Species richness and biomass-how are they related?

Submitted by editor on 13 September 2016.Get the paper!

Figure 1: More diverse assemblages of herbivores, like these algal grazers living on seagrass, produce more grazer biomass.

Hundreds of experiments have shown that changes in species richness affect the biomass of a species assemblage. Have they converged on a single quantitative relationship, or have they simply shown that diversity matters, but its effects depend on context? Our analyses indicate that there is convergence, at least within trophic levels, suggesting that a common, empirically-derived scaling relationship between biodiversity and ecosystem functioning may exist that could be applied to predict consequences of changing biodiversity. 

As a first step toward answering this question, we used a hierarchical mixed effects modeling approach to re-analyze a database of hundreds of experiments that manipulated species richness and measured standing biomass. We found that for nearly all tests, a relationship of Y = a x Sb describes the change in biomass (Y) with a change in species richness (S) for plants and algae, but that different exponents (b values) are required for herbivores and detritivores (Figure 2). Thus, the empirical evidence supports a repeatable relationship between diversity and function that is significant, and does not vary systematically with ecosystem type, experimental design, level of species richness tested, or other potential drivers. 

Figure 2. (Panel 1C from the main manuscript). Power functions plotted with b values shown in panel B for primary producers, aquatic herbivores and aquatic detritivores.

The finding that the exponent of the power function (b), sometimes called a scaling exponent, has a common value for most studies within a trophic level, highlights one aspect of biodiversity-ecosystem functioning relationships relevant to understanding biodiversity change. Specifically, for a positive value of b that is less than one, this relationship, combined with Jensen’s inequality theorem, implies that species losses have greater consequences for changing biomass than do species gains of the same magnitude (Figure 3). Thus, even if observations of change in species richness include increases and decreases over time, but are centered on zero, we expect a mean decline in biomass due to the general diversity-function relationships reported here.

Figure 3: Expected change in biomass associated with changes in species richness. Distribution of species richness changes (top histogram), expressed as a response ratio (ln(STime1/STime2)), and the distribution of associated change in biomass (vertical histogram), expressed as ln(YTime1/YTime0) expected for primary producers. The distribution of expected function was produced using Y = aSb (the plotted curve) for values of b = 0.26. Solid blue lines indicate response ratios of 1 = no change in richness; and the red lines indicate the mean expected function. Dashed lines identify a 10% decline in standing biomass, and the intersection with the BEF curve identifies the change in plant richness (35%) expected to cause a 10% change in biomass.

Our analysis of the richness-biomass relationship can allow practitioners to apply an empirically-derived, a priori prediction for the species richness-biomass scaling relationship as a quantitative estimate for the importance of a change in biomass potentially resulting from a change in richness. This estimate provides a starting point for applications of the experimental evidence for a relationship between diversity and function to broader scenarios of biodiversity change.

The authors through Mary O'Connor

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