Worldwide evidence of a unimodal relationship between productivity and plant species richness

Abstract

One Sentence Summary: Empirical evidence from grasslands around the world demonstrates a humped-back relationship between plant species richness and biomass at the 1 m 2 plot scale.Abstract: One of the central problems of ecology is the prediction of species diversity. The humped-back model (HBM) suggests that plant diversity is highest at intermediate levels of productivity; at low productivity few species can tolerate the environmental stresses and at high productivity a small number of highly competitive species dominate. A recent study claims to have comprehensively refuted the HBM. Here we show, using the largest, most geographically diverse dataset ever compiled and specifically built for testing this model that if the conditions are met, namely a wide range in biomass at the 1 m 2 plot level and the inclusion of plant litter, the relationship between plant biomass and species richness is hump shaped, supporting the HBM. Our findings shed new light on the prediction of plant diversity in grasslands, which is crucial for supporting management practices for effective conservation of biodiversity. 4Main Text: The relationship between plant diversity and productivity is a topic of intense debate (1-6). The HBM states that plant species richness peaks at intermediate productivity, taking above-ground biomass as a proxy for annual net primary productivity (ANPP) (7-9). This diversity peak is driven by two opposing processes; in unproductive and disturbed ecosystems where there is low plant biomass, species richness is limited by either stress, such as insufficient water and mineral nutrients, or high levels of disturbance-induced removal of biomass, which few species are able to tolerate. In contrast, in the low disturbance and productive conditions that generate high plant biomass it is competitive exclusion by a small number of highly competitive species that is hypothesized to constrain species richness (7-9). Other mechanisms proposed to explain the unimodal relationship between species richness and productivity include disturbance (10), evolutionary history and dispersal limitation (11,12), and density limitation affected by plant size (13).Different case studies have supported or rejected the HBM, and three separate meta-analyses reached different conclusions (14). This inconsistency may indicate a lack of generality of the HBM, or it may reflect a sensitivity to study characteristics including the type(s) of plant communities considered, the taxonomic scope, the length of the gradient sampled, the spatial grain and extent of analyses (14,15), and the particular measure of net primary productivity (16). Although others would argue (6), we maintain that the question remains whether the HBM serves as a useful and general model for grassland ecosystem theory and management. 5 We quantified the form and strength of the richness-productivity relationship using novel data from a globally-coordinated (17), distributed, scale-standardized and consistently designed survey, in which plant richness and biomass were m...

Interactive effects between plant functional types and soil factors on tundra species diversity and community composition

“…Aboveground biomass has been widely used as a proxy of productivity (Chiarucci, Wilson, Anderson, & De Dominicis, 1999; Fraser et al., 2015). In tundra, aboveground biomass and production are highly correlated (Webber, 1978).…”

Consistent drivers of plant biodiversity across managed ecosystems

“…Indeed, there are various ecological drivers of plant species richness that cannot be accounted for using bivariate models alone, and this has been recognized by many studies [14,16,[51][52][53][54]. Our study used structural equation modelling to compare three habitat types managed by grazing and mowing to test an integrated causal hypothesis from management via total plant biomass and productivity to observed species richness.…”

“…At sites with high total biomass, species co-occurrence should be limited [46,47], hence we expected a negative relationship between total biomass and observed species richness. We formulated no initial hypothesis for the relationship between productivity and observed species richness, but instead present opposing assumptions: (i) the relationship should be negative: high above-ground biomass allocation in productive communities leads to a competition for light, excluding competitively subordinate species [48,49]; (ii) the relationship should be significant, but without pre-formulated direction, as, following the hump-shaped model, the relationship between productivity and species richness should be highest at intermediate levels of productivity [4,14,50]; (iii) there should be no relationship between productivity and observed species richness, following Adler et al's [15] conclusion of 'weak and variable' productivity-richness relationships [15].…”

The influence of balanced and imbalanced resource supply on biodiversity–functioning relationship across ecosystems

“…This effect is attributed to more complete resource use in communities with a higher number of competing species [4,5] or to a greater chance of including a highly productive species in a more diverse community [6]. The influence of productivity on diversity, on the other hand, has a long history of debate in ecology, in particular regarding the general presence or absence of hump-shaped patterns of biodiversity across gradients of productivity [7][8][9][10][11][12].…”

“…Concerning (ii), the evidence that biodiversity not only responds to potential productivity but also influences realized productivity [13,14] negates the relevance of simple bivariate analyses, although they are still commonly used in ecology [8,9]. Instead, multivariate frameworks with resource availability (potential productivity), biodiversity and realized productivity as causally connected components promise greater mechanistic insights regarding biodiversity-productivity relationships.…”