The fate of biological soil quality : Functional diversity and realised niche width

Abstract

The ecosystem services that humans obtain from the soil are strongly linked to the soil's biota. There is ample evidence that intensive agriculture has a negative effect on the soil's biological diversity. While in other ecosystems habitat specialists are at a higher risk of extinction due to human impacts than generalists, we have no evidence of whether this holds true for soil biota. We calculated the realised niche width for soil nematodes using co-occurrence data. We compared these with ecological traits. We then calculated an index of community specialisation and tested whether land use intensity leads to decreases in the index of community specialisation, taxon richness, diversity, and to changes in nematode abundance.The resulting realised niche widths did not correlate with ecological traits such as feeding group, body mass or c-p class. While it is possible that there are no relationships between these traits and the realised niche width, it is likelier that food availability, pH tolerance, or host breadth are more important factors in explaining niche width.Contrary to our expectations, the lowest community specialisation levels were found in soils with the lowest human intervention (shrubland-woodland ecosystems), while grasslands, dairy farms, and arable farms had an overall higher level of specialisation. Weather variables and land use intensity explained 66% of the variation in the index of community specialisation in sandy soils. We found highest richness and diversity at intermediate levels of disturbance (grasslands and dairy farms). The lowest abundances were found on shrubland-woodland systems. Dairy farms on sand and clay had similar indices of community specialisation, whereas peaty soils fostered a higher proportion of habitat specialists.We argue that farmland supposes a stable environment for organisms with shorter lifespans. Agricultural management strives to lower disturbances, allowing shorter lived organisms to escape pressures otherwise present in nature, such as drought or nutrient deficiencies during the growing season. In very disturbed systems, however specialists may also suffer from negative effects of land use intensity.This co-occurrence method to assess niche width opens the door to estimating the soil community's niche breadth, for which resource-based methods are difficult to implement.