Secondary production increases with species richness but decreases with species evenness of benthic invertebrates

Biodiversity is known to regulate ecosystem functioning under controlled experimental conditions. However, the 'real-world' consequences of biodiversity change remain uncertain, as biodiversity-ecosystem function (BEF) relationships observed in nature may be influenced by other drivers. Attempts to disentangle BEF relationships from the effects of confounding factors have so far focused mainly on primary producers, leaving comparatively little known about the impact of changes in consumer diversity despite ecosystems experiencing species extirpations and introductions across trophic levels. Using data from 176 benthic invertebrate assemblages distributed throughout the North Sea, we studied how a fundamental ecological function - secondary production - varies in relation to two components of biodiversity - consumer species richness and evenness - while statistically controlling the effects of abiotic and biotic covariates. Production was enhanced as richness increased or evenness decreased. The relationship with evenness was attributable to its negative covariance with the abundance of small organisms; however, the relationship with richness could not be fully explained by other drivers. Our study reaffirms experimental findings about the functional importance of biodiversity and suggests that losing or gaining consumer species will affect secondary production over a broad range of species richness (20-118 species) in natural ecosystems.