Ecosystem Functioning

Ecosystem functioning reflects biotic effects on the physical and chemical properties of the environment, and an emerging theme has been to link this functioning to metacommunity properties. A common theme in research on ecosystem functioning is the diversity-productivity relationship. Nonspatial models generally predict a positive relationship between diversity and productivity due to increasing niche complementarity, with eventual deceleration due to niche overlap (Figure 2a). Though there is experimental evidence to support these predictions, there are also studies that show a neutral, unimodal, or negative relationship between diversity and productivity. Some of these conflicting results may be reconciled through a metacommunity framework that accounts for the spatial exchanges known to occur in real ecosystems.

Varying rates of dispersal between patches may explain a unimodal diversity-ecosystem functioning relationship at local scales and a positive linear relationship at regional scales. At the local level, diversity may increase productivity because of niche complementarity or sampling effects. This relationship may peak and become negative, however, if dispersal allows inferior competitors to interfere with superior competitors through source-sink dynamics (Figure 2b). At larger scales, increased diversity may allow species to coexist and complement each other regionally, increasing productivity linearly so long as the number ofspecies does not exceed the number of limiting resources. A study examining productivity at the pond (local) and watershed (regional) scale found this scale-dependent relationship between diversity and productivity in nature, evidenced by low local diversity and increasing species dissimilarity between ponds in watersheds with higher productivity.

A form of spatial insurance could also result in a scale-dependent diversity-productivity relationship. If the

Figure 2 Theoretical relationships between productivity and species diversity. (a) Diversity as a result of niche complementarity or sampling effects. (b) Diversity as a result of source-sink dynamics.

dynamics of local communities and environments fluctuate asynchronously, dispersal will supply immigrants to localities where the environment has changed to be unsuitable for the current residents. Increasing regional diversity will increase the likelihood that there will be species appropriate for the various environmental conditions, which will enhance resource-use efficiency and productivity. When levels of dispersal are intermediate, supplying enough immigrants to fill vacancies without leading to dominance by a species with intermediate traits, the temporal mean of productivity across the region will be higher and exhibit lower variability. When the system involves more than one trophic level, consumers that track resources can increase variability in productivity at a local scale, but decrease variability at the metacommunity scale by creating a heterogeneous environment that hosts a more diverse community capable of spatial insurance.

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