Submitted to: Ecological Society of America Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 5/11/2004
Publication Date: 10/29/2004
Citation: Polley, H.W., Wilsey, B.J., Tischler, C.R. 2004. Is the outcome of species interactions sensitive to species relative abundances?. Ecological Society of America Abstracts. p. 403.
Technical Abstract: Species relative abundances usually are not rigorously controlled in experiments to test relationships between plant production and species richness. What effect, if any, uncontrolled variation in relative abundances has on richness experiments depends on how sensitive species interactions are to differences in relative abundances. We grew perennial grasses and forbs that co-occur in grasslands in central Texas in field plots (1 m X 1 m) as equal-density monocultures and two-species mixtures. Evenness (1:1 or 3:1 ratio of plants by species) and the identity of the dominant species in 3:1 assemblages was varied in replicated mixtures. After one growing season, we calculated the relative yields of species in mixtures to determine the sensitivity of the net biodiversity effect [NBE; Loreau and Hector (2001)] to differences in species relative abundances. In only one of six species combinations, a grass/grass assemblage, was aboveground biomass greater in mixtures than expected from monocultures (NBE > 0). The NBE did not differ from zero in a second grass/grass combination or in forb/forb or forb/grass assemblages. The NBE varied significantly with species abundances in two of six species combinations when calculated using aboveground biomass and in one of the two grass/forb combinations in which roots could be distinguished by species and total biomass was used in the calculation. In all combinations in which the NBE varied with either species evenness or identity of the dominant species, the variation was caused by a change in the mean of species relative yields, i.e. the complementarity effect. Our results illustrate that the outcome of interactions between some pairs of species varies with species abundances. If the same is true of some multi-species mixtures, richness experiments may be affected by a previously-unrecognized sampling effect for species combinations in which the expression of interactions is sensitive to relative abundances.