size, whereas trait diversity more strongly to changes in niche. Together, our study demonstrates the roles of competition for biodiversity at multiple levels.