American Society of Naturalists

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“Larger area facilitates richness-function effects in experimental microcosms”

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John P. DeLong and Jean P. Gibert (May 2019)

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Biodiversity ecosystem function and species area curves are linked in experimental island communities

Microcosm sizes.<br />(Credit: John P. DeLong)
Microcosm sizes.
(Credit: John P. DeLong)

Patterns of species diversity, abundance, and their ability to turn over energy and matter (or “function”) at the level of collections of species and their interactions – i.e., ecological communities – are generally viewed as separate properties that require separate explanations. This is strange because the underlying processes determining which individuals of which species conducting which ecological functions in any community must drive the aggregate properties however they are measured. For example, the increase in the number of species with an increase in the area sampled (or a species-area (SAR) curve) or the increase in ecosystem function with an increase in the number of species (or a biodiversity-ecosystem function (BEF) relationship), are central patterns in ecology and are generally treated as separate phenomena. However, the processes determining how many species occur in a place and how much they can do (eat and digest things, turn around energy and matter, etc.) must, at some level, be related. John DeLong and Jean-Philippe Gibert of the University of Nebraska–Lincoln show that these two patterns are indeed linked and together likely emerge from the same underlying processes.

The authors constructed experimental microcosms (i.e., tiny constructs that simulate the conditions of freshwater ecosystems in the lab) of protists across a range of Petri dish sizes and measured species richness, abundance, functional diversity, and functioning (carbon and nitrogen content, total biovolume, and oxygen consumption). They found an increase in species richness and oxygen consumption with increasing dish area, indicating that SARs and BEF patterns occur together. Digging into the abundance and functional diversity data, the authors found that the most likely source of both patterns was diversification of niche use (e.g., food types, places to hang out, species they interact with) in the larger dishes that allowed rarer species not to go extinct as often as they do in smaller dishes. The authors also found evidence that higher nutrient turnover – possibly through the action of bacteria – may have contributed to the higher richness and function in larger dishes.


Species-area (SAR) and biodiversity-ecosystem function (BEF) relationships are central patterns in community ecology. Although research on both patterns often invokes mechanisms of community assembly, both SARs and BEFs are generally treated as separate phenomenon. Here we link the two by creating an experimental SAR in microcosm communities and show that greater species richness in larger areas is accompanied by greater ecosystem function. We then explore mechanisms of community assembly by determining whether rare, large, or high biomass species are more likely to persist in the larger microcosms. Our results indicate that larger areas harbor more rare species of a wider range of body sizes and have higher functional diversity, implying that the addition of niche space that supports rare species underlies the effect of area on species richness and function. Our results suggest that the preservation of large areas is a potentially useful way of maximizing the provisioning of ecosystem services through the maintenance of biodiversity.