Soil biodiversity and function: A historical perspective
It has been known for over half a century that interactions within the soil biota contribute to a suite of ecological processes both below- and above the ground. The first “functional” steps were taken in the late 1960’s when the International Biological Programme (IBP) launched research projects, mostly in Europe, focusing on energetic aspects of the detrital food web. Based on theoretical calculations, these studies concluded that the functional importance of a given decomposer group in decomposition processes stems from its activity and number of interactions with other groups, rather than its biomass or number of individuals. In particular, the fundamental role of the microbial-feeding soil fauna in belowground processes was acknowledged. Although studies on the resolution of biological complexity were coarse (mostly at the trophic group level) and little experimental evidence existed, the IBP era can be considered as a stepping stone to soil biodiversity-ecosystem function (BD-EF) research.
Not until the late 1970’s did soil ecologists start to unravel the role of soil biological complexity at higher resolutions. This was boosted by microcosm studies, mostly conducted in Europe, confirming the indisputable importance of the microbial feeding fauna in decomposition processes. The experimental evolution of microcosm techniques was rapid, allowing detailed manipulation of both the biological diversity of the decomposer biota and the complexity of the experimental environment. Results clearly showed that many of the soil processes as well as plant-soil interactions are regulated at the functional group or guild level, while little empirical evidence suggested the importance of species diversity in below-ground processes.
The absence of a universally clear species BD-EF relationship suggests relatively high functional redundancy among soil organisms, which may result from generalistic, even omnivorous feeding habits/resource use of the biota. It is also possible that high functional redundancy stems from macroecological rules/patterns, in which variation in dispersal ability and meta-community dynamics, and the degree of species-packing and community saturation among decomposers, set limits to redundancy.
Urban systems, typified by high habitat heterogeneity, highly disturbed soils and potentially impoverished soil communities, can be suitable to unravel factors causing functional redundancy. In theory, urban soil communities are unstable and, due to weak biotic interactions, unsaturated, which should display low functional redundancy. In such systems, dispersal between patches should be common, even at the regional scale. This, in turn, should make urban soils prone to alien invasion.