University of Pittsburgh, Pennsylvania, United States
Abstract: Evolution during competition can help or hinder coexistence among competing species, but field-based experimental tests of this idea remain rare. It is possible that different outcomes could be driven by differences in evolutionary potential in the determinants of species coexistence. To address this, we conducted a series of field mesocosm experiments using multiple allopatric and sympatric populations of two species of duckweed. Competition experiments, at the population and genotype-within-population level, manipulated intraspecific and interspecific competitor densities. Analysis of variance revealed differences in the demographic and competitive parameters (growth rate when alone, intra- and interspecific competition coefficients) in sympatric and allopatric populations and among geographic sites. For example, preliminary analyses indicated that growth rates (lambda) were significantly higher for genotypes from sympatric compared to allopatric populations (F1 = 16.05, p = 0.0004) for one species. Evolutionary potential for growth rate, assessed by the coefficient of variation, was also significantly larger in sympatric populations (p = 0.0062). Then by integrating these parameters, we found that niche differences among genotypes varied across geographic locations (F3 = 10.16, p < 0.0001), but whether niche differences were larger for sympatric or allopatric populations depended upon geographic location (F3 = 7.23, p = 0.0004). Thus, we observed that predicted stable coexistence varies among populations. Such variation is expected to impact evolution and may influence coexistence.