What is the mechanism of submerged macrophyte biodiversity affecting biomass productivity along water depth: Niche complementarity or selection effect?

Abstract

The impact of biodiversity on ecosystem functions and services remains a hot topic in ecology, especially under increasing biodiversity loss. Studies on the relationship between biodiversity and ecosystem functioning have identified two regulatory mechanisms: i.e., “niche complementarity” and “selection effect”. However, the relative importance of these mechanisms within communities and how they change along ecological gradients remain unclear. Moreover, most studies have focused on relatively stable ecosystems (e.g., forest, grassland), with limited attention paid to ecosystems with obvious seasonal changes, e.g., freshwater ecosystems. In this study we conducted a seasonal survey of submerged macrophyte communities to clarify the mechanisms of biodiversity on biomass productivity accounting for seasons and water depth. Our results showed that (1) seasonal variations of community indicators exhibited different trends along the water depth gradient. Biomass productivity showed high seasonal variation at intermediate depth while temporal beta diversity showed the opposite pattern. (2) Water depth not only inhibited biomass productivity directly but also indirectly by reducing species richness and enhancing temporal beta diversity. (3) The positive effect of species richness on biomass productivity was lower than the negative effect of evenness (path coefficients: 0.13 vs. 0.27), indicating that “selection effect” played a more important role in the relationship. (4) In shallow areas, the mechanism underlying the effects of biodiversity on biomass productivity was more from “selection effect”, whereas, in deep areas, the effects were related to both “niche complementarity” and “selection effect”. Our findings suggest that “niche complementarity” and “selection effect” are two complementary mechanisms underpinning the relationship between biodiversity and biomass productivity, and their relative importance varies with specific ecological gradients. Our results also provide a reference for studies on other freshwater organisms with analogous distribution patterns as submerged macrophytes with water depth. Scale dependence of the above relationship should be considered in future studies.