A scale-dependent framework for predicting losses and gains in ecosystem functions from shifts in biodiversity patterns

Background/Question/Methods

Recent syntheses of small-scale observational and experimental studies clearly show (despite variation) that biodiversity plays a key role in ecosystem functioning (EF) and ecosystem services (ES). These studies typically use species richness as the primary variable to describe biodiversity, treating richness as an independent variable that drives ecosystem patterns.  However, species richness is a poor descriptor because its values are highly sensitive to spatial scale.  Given this limitation in defining biodiversity, it is currently unclear how to ‘scale up’ biodiversity and EF/ES relationships to understand the value of preserving larger habitat areas.

We developed a framework to determine how changes in biodiversity patterns that underlie species richness translate into the change in EF/ES with increasing scale.  Using simulation models and varying biodiversity extinction scenarios, we explored links between richness and EF/ES as determined by scale, species abundance patterns (evenness and total abundance), the intra- and interspecific aggregation of species, and species trait distributions across a landscape.  We test this framework by partitioning the role of these factors in driving function-area relationships in tropical forest systems, particularly the Barro Colorado Island 50-ha forest plot in Panama.

Results/Conclusions

Our model investigated the gains and losses in simulated EF/ES across spatial scales in forested landscapes.  Results revealed that a single species-area relationship results in, not surprisingly, drastically different function-area scaling relationships based on the underlying biodiversity patterns.  For example, high intra-specific species aggregation caused non-linear dynamics in the scaling of EF when rare species were associated with functional traits that contributed disproportionately more to EF than common species.  However, these non-linear scaling effects were dampened, and changes in tree species evenness caused less variation in total EF across scales, when dominant species were associated with functional traits that contributed disproportionately more to EF.  We applied this model to partitioning the role of biodiversity patterns in function-area relationships in tropical forests.  For example, at Barro Colorado Island, ecosystem function, measured as total above ground biomass (AGB), scaled linearly with habitat area. Total abundance patterns, like tree basal area, overrode the effects of species inter- and intra-specific aggregation.  However, these relationships were not consistent when considering EF/ES that do not scale additively with each individual trees.