Variation in environmental heterogeneity and ecosystem volume have little effect on the diversity-functioning relationship in a freshwater rock pool system
Over the last two decades, we have learned that changes in biodiversity generally affect a range of ecosystem processes and ecosystem services. However, our inference comes mainly on highly controlled experiments that often only vaguely mirror natural communities. Experimental units are often identical in scale (i.e. size or volume) and relatively homogenous in terms of their environmental conditions. To increase our understanding of how scale and environmental heterogeneity can affect the relationship between biodiversity and ecosystem processes, I deployed an outdoor experiment in 2013 using freshwater rock pools as model system. Experimental pools can be designed to mimic natural pools to a high degree, and the biodiversity, scale and characteristics of whole rock pool ‘ecosystems’ can be manipulated. In 42 outdoor pools, I manipulated the volume, bottom heterogeneity and initial diversity of five metazoan species (3 cladocerans, 1 copepod, 1 ostracod), and monitored the abundance and biomass of the metazoans over time. The volume of the pools varied from 7 to 103 liters, and combinations of four different benthic substrates were used to create a gradient in environmental heterogeneity.
Results from the first year of sampling the experiment show that the realized diversity of the metazoans is related to the abundance of the same focal metazoan species. The relationship varies slightly over time but is generally positive. Interestingly, spatial scale and heterogeneity interacts only weakly with diversity to affect animal abundance, and there is no evidence that the role of diversity strengthens over time. The lack of strong interactions between diversity, scale and heterogeneity indicates that the substrate heterogeneity is not a key factor in mediating the coexistence of the animals. Even though theory and some previous empirical studies suggest that the role of biodiversity is more important in environments with heterogeneous physical structure, compared to homogenous environments, this is not supported by the current rock pool study.