Ecological communities consist of small abundant and large non-abundant species. The energetic equivalence rule is an often-observed pattern that could be explained by equal energy usage among abundant small organisms and non-abundant large organisms. To generate this pattern, metabolism (as an indicator of individual energy use) and abundance have to scale inversely with body mass, and cancel each other out. In contrast, the pattern referred to as biomass equivalence states that the biomass of all species in an area should be constant across the body-mass range. In this study, we investigated forest soil communities with respect to metabolism, abundance, population energy use, and biomass. We focused on four land-use types in three different landscape blocks (Biodiversity Exploratories). The soil samples contained 870 species across 12 phylogenetic groups. Our results indicated positive sublinear metabolic scaling and negative sublinear abundance scaling with species body mass. The relationships varied mainly due to differences among phylogenetic groups or feeding types, and only marginally due to land-use type. However, these scaling relationships were not exactly inverse to each other, resulting in increasing population energy use and biomass with increasing body mass for most combinations of phylogenetic group or feeding type with land-use type. Thus, our results are mostly inconsistent with the classic perception of energetic equivalence, and reject the biomass equivalence hypothesis while documenting a specific and nonrandom pattern of how abundance, energy use, and biomass are distributed across size classes. However, these patterns are consistent with two alternative predictions: the resource-thinning hypothesis, which states that abundance decreases with trophic level, and the allometric degree hypothesis, which states that population energy use should increase with population average body mass, due to correlations with the number of links of consumers and resources. Overall, our results suggest that a synthesis of food web structures with metabolic theory may be most promising for predicting natural patterns of abundance, biomass, and energy use.

Analysing the structure and dynamics of biotic interaction networks and the processes shaping them is currently one of the key fields in ecology. In this paper, we develop a novel approach to gut content analysis, thereby deriving a new perspective on community interactions and their responses to environment. For this, we use an elevational gradient in the High Arctic, asking how the environment and species traits interact in shaping predator-prey interactions involving the wolf spider Pardosa glacialis. To characterize the community of potential prey available to this predator, we used pitfall trapping and vacuum sampling. To characterize the prey actually consumed, we applied molecular gut content analysis. Using joint species distribution models, we found elevation and vegetation mass to explain the most variance in the composition of the prey community locally available. However, such environmental variables had only a small effect on the prey community found in the spider's gut. These observations indicate that Pardosa exerts selective feeding on particular taxa irrespective of environmental constraints. By directly modelling the probability of predation based on gut content data, we found that neither trait matching in terms of predator and prey body size nor phylogenetic or environmental constraints modified interaction probability. Our results indicate that taxonomic identity may be more important for predator-prey interactions than environmental constraints or prey traits. The impact of environmental change on predator-prey interactions thus appears to be indirect and mediated by its imprint on the community of available prey.

Anthropogenic land use shapes the dynamics and composition of central European forests and changes the quality and availability of resources of the decomposer system. Th ese changes likely alter the structure and functioning of soil animal food webs. Using stable isotope analysis ( 13 C, 15 N) we investigated the trophic position and resource use of soil animal species in each of four forest types (coniferous, young managed beech, old managed beech and unmanaged beech forests) across three regions in Germany. Twenty-eight species of soil invertebrates were analyzed covering three consumer levels and a representative spectrum of feeding types and morphologies. Data on stable isotope signatures of leaf litter, fi ne roots and soil were included to evaluate to which extent signatures of soil animals vary with those of local resources. Soil animal δ 15 N and δ 13 C signatures varied with the respective signatures of leaf litter and fi ne roots. After calibration to leaf litter signatures, soil animal stable isotope signatures of the diff erent beech forests did not diff er signifi cantly. However, thick leaf litter layers, such as those in coniferous forests, were associated with low animal stable isotope signatures presumably due to reduced access of decomposer animals to root-derived resources, suggesting that the decomposer food web is shifted towards leaf litter based energy pathways with the shift aff ecting all consumer levels. Variation in stable isotope signatures of soil animal species with litter quality parameters suggests that nutrition of third level but not fi rst and second level consumers is related to litter quality, potentially due to microorganisms locking up litter resources thereby hampering their propagation to higher trophic levels.

Summary1. Generalist predators such as carabid and cantharid beetles form an important component within natural enemy communities in arable land. Usually, predator-prey interactions are examined during the vegetation period, largely ignoring food web interactions occurring in the cold season. This is, however, when the larval stages of many polyphagous beetles develop whose survival critically depends on the availability of suitable prey. 2. In this study, we examined intra-and extraguild feeding links in larval Cantharis spp. (Coleoptera: Cantharidae) and Nebria brevicollis (Coleoptera: Carabidae), both abundant cold-adapted invertebrate predators in European arable land. As these immature beetles are fluid feeders, which impedes a microscopic analysis of their gut content, multiplex polymerase chain reaction assays were developed to examine trophic linkages in the field. 3. Collembolan DNA was detected in 49% and 54% of cantharid and carabid larvae, respectively. Earthworms were consumed by 34% of cantharid and 24% of carabid larvae. Significant differences in lumbricid and collembolan prey detection rates occurred between sampling dates in Cantharis spp. and N. brevicollis larvae, respectively, suggesting that these predators utilized different feeding strategies. In both predator taxa, however, only few individuals (0AE2-1%) tested positive for DNA of intraguild prey, indicating that predator-predator trophic interactions were scarce in this community. 4. Synthesis and applications. We present conclusive evidence that cold-adapted predatory beetle larvae are strongly linked to the detrital food chain by feeding on collembolans and earthworms. By improving the habitat conditions for detritivores in arable land by mulching, compost applications, or provision of plant cover during winter, their densities can be increased easily. Applying these measures year round will retain and sustain predatory beetles during their whole life cycle in arable land. Ultimately, we expect that these measures will enhance the ability of polyphagous beetle predators to provide their fundamental ecosystem service as regulators of agricultural pests.

Predation is an important ecological factor driving animal population structures, community assemblages and consequently ecosystem stability and biodiversity. Many environmental factors influence direction and intensity of predation, suggesting that trophic linkages between animals vary between different habitats. This in consequence has particular relevance in anthropogenically altered habitats such as managed forests, where disturbance regime, tree composition and stand age may change the natural food web structure. We investigated how prey consumption of three common centipede predators (Lithobius spp., Chilopoda), representing two body sizes varies between four differently managed forest types in two regions across Germany. We hypothesized that prey preference of these generalist predators is independent of forest type but rather driven by habitat structure, prey abundance and predator body size. Applying specific PCR assays to test for DNA of three abundant prey groups, i.e. Collembola, Diptera and Lumbricidae, in the predators’ guts, we tracked trophic interactions. The results showed that management type indeed has no influence on centipede prey consumption but depth of litter layer and soil pH. Trophic interactions varied between the two sampled forests regions mainly due to changes in the detection of Lumbricidae and Diptera. Also, effect of litter layer and prey abundance significantly differed between the smaller L. crassipes and the larger L. mutabilis, indicating a body size effect. The results complement food web analyses using fatty acids and stable isotopes by elucidating trophic interactions in soil in unprecedented detail.

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Analysis of predator–prey interactions is a core concept of animal ecology, explaining structure and dynamics of animal food webs. Measuring the functional response, i.e. the intake rate of a consumer as a function of prey density, is a powerful method to predict the strength of trophic links and assess motives of prey choice, particularly in arthropod communities. However, due to their reductionist set‐up, functional responses, which are based on laboratory feeding experiments, may not display field conditions, possibly leading to skewed results. Here, we tested the validity of functional responses of centipede predators and their prey by comparing them with empirical gut content data from field‐collected predators. Our predator–prey system included lithobiid and geophilomorph centipedes, abundant and widespread predators of forest soils and their soil‐dwelling prey. First, we calculated the body size‐dependent functional responses of centipedes using a published functional response model in which we included natural prey abundances and animal body masses. This allowed us to calculate relative proportions of specific prey taxa in the centipede diet. In a second step, we screened field‐collected centipedes for DNA of eight abundant soil‐living prey taxa and estimated their body size‐dependent proportion of feeding events. We subsequently compared empirical data for each of the eight prey taxa, on proportional feeding events with functional response‐derived data on prey proportions expected in the gut, showing that both approaches significantly correlate in five out of eight predator–prey links for lithobiid centipedes but only in one case for geophilomorph centipedes. Our findings suggest that purely allometric functional response models, which are based on predator–prey body size ratios are too simple to explain predator–prey interactions in a complex system such as soil. We therefore stress that specific prey traits, such as defence mechanisms, must be considered for accurate predictions.

The input of plant leaf litter has been assumed to be the most important resource for soil organisms of forest ecosystems, but there is increasing evidence that root-derived resources may be more important. By trenching roots of trees in deciduous and coniferous forests, we cut-off the input of root-derived resources and investigated the response of microorganisms using substrate-induced respiration and phospholipid fatty acid (PLFA) analysis. After one and three years, root trenching strongly decreased microbial biomass and concentrations of PLFAs by about 20%, but the microbial community structure was little affected and the effects were similar in deciduous and coniferous forests. However, the reduction in microbial biomass varied between regions and was more pronounced in forests on limestone soils (Hainich) than in those on sandy soils (Schorfheide). Trenching also reduced microbial biomass in the litter layer but only in the Hainich after one year, whereas fungal and bacterial marker PLFAs as well as the fungal-to-plant marker ratio in litter were reduced in the Schorfheide both after one and three years. The pronounced differences between forests of the two regions suggest that root-derived resources are more important in fueling soil microorganisms of base-rich forests characterized by mull humus than in forests poor in base cations characterized by moder soils. The reduction in microbial biomass and changes in microbial community characteristics in the litter layer suggests that litter microorganisms do not exclusively rely on resources from decomposing litter but also from roots, i.e. from resources based on labile recently fixed carbon. Our results suggest that both bacteria and fungi heavily depend on root-derived resources with both suffering to a similar extent to deprivation of these resources. Further, the results indicate that the community structure of microorganisms is remarkably resistant to changes in resource supply and adapts quickly to new conditions irrespective of tree species composition and forest management.