Response to Comment on “Worldwide evidence of a unimodal relationship between productivity and plant species richness”

Abstract

Laanisto and Hutchings claim that the local species pool is a more important predictor of local plant species richness than biomass and that when the species pool is considered, there is no hump-backed relationship between biomass and richness. However, we show that by calculating a more appropriate measure of species pool, community completeness, both regional and local processes shape local richness. Species diversity varies from habitat to habitat. Because species diversity has been related to ecological processes and ecosystem functions, it is important to predict whether species diversity may be high or low in any given environment (1). Our recent Report (2) provides evidence from data collected from grasslands in 19 countries and six continents of a unimodal relationship between herbaceous species richness and aboveground plant biomass plus plant litter, such that the maximum potential species richness occurs at intermediate levels of biomass production. We tackled the question of the diversity-productivity relationship because of its foundational role in ecological research and its controversial history (2-7). The data were collected at the 1-m 2 scale, within 8-m-by-8-m grids, so that we could test linear and quadratic regressions at 1, 2, 4, 9, 16, 25, and 64 m 2 scales. At each scale, the best descriptor of the relationship was a concave quadratic regression, but explanatory power diminished with increasing scale (2).We point to the relevance of our findings and the scaling approach in our analysis because it provides a comparative to Laanisto and Hutchings (8). Through a reanalysis of our data, Laanisto and Hutchings (8) argue that the local species pool has a stronger effect on species richness than primary productivity. We are in agreement that large-scale processes play a role in governing local-scale diversity, but it is challenging to account for large-scale processes in an empirical study such as ours. Thus, instead of addressing whether it is more reasonable to seek for a relationship between species richness and primary productivity, or local species pool, we would rather ask what the relative role of local and regional factors is in shaping the empirically observed relationship between diversity and productivity.Laanisto and Hutchings emphasize geographic variation in the shape of the response curve ("While the HBM [hump-backed model] certainly applies in some regions and habitats, such as high-latitude grasslands … richness-productivity relationships tend to be positive in tropical areas"). In fact, there are two confounding aspects in this statement: geographical region (with different biogeographic history) and ecosystem type (grassland versus forest). Positive relationships come predominantly from forests (9). We chose to avoid mixing different ecosystems because of the different processes that may be acting.In their figure 1, Laanisto and Hutchings's regression of local richness versus species pool is problematic because these two variables are inherently related, not independent, and ...

Plant distribution, stature, rarity, and diversity in a patterned calcareous fen: tests of geochemical and leaf‐height models

“…() claiming there was no global regularity in the plant diversity–productivity relationship, and Fraser et al. (, b) arguing instead that, if extremely productive and unproductive sites are included, grasslands show the hypothesized unimodal relationship. Our data support a simple decline of species richness with coverage (Fig.…”

Macroecology of biodiversity: disentangling local and regional effects

“…Similarly, in a global study of grasslands, Fraser et al . () found lower community completeness in high productivity habitats, where strong competition for light is expected to limit coexisting plant species.…”

Estimating probabilistic site‐specific species pools and dark diversity from co‐occurrence data

“…The species pool specific for a site includes two complementary parts: present species (observed diversity) and species from the region that are theoretically able to live in the site’s particular ecological conditions (dark diversity, Figure 1; Pärtel et al., 2011). Dark diversity provides a link between the site (affected by local dynamics) and the regional scale (more likely to be affected by large‐scale dynamics, such as immigration from other regions) (Fraser et al., 2015; Miličić et al., 2020; Moeslund et al., 2017; Trindade et al., 2020). Unlike present species, the size and composition of dark diversity are, by definition, unobservable and must be estimated.…”