Evolution due to competitive interactions is generally considered between pairs of species, abstracted from any larger species assemblage or community. How evolution among competitors may differ in a community context is less clear and may be dependent on the symmetry of species interactions. The ratio of intra- to interspecific competition may decrease with the degree of competitive dominance between species, such that good competitors experience stronger selection for intra- rather than interspecific competitive abilities and this effect may be magnified in less vs. more diverse communities. These questions were tested using the protozoa that occur in the water filled leaves of the pitcher plant, Sarracenia purpurea. Competitive abilities and traits of four different protozoa species were measured before and after >200 generations of selection for growth in different species combination, including monocultures, pairwise mixtures, and a 4-species mixture. All initial and final lines were grown over a gradient of nutrient availability to determine minimum resource levels at which species can maintain growth. Simultaneously, microbial abundances were determined with flow cytometry and relative abundances were quantified using 16S rRNA gene sequencing.
Results/Conclusions
These species demonstrate a strong hierarchy of competitive effect and response. A Tetrahymena species both strongly suppresses other species and is the least affected by other species, while at the other end, Colpoda steinii has the least effect on and is the most suppressed by the other three species. Competitive asymmetry would predict that dominant species should evolve to be better at intraspecific competition, perhaps at the cost of interspecific competitive ability, while poor competitors should do the converse. Dominant competitors were found to evolve higher cell densities in monoculture if selected in monoculture, but show no evolution of improved growth in 4-species mixtures. Subordinate species selected in 4-species mixtures have higher growth under all other conditions, including mixtures and monocultures. These changes are associated with increases in the rate of bacterial cell consumption, but do not appear to be correlated with any changes in the composition of the microbial communities. Nutrient experiments find dominant competitors require high nutrient levels. While minimum resource levels do evolve, they do not appear to convey increased competitive abilities. These results suggest that competitive abilities evolve through changes in rates of resource uptake, rather than tolerance to low resource levels or niche partitioning.