Emergent relationships between the functional diversity of marine zooplankton and ecosystem functioning in the global ocean

Copepods are a major group of the mesozooplankton and thus a key part of marine ecosystems worldwide. Their fitness and life strategies are determined by their functional traits which allow different species to exploit various ecological niches. The range of functional traits expressed in a community define its functional diversity (FD), which can be used to investigate how communities utilize resources and shape ecosystem processes. However, the spatial patterns of copepod FD and their relation to ecosystem functioning remain poorly understood on a global scale. Here, we use estimates of copepod community composition derived from species distribution models in combination with functional traits and indicators of ecosystem functioning to investigate the distribution of multiple facets of copepod FD, their relationships with species richness and ecosystem processes. We also project how anthropogenic climate change will impact the facets of copepod FD. We find that the facets of FD respond to species richness with variable strength and directions: functional richness, divergence and dispersion increase with species richness whereas functional evenness and trait dissimilarity decrease. We find that primary production, mesozooplankton biomass and carbon export efficiency decrease with species richness, functional richness, divergence and dispersion. This suggests that ecosystem functioning may be disproportionally influenced by the traits of a few dominant species in line with the mass-ratio hypothesis. Furthermore, climate change is projected to promote trait homogenization globally, which may decrease mesozooplankton biomass and carbon export efficiency globally. The emergent covariance patterns between FD and ecosystem functions we find here strongly call for better integrating FD measurements into field studies and across scales to understand the effects of changing zooplankton biodiversity on marine ecosystem functioning.