Main

Bai et al.1 present data on above-ground biomass of the total community and on various plant groups, but they provide no time series to indicate how diversity, productivity and stability might have changed during succession. The two communities compared at their site A represent two extreme degrees of disturbance: one undisturbed and the other heavily degraded. Although the undisturbed community did support relatively higher diversity and above-ground biomass than the highly degraded community, intermediate amounts of disturbance or transitional stages, which may well support even higher diversity and productivity, were not accounted for2,3,4,5,6 (Fig. 1).

Figure 1: Changes during succession.
figure 1

Model of temporal changes in diversity (species richness; green curve), productivity (blue), biomass (turquoise) and stability (red) during succession, based on an extensive literature review5,6. In the transitional stage, both early (short-lived) and late (long-lived) species coexist, leading to high species diversity. In this stage, biomass is relatively low and resource level is still high, promoting higher productivity. In the late stage, stability may be high but diversity and productivity may be low because of competitive exclusions7 and sequestration of limited resources by accumulated litter and biomass5,12.

Full size image

Over space (that is, within one habitat), the ‘intermediate disturbance’ hypothesis7 predicts that diversity will be highest at an intermediate level of disturbance2,3. Extensive evidence2,3,4,5,6,7,8,9,10,11,12 in support of this hypothesis is not in agreement with the conclusions of Bai et al.

Finally, over time at one locality, successional studies from various ecosystems — particularly those covering entire successional cycles — reveal that biodiversity and productivity are highest in the mid- or transitional-stage of succession, when both early- and late-stage species coexist. The high diversity and productivity then gradually decline owing to accumulated biomass and litter and therefore to increasing competition2,3,4,5,6,7,8,9,10,11,12 (Fig. 1).

Although each particular case would be expected to show some deviation from the general patterns in Fig. 1, because of the life history of dominant species, less destructive disturbance, or variation in resources available over time13, for example, it is not clear why the Inner Mongolia grassland should be so different1. If community stability, whose estimate depends on how it is measured, and biomass both increase with succession and are really high in undisturbed mature ecosystems, then the contrasting patterns between diversity and stability call their relationship into question.

The high stability in mature, or late-stage succession, grassland may be at least in part caused by the longer lifespan of the remaining, competitive perennial species (unlike annuals or short-lived plants in early succession) and by the high accumulated biomass, rather than by species diversity. For example, if community stability is measured as the coefficient of variation in biomass (CV, variance/mean)1, then the CV, which is not independent of mean biomass14, will be lower when the biomass is higher, so the stability should be higher. Long-term, simultaneous monitoring of these variables in both above- and below-ground communities over the entire successional cycles of the grassland would help to clarify this point.