Within and among species diversity as insurance against extreme drought in a coordinated distributed mesocosm experiment across Europe

Background/Question/Methods:

Biodiversity may insure stability of ecosystem functioning in the face of environmental change as predicted by the insurance hypothesis. This assumption is particularly important in times of climate change and the associated increase in the frequency of extreme events such as drought. Empirical evidence for biodiversity effects on functional stability in the face of extremes, however, is mixed and has up to now mainly been tested for species richness at single locations. Here, we used a mesocosm approach at four climatically different field sites across Europe and directly compared the insurance effect of increased species richness (number of grassland species from local species pools) with the insurance effect of increased within-species richness (number of ecotypes within a single species from various origins across Europe) in face of a severe drought (>72 days without rainfall versus control conditions). We quantified species-/ecotype-specific biomass production before the drought, directly after the drought, and at the end of the growing season, apparent mortality, weekly NDVI, and soil moisture. Mesocosms filled with local versus standardized soil and local versus standardized community compositions allowed for a separation of treatment effects to local climate, local soil, and plant community composition.

Results/Conclusions:

Biomass production before the drought was increased by species richness, but not by within-species richness. Effects, however, were site-specific with some sites showing a clear diversity effect while others did not. The insurance hypothesis was also supported only at specific sites and in specific response parameters with no apparent effect of increased biodiversity in NDVI but reduced mortality after the extreme drought with increasing within-species and among-species diversity at some sites. Further analyses will show if the first trend of these effects being caused by the differences in climate rather than the differences in soil substrates or species identities among the single sites will hold true. Our results confirm that within-species diversity can contribute to diversity effects similarly as species richness. Concerning the biodiversity-ecosystem function and biodiversity-stability debate, the coordinated distributed design of this experiment offers the opportunity to explore potential causes of the mixed results obtained.