Submitted to: Ecological Society of America Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 7/1/2002
Publication Date: 8/4/2002
Citation: Wilsey, B., Polley, H.W. Effects of species evenness and richness on ecosystem process rates: first year results. Ecological Society of America Annual Meeting Abstracts. 2002. p. 299.
Technical Abstract: Species diversity contains two components: richness (number of species) and evenness (how well distributed abundance is among species). Previous experiments that varied species richness have been seeded with equal species abundances (maximum evenness). This has been hypothesized to overestimate richness effects. Furthermore, richness effects are hypothesized to be much smaller in systems without legumes. We planted equal-sized transplants from 1, 2, 4 and 8 species into 1 m2 field plots either in an even distribution of species (equal abundance, maximum evenness) or a geometric distribution of species (k=2, low evenness), which is a more realistic distribution of abundance in grasslands. With a design such as this, diversity is varied in a manner that removes the sampling effect (evenness) or allows the potential for a sampling effect (richness). Species were selected randomly from 13 C3 and C4 bunch-grass and forb species that are common in Texas grasslands, and each was grown in monocultures. After the first growing season, aboveground peak biomass did not differ between evenness treatments. Biomass increased 71% between 2 and 4 species and 90% between 2 and 8 species per plot (P<0.01) and this relationship was consistent between evenness treatments (P=0.97). Relative yield totals (RYT's) increased with species richness (2 sp.: 1.08, 4 sp.: 1.23, and 8 sp.: 1.64). However, RYT's were dominated by the RY of one productive grass species (mean RY=2.6). Richness effects were not significant in plots where this grass was absent, and productivity of mixtures was much smaller than the highest yielding monocultures (i.e. transgressive overyielding was not observed). These results suggest that 1) relationships between aboveground productivity and species richness may be robust to changes in species evenness, and 2) relationships are caused primarily by the sampling effect in developing communities.