H. Olff

Grasslands are often characterised by small‐scale mosaics in plant community composition that contribute to their diversity. Although above‐ and belowground biota can both cause such mosaics, few studies have addressed their interacting effects. We studied multi‐trophic interactions between aboveground vertebrate grazers, subterranean ants, plant‐pathogenic soil biota (especially nematodes) and the vegetation in a temperate grassland. We found that when rabbits and cattle locally omit vegetation patches, yellow ants ( Lasius flavus ) respond to the taller vegetation by digging up more sand from deeper soil layers (hence making taller nest mounds), probably to maintain sufficiently high soil temperatures. We found that this ant digging affects other soil biota, as the mounds contain fewer plant‐parasitic and fungivore nematodes. Also, the mounds have lower moisture content and soil bulk densities, and higher pH and available nutrient content than the directly surrounding soil. The clonal sedge Carex arenaria grows vigorously on the mounds, producing more shoots and shorter rhizome internode lengths than in surrounding vegetation. Other plant species, such as the grass Festuca rubra , dominate the surrounding vegetation. A greenhouse bioassay experiment revealed that harmful soil organisms (as plant‐parasitic nematodes and pathogenic fungi) outweighed the effect of beneficial organisms (e.g., mycorrhizae) in this system. Rhizome biomass and shoot production of C. arenaria were indeed inhibited less by biota in soil from ant mounds than by biota in soil from the surrounding vegetation . However, the total biomass production of C. arenaria was inhibited as strongly in both soil types. F. rubra was inhibited more strongly by biota in the surrounding soil. We suggest that various direct and indirect interactions between above‐ and belowground biota can contribute to community mosaics and hence diversity in grasslands.

Free-ranging large grazers, such as cattle and horses, are increasingly reintroduced to former agricultural areas in Western Europe in order to restore natural and diverse habitats. In thi...