Editor’s Choice: Elegance in simplicity – a useful new way to quantify plant diversity dynamics

The Editor’s Choice article for Volume 107 Issue 5 is a study by Zhang et al., which outlines their new SRUD approach for measuring plant diversity dynamics. Associate Editor, Hans Cornelissen, discusses the importance of this new method.

Many thousands of researchers have been concerned with what drives and threatens biodiversity. They have investigated whether biodiversity provides benefits to ecosystem functions & people and have explored how it does so (Cardinale et al. 2012). No doubt, many more thousands will continue this field of research, so long as humankind hasn’t wiped out its very study object (and thereby itself).

It is therefore remarkable that we are still not very good at describing biodiversity within a given section of vegetation. Long gone are the days that species richness alone (i.e. the number of species found) was a sufficient indicator of biodiversity – even though it can still play an important role.  More advanced measures of biodiversity consider the relative abundance of the different species in a vegetation plot. A classic example of this kind is the Shannon-Wiener index, which expresses the relative “evenness” of species abundance. The rationale for these indices is, firstly, that greater evenness is inherently richer, more “beautiful” and more “desirable” (certainly to the nature conservation-minded). Secondly, more abundant species tend to play a proportionally larger role in key ecosystem functions (mass-ratio effect: Grime 1998). This impacts not only productivity, as the most obvious function, but also other functions related to carbon, nutrient and water cycling. For instance, decomposition rates, soil water retention capacity and fire regime. So far so good. But the incredible thing is that, regardless of these indices of diversity, we still do not really know how to best express relative abundance. Some count the number of individuals, others measure or estimate percentage cover of the ground surface in vertical projection and others determine (aboveground or total) biomass. The latter is hard to get without damaging the vegetation, unless mowing residues are used. Each of these methods have drawbacks in what they signify and how exactly to measure them. Competition is one of the key mechanisms determining diversity at the plot scale, but do any of the current abundance indicators get to the crux of how competition really works? Asking the question is answering it: no.

(a) Space resource utilization (SRU) = product of the maximum species height, species cover (%) & plot area. (b) Conceptual abundance curve (for cover/biomass) for all species in the community. (c) Conceptual graph illustrating relationship between changes in SRU/biomass & changes in plant species richness, in response to human disturbance. See Figure 1 in Zhang et al. (2019) for the full legend.

In this issue’s Editor’s Choice paper, Zhang et al. come to the rescue, by elaborating on an index that expresses the relative abundance of a species in the community, as the outcome of competition: Species Resource Utilization (SRU). In a previous paper Zhang et al. (2015) introduced SRU generally, as the simple product of plant average maximum height, percentage cover and plot area. The idea behind this is that aboveground competition is all about fighting for light and physical space (Campbell et al. 1992); and SRU, being 3-dimensionsonal, reflects just that. However, simply combining the SRU values of all species in a plot still doesn’t provide an index that best expresses the outcome of competition and its consequences in terms of diversity.

In this new paper, Zhang et al. demonstrate that quantifying the summed SRUs of the dominants in a community only (SRUD) is a good negative predictor of species richness in grasslands, across various parts of the world. It does a better job at this prediction than community level SRU or biomass (per unit area) and is also better than measuring the biomass of the dominants only. In other words, how much physical space the dominant species in a community occupies, both in vertical and horizontal dimension, gives the best expression of their aboveground competitive strength and the best (negative) prediction of species richness among the measures available. Given the ease at which SRUD can be quantified in the field by eye with only a ruler (with or without a point-quadrating device), without having to harvest biomass and without having to recognise each rare species by its leaves only, this new approach is elegant in its simplicity. I think SRUD will turn out to be a useful and popular tool for vegetation surveyors and biodiversity researchers in many parts of the world. It may also turn out to be a good predictor of ecosystem functions other than species richness support. For instance, fire regimes – as fire behaviour depends not only on fuel mass but also on the 3-dimensional structure (size, shape, density) of the fuel.

Pity we do not yet have a matching easy-to-measure SRUD to represent belowground competition and diversity dynamics, as plants obviously do not only fight for resources and space aboveground…This could be a next holy grail in biodiversity research.

Hans Cornelissen, Associate Editor, Journal of Ecology

Read the full article online: “SRUD: A simple non‐destructive method for accurate quantification of plant diversity dynamics” by Zhang et al. (2019)

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