Biological soil crusts as a model system in community and landscape ecology

Background/Question/Methods

Despite the fact that soils may be the greatest repository of biodiversity on Earth, and that myriad terrestrial ecosystem functions occur in the soil, research on the role of the community properties of soil biota in determining ecosystem multi-functionality has lagged behind corresponding research on aboveground organisms. Soil organisms pose special problems, including the fact that we usually do not know their identity nor role in soil ecosystems, we cannot easily estimate their biodiversity, and cannot culture the majority of the organisms for use in manipulative experiments. We propose here that biological soil crusts (biocrusts) of deserts and many other ecosystems may serve as a useful model system because –at least within some groups of biocrust biota- the species concept is relatively well-defined, their functional attributes are relatively well-known, and estimation and manipulation in experiments are feasible. Their diminutive size also contributes to their utility in an experimental setting. Using a biocrust model, we ask the question: what is the relative importance of biodiversity (a community ecology concept) compared to spatial patterning (a landscape ecology concept), in determining ecosystem multi-functionality? We review our recent work from both small and large scale observational studies, and experimental microcosms in the USA, Spain and Australia.

Results/Conclusions

We found that, even when total biocrust abundance and key environmental gradients were controlled for, direct and approximately linear relationships between species richness and/or evenness and indicators of ecosystem functioning are the norm in the field. Such relationships appear independent of region or spatial scale, but their strength seems to differ in every dataset. Further, a higher number of species appear to be required to sustain higher levels of multi-functionality, with a lower number needed to sustain any single function. These results are supported by experiments that find richness and species composition to be most responsible for determining multiple indices of nutrient cycling rates. Regarding spatial patterning, we find that spatial aggregation, spatial heterogeneity and patch size distribution characteristics may occasionally impact some individual ecosystem function indicators, but less so than biodiversity.