Understanding earthworm – Collembola interactions and their importance for ecosystem processes needs consideration of species identity

Abstract

Soil animals and their interactions exert strong effects on ecosystem processes, such as leaf litter decomposition and nitrogen (N) cycling, thereby contributing to ecosystem functioning and stability. The understanding of how and why certain species interact is important to predict the effect of soil animal communities on ecosystem processes. Species interactions are discussed as being facilitative, antagonistic or neutral. We investigated interactions between two earthworm (Lumbricus terrestris, Aporrectodea caliginosa) and two Collembola species (Heteromurus nitidus and Protaphorura armata), representing major soil decomposer taxa. The two earthworm species are representatives of the soil macrofauna, with L. terrestris living in permanent vertical burrows and feeding on leaf litter, and A. caliginosa living in non-permanent horizontal burrows and feeding predominantly on resources in organo-mineral soil. The Collembola species are representatives of the soil mesofauna, with H. nitidus predominantly colonizing the soil litter interface and feeding on litter associated resources, whereas P. armata colonizes deeper soil layers and mainly feeds on resources in organo-mineral soil. Therefore, the species were assumed to differ in two major traits, i.e. body size (macrofauna vs. mesofauna) and food microhabitat association (“litter-associated” vs. “soil-associated”). Mesocosms with natural forest floor containing one beech (Fagus sylvatica) sapling were set up and incubated in the laboratory for three months. 15N labeled beech litter was added to follow the effect of detritivore animals on N cycling and N uptake by beech saplings. We hypothesized antagonistic interactions to dominate in species with similar body size or food microhabitat association via hampering the performance (biomass, abundance) of each other thereby reducing effects on leaf litter decomposition and N cycling. On the contrary, we expected species of different body size or food microhabitat association to facilitate each other's effects on ecosystem processes. In contrast to our expectation there was no clear relationship between similarity of body size or food microhabitat association and soil fauna effects on each other. Interactions between detritivores were not consensual with L. terrestris facilitating biomass gain of A. caliginosa, while its own biomass was reduced in presence of A. caliginosa. Additionally, 15N incorporation into A. caliginosa and the two Collembola species decreased in presence of L. terrestris, irrespective of similarity of body size or food microhabitat association. Leaf litter decomposition was increased by L. terrestris, while none of the other species affected litter decomposition. Generally, 15N incorporation into beech saplings was significantly increased in presence of L. terrestris or H. nitidus, but reduced in two species treatments due to antagonistic interactions. Interestingly, it was increased if L. terrestris, A. caliginosa and P. armata were present together, indicating facilitative interactions between these species. The results suggest that soil fauna interactions mainly vary with the identity of species and community composition rather than with similarity of traits. This highlights the complexity of soil fauna interactions and the difficulty to predict their effects on ecosystem processes, such as litter decomposition and N cycling, in species rich communities.