Short-term studies of the relationship between diversity and productivity have found that biomass production levels saturate in response to species richness. They suggest that losing one or a few species at high diversity levels would have minimal effects on productivity. However, these studies do not account for important temporal effects only observable after long periods of time. To examine the long-term relationship between diversity and productivity, Reich et al. (2012) compared two experiments on grassland biodiversity that lasted over 13 years. Their results indicate that the relationship between diversity and productivity increases at a positive rate due to distinct niches in resource acquisition and positive diversity effects on nitrogen in the ecosystem. They also found that changes in diversity have larger long-term than short-term effects, suggesting that losing even a few species in diverse communities may eventually lead to decreased biomass production.–Katie Huang
Reich, P.B., Tilman, D., Isbell, F., Mueller, K., Hobbie, S.E., Flynn, D.F.B., Eisenhauer, N., 2012. Impacts of biodiversity loss escale through time as redundancy fades. Science 336, 589–592.
Reich et al.presented data from two independent experiments on grassland biodiversity that shared overlapping but non-identical species pools consisting of various grasses and herbs. The species richness in both ranged from one to 16 species, covering the typical range found in disturbed to native grassland. To analyze the experimental results, the authors used statistical analyses to compare the biomass yields in both grasslands with two types of functions, saturating and decelerating, to see which was the more appropriate model. They also directly compared species-richness treatments in order to analyze the effect of richness on diversity at different times during the study. In their analyses, they factored in potential result-influencing conditions such as differences in aboveground and belowground biomass and the proportions of nitrogen-fixing legumes present in the ecosystem.
In their statistical analyses, the authors found that the saturating function, which was found to be appropriate in short-term studies of diversity and productivity, was not as effective at modeling data collected in later years as the decelerating function was. They also found that the diversity-productivity relationship became more linear and less strongly decelerating as time went by and that estimates of the number of species required to generate most of the diversity effect on biomass increased over time. Reich et al. suggest that these results can be explained by different species acquiring distinct resource acquisition niches over time, possibly driven by increased functional diversity or the replacement of similar species with dissimilar ones. Diverse plots were able to accumulate more nitrogen than less rich ones, allowing more nutrients to recycle through the ecosystem with the decomposition of plant litter and soil organic matter. As a result, greater levels of nitrogen were available to the entire ecosystem over time. In both experiments, species-rich plots had greater nitrogen stocks, uptake, and/or supply rates in comparison with species-poor plots, and the additional resources helped boost biomass production in later years. The results suggest that short-term experiments, which do not account for these factors, are limited in their ability to predict long-term effects of diversity on biomass production.
In their species-richness treatment comparisons, Reich et al. observed that in the first four years of both experiments, increasing richness from one to four species led to strong increases in biomass while increasing richness from medium to high levels did not produce any significant effects. However, as time went by, the effect of diversity on biomass remained stable for plots that did not increase very much in species richness, while it increased for plots changing from low to high diversity. The latter effect was observed because at higher species-richness levels, all species contribute to increasing biomass over time. Thus, losing even one or a few species in high diversity ecosystems may lead to decreases in productivity, a different result than what was seen in short-term studies.