A new synthesis of models of biodiversity-ecosystem-function relationships based on pairwise species interactions

Background/Question/Methods

The delivery of essential ecosystem functions (primary productivity, maintenance of soil fertility, resistance to weed invasion, purification of water etc) may be compromised by global declines in biodiversity. There is still controversy about the description of, and mechanisms behind, Biodiversity-Ecosystem Function (BEF) relationships. The Diversity-Interactions (DI) model introduced by Kirwan et al., (J Ecol, 2007,95:530-539, Ecology, 2009, 90:2032–2038 ) quantified ecosystem functional response in terms of identity effects of the species in a community, community density and of interactions between all pairs of species in the community.  The model proposed that the contribution of two species (i and j) to the functional response in a community is δ_ijP_iP_j, where δ_ij reflects the potential of the two species to contribute to the response and its actual contribution depends also on P_i and P_j, the relative abundance of the two species in the community.  A generalised version (GDI) introduces a more complex formulation of pairwise interaction as δ_ij(P_iP_j)^θ.  The GDI model reduces to the DI model when the value of θ = 1

Results/Conclusions

This model and parsimonious variants were fitted to biomass data from 4 grassland diversity experiments (2 natural and 2 agronomic), to respiration data from a microbial diversity experiment and to nitrate concentration in the soil in a worm diversity experiment. The DI model fitted well to functional responses from several, but not all, experimental datasets examined. The GDI fitted much better than the DI model in four datasets and in five out of the six experiments there was no need for higher order interactions.

The coefficient θ suggests new insight into the way species interact at low levels of relative abundance. The GDI model integrates many of the features of DI and other models of BEF relationships. The value of θ relates to the rate at which the diversity effect increases with species richness and particular values of this coefficient suggest, among other possibilities, a linear, a loglinear, a square root and a saturating form for this relationship.