Quantifying the contribution of community trait mean and diversity to
ecosystem functioning
Abstract
Hundreds of experiments conducted over the last decades demonstrate a
positive relationship between species diversity and ecosystem functions.
Following good experimental practice, most of these studies have
manipulated species richness artificially by assembling communities
randomly. Yet, natural communities along ecological gradients often show
niche-based responses to selection gradients and species extinction
order is generally not a random process. Such responses are commonly
trait-mediated and the effects of communities on ecosystems’ functions
also depend on species traits. In an effort to disentangle the
relationship of trait mean value and diversity with ecosystem
functioning, we revisited a community assembly mesocosm experiment that
simulated habitat heterogeneity and a typical gradient of productivity
to test how body size diversity and composition of cladocerans responded
to such gradients and whether and how such trait responses impacted
top-down control of unicellular algae, a key ecosystem function in
aquatic systems. Nutrient addition and habitat heterogeneity lead to an
increase in community average size (CAS), which ultimately resulted in
an increased zooplankton resource use efficiency
(RUEZP). This increase in CAS acted as a buffer for
ecosystems functions despite the decline in species richness under high
phosphorus levels. Habitat heterogeneity also increased size diversity
(SD) and species richness (S) and modified CAS and SD responses along
the nutrient gradient. CAS proved to be more powerful in explaining
variation in RUEZP than SD and S. Our results suggest
that exploring the response of the mean value of a key trait, such as
zooplankton body size, to eutrophication might offer a much better
understanding of variations in the top-down control of algae compared to
relying solely on taxonomic and functional diversity metrics.