Effects of fungivorous and predatory arthropods on nematodes and tardigrades in microcosms with coniferous forest soil

Effects of fungivorous and predatory soil arthropods on free-living nematodes and tardigrades were studied in a factorial microcosm experiment. A stepwise increase in faunal complexity was obtained by adding soil arthropods to defaunated humus samples from an irrigated+fertilized and an untreated stand of Scots pine. The effects were assessed after 103 and 201 days at 15°C and a soil moisture content of 50% water-holding capacity. The study showed that a diverse community of fungivorous arthropods (collembola and oribatid mites), present in numbers similar to those in the field, reduced the abundance of nematodes. A complete community of fungivorous and predatory arthropods (e.g., gamasides, spiders, and cantharid larvae) further strengthened this repressive effect. Certain nematode genera were more affected than others. Tardigrades seemed to be efficient predators on nematodes, but their numbers were, in turn, strongly reduced by predatory arthropods. Because predatory arthropods fed on both nematodes and their tardigrade predators, the impact of arthropod predators on nematode regulation was greater than it appeared to be on the basis of nematode numbers. Humus type also interacted with the other factors. Nematode numbers were initially higher in the untreated humus than in the irrigated+fertilized humus. However, because tardigrade populations increased only in the untreated humus, nematode numbers decreased more in this humus than in the irrigated+fertilized humus. The study demonstrates that nematode abundance can be regulated by a number of types of interacting predators.