Tree identity surpasses tree diversity in affecting the community structure of oribatid mites (Oribatida) of deciduous temperate forests

The role of tree diversity and identity as determinants of soil animal community structure is little understood. In a mature deciduous forest dominated by beech we identified clusters of one, two and three tree species of beech, ash and lime allowing to investigate the role of tree species diversity and identity on the density and community structure of oribatid mites. To relate oribatid mite community structure to environmental factors we measured leaf litter input, fine root biomass, mass of organic layers, topsoil pH and C and N content. We expected oribatid mite density to increase with increasing tree diversity, but we expected the effects of tree species identity to override effects of tree diversity. Further, we hypothesized the density of oribatid mites to be reduced by the presence of beech but increased by the presence of lime and ash. As expected tree diversity little affected oribatid mite communities, whereas tree species identity strongly altered density and community structure of oribatid mites. However, in contrast to our expectations the density of oribatid mites was highest in presence of beech indicating that many oribatid mite species benefit from the presence of recalcitrant litter forming thick organic layers. Especially Oppioidea benefited from the presence of beech presumably due to an increased availability of food resources such as fungi and nematodes. Lower density of oribatid mites in monospecific clusters of lime and ash suggests that oribatid mites did not benefit from high quality litter of these species. Notably, large and strongly sclerotized oribatid mite species, such as Steganacarus magnus and Chamobates voigtsi, benefited from the presence of ash and lime. Presumably, these large species better resist harsh microclimatic conditions in shallow organic layers.     

Little effect of forest age on oribatid mites on the bark of trees

This study investigates the effect of forest age (20-, 50-, 160-year-old, and primeval forest) on oribatid mite communities on the bark of oak trees in the National Park Pusza Białowieża in eastern Poland. We hypothesized that oribatid mite diversity on bark peaks at forests of intermediate age and that the number of parthenogenetic species of oribatid mites is highest in young stands. In contrast to these hypotheses, the diversity, density, number of juveniles, community structure and the mode of reproduction of oribatid mite species were not significantly affected by forest age. None of the oribatid mite species occurred exclusively on trees of a specific age. The results suggest that oribatid mite communities on the bark of trees are minimally affected by tree harvesting regimes. In contrast to oribatid mites in soil, communities on bark appear to be less sensitive to disturbances.     

A comparison of microarthropod assemblages with emphasis on oribatid mites in canopy suspended soils and forest floors associated with ancient western redcedar trees

Microarthropod abundance, oribatid mite species richness and community composition were assessed in the high canopy (ca. 35 m) of an ancient temperate rainforest and compared with microarthropod communities of the forest floor. Microarthropods were extracted from 72 core samples of suspended soils and 72 core samples from forest floors associated with six western redcedar trees in the Walbran Valley on the southwest coast of Vancouver Island, Canada. Total microarthropod abundances, mesostigmatid and astigmatid mites, Collembola and other microarthropod abundances were significantly greater in forest floors compared to canopy habitats. Oribatid and prostigmatid mite abundance were not significantly different between habitats. The relative abundances of all microarthropod groups considered in this study differed significantly between habitats. Eighty-eight species of oribatid mites were identified from the study area. Eighteen of the 53 species observed in suspended soils were unique to the canopy. Cluster analysis indicates that the arboreal oribatid mite community is distinct and not a taxonomic subset of the forest floor assemblage, however, canopy oribatid mite communities are more heterogeneous in species composition than in the forest floor.     

Do oribatid mites live in enemy-free space? Evidence from feeding experiments with the predatory mite Pergamasus septentrionalis

To examine whether their strongly hardened cuticle permits adult oribatid mites (Acari) to live in enemy-free space, we investigated (1) if Pergamasus septentrionalis, a widespread and abundant predatory mesostigmate mite species, is able to feed on oribatid mites, (2) if this predator preferentially feeds on certain oribatid mite species and (3) to what extent oribatid mites are consumed compared to collembolans and juvenile Mesostigmata. Single adult individuals of six different oribatid mite species (Steganacarus magnus; Nothrus silvestris; Damaeus riparius; Liacarus coracinus; Eupelops plicatus; Achipteria coleoptrata), one collembolan species (Folsomia quadrioculata) and juvenile Pergamasus spp. were offered separately to adult P. septentrionalis in a no-choice feeding experiment. The predators quickly and preferentially fed on collembolans and juvenile Pergamasus; three oribatid mite species were occasionally eaten (L. coracinus, N. silvestris, A. coleoptrata); the other oribatid mite species were rejected as food (E. plicatus, S. magnus, H. riparius). When preying on oribatid mites, P. septentrionalis typically first cut off the legs of the mite, then opened the body through the region of the genital plates or the mouthparts. The results suggest that predator pressure on adult oribatid mites in the field is low, since few relevant predators at the study site are more abundant and powerful than P. septentrionalis. Adult oribatid mites therefore likely indeed live in enemy-free space, i.e. are little affected by predators, but that may not apply to soft-bodied immatures. Collembolans were quickly consumed indicating that they comprise a major part of the diet of P. septentrionalis. Strong feeding on juveniles of Pergamasus suggests that P. septentrionalis also functions as an intra-guild predator.     

Oribatid mite communities and foliar litter decomposition in canopy suspended soils and forest floor habitats of western redcedar forests, Vancouver Island, Canada

Litter decomposition and changes in oribatid mite community composition were studied for 2 years in litterbags collected from arboreal organic matter accumulations (canopy suspended soils) and forest floors associated with western redcedar trees on Vancouver Island, British Columbia. We tested the hypotheses that lower rates of mass loss, higher nutrient levels, and different patterns of oribatid mite richness and abundance in decomposing western redcedar litter would be observed in litterbags associated with canopy suspended soils compared to forest floors. Decomposition, measured by mass loss of cedar litter in litterbags, was not significantly different in canopy and forest floor habitats, although reduced in the canopy. Abundance and richness of oribatid mites inhabiting litterbags were significantly greater on the forest floor compared to the canopy suspended soils. Canopy suspended soils had higher levels of total nitrogen, available phosphorus and potassium than the forest floor, but moisture content was significantly lower in the suspended soils. Higher nutrient levels in the canopy system are attributed to differences in coarse woody debris input (but not foliar litter), combined with reduced nutrient uptake by roots and lower mobilisation rates of nutrients by detritivorous and fungivorous microarthropods. Moisture limitation in the canopy system possibly contributed to lower mass loss in litterbags, and lower abundance and richness of oribatid mites in litterbags placed on canopy suspended soils. Patterns of oribatid mite community composition were related to mite communities associated with the underlying substrate (forest floor or canopy suspended soil) which act as source pools for individuals colonising litterbags. Successional and seasonal trends in oribatid mite communities were confounded by moisture limitation at 24 months, particularly within the canopy habitat.     

Spruce forest conversion to a mixed beech-coniferous stand modifies oribatid community structure

We investigated oribatid mite community diversity and structure in the managed conversion of coniferous stands into semi-natural montane forests that are composed of a small-scale mosaic formed by different age classes of silver fir, Norway spruce and European beech in the southern Black Forest area, South-Western Germany, using the space-for-time substitution method. The core hypothesis was that changing tree composition and management practice will affect functional structure and diversity of oribatid mite community through changing substrate quality and litter diversity. Three forest districts were selected within the research region. Four stand types representing the major stages of forest conversion were selected within each forest district: (i) even-aged spruce monocultures, (ii) species enrichment stage, (iii) forest stand structuring stage, at which fur and beech and other deciduous trees penetrate the upper storey of the forest and (iv) a diverse continuous cover forest respectively. Oribatid mite abundance, species richness and composition, biomass, ecomorphs and feeding groups relative abundance were determined. An overall increase in species richness moving from the spruce monoculture to a continuous cover forest was detected. However, the herbivorous and litter-dwelling mites were most sensitive to forest conversion demonstrating significant differences in abundance between conversion stages. Almost all changes in the oribatid community were associated with the properties of the changing litter layer. Abundance of soil-dwelling mites remained very stable what is in contradiction with the response of the other soil fauna groups found at the same sites. Overall oribatid community seemed to be more dependent on total microbial biomass than fungi. However, observed effects were overshadowed by considerable district-induced differences.     

Oribatid mite community structure and tree species diversity: A link?

Differences in tree species may lead to contrasting soil environments via differences in litter chemical quality and physical environmental factors, such as soil type and soil moisture. However, separating the effects of litter quality and physical environment is difficult under field conditions. Both litter quality and soil environment affect the species composition of the soil animal community. A diversity gradient of canopy tree species (11–25 species) located on homogeneous soil substrate at Tomakomai Experimental Forest of Hokkaido University was used to analyse the relationship between tree species diversity and oribatid mite community structure. Soil samples were collected from three levels of tree species richness (high, intermediate and low) with three replicates each, in July 2000. Leaf area index (LAI) was positively correlated with tree species diversity suggesting higher litter input into the soils with increasing tree diversity. However, the tree species diversity gradient affected neither accumulation of litter on the forest floor nor abundance and species richness of oribatid mites. Canopy and understory plant species richness, LAI, total soil carbon and biomass of epigeic and endogeic earthworms did not significantly affect mite community structure as indicated by redundancy analysis (RDA). The results suggest that oribatid mite community structure is minimally affected by tree species diversity and associated changes in litter diversity.     

Diversity of soil mite communities when managing plant communities on set-aside arable land

When restoring former agricultural land to more low-nutrient input ecosystems, the establishment of a plant community can be enhanced by sowing desirable species. In this study our aim was to determine whether management of the plant community influences the microarthropod community. We carried out a field experiment in three European countries on set-aside arable land and determined soil mites from the sites in Sweden, The Netherlands and Spain. Experimental plots on set-aside arable land were sown with high (15 species) or low (4 species) plant species seed mixtures; other plots were colonized naturally. A field with continued agricultural practices and a later successional site (target site) were used for comparison with the experimental plots. Soil from the later successional site was inoculated into half of the plots. Abandoning agricultural practices increased the density of mites at one site while the number of mite species was not affected. Sowing plant seeds had no effect on mite densities at any of the sites. The community composition of mites changed in response to management of the plant community, as shown by canonical correspondence analysis. Among the functional groups of mites, saprophytes generally dominated on all plots at all sites. Mites parasitic on insects were not present on fields with continued agricultural practice in Sweden and The Netherlands, and might thus be regarded as an indicator of an increase in trophic complexity in the sown and naturally colonized treatments. Predatory and plant parasitic mites showed no consistent pattern in relation to the treatments of the three sites. Soil inoculation treatment had only a minor impact on the soil mite communities.     

Do oribatid mites (Acari: Oribatida) show a higher preference for ubiquitous vs. specialized saprotrophic fungi from pine litter?

Previous studies of oribatid mite feeding preferences for different saprotrophic fungi were limited to ubiquitous fungal species, whereas saprophytes specialized to decompose particular substrates have been neglected. We examined the preference of seven oribatid mite species (Adoristes ovatus, Eniochthonius minutissimus, Eueremaeus silvestris, Nothrus silvestris, Oppiella subpectinata, Porobelba spinosa and Spatiodamaeus verticillipes) for nine autochthonous saprotrophic fungi from Scots pine litter (Pinus sylvestris). Among the fungal species offered were specific coniferous litter colonizers (Allantophomopsis lycopodina, Ceuthospora pinastri, Hormonema dematioides, Scleroconidioma sphagnicola, Verticicladium trifidum, Marasmius androsaceus and Sympodiella acicola) and two ubiquitous species (Cladosporium herbarum and Oidiodendron griseum). The fungi were inoculated on fragments of pine needles and offered simultaneously and separately to the mites. Our main hypothesis, that oribatid mites (usually occurring in more than one type of ecosystems) would prefer the ubiquitous fungal species rather than those specific to pine litter, was supported only partly. The ubiquitous C. herbarum was highly preferred by all studied mites, but most of them preferred one or more of the specialized fungi with similar intensity. The basidiomycete M. androsaceus along with sterile needles were consistently avoided by all mites in all experiments. Our results do not support the hypothesis, that the “true” fungivorous oribatid mites in traditional sense are more selective fungal feeders than are the “unspecialized” panphytophagous ones. We observed no gradation in preference of fungi for oribatid mites as a group, but rather a discontinuous and dynamic mosaic with particular mites preferring particular fungal species. This heterogeneous mosaic shapes the feeding niches occupied by particular oribatid mite species and probably reduces competition for food source among numerous species coexisting in a given habitat and time.     

Community structure of oribatid mites in relation to elevation and geology on the slope of Mount Kinabalu,Sabah, Malaysia

The patterns of oribatid communities were investigated in relation to elevations (700, 1700, 2700 and 3100 m a.s.l.) and geological substrates (i.e. non-ultrabasic and ultrabasic rocks) on the slopes of Mt. Kinabalu, Sabah, Malaysia. The density and morphospecies richness of oribatid mites were greater in the non-ultrabasic plot than in the ultrabasic plot at each of the same elevations. The density and richness of Oribatid mites decreased with elevation on both substrates, but the effects of elevation on the density on non-ultrabasic were less significant than on the ultrabasic substrate. Oribatid mite density correlated positively with the concentration of soil organic phosphorus and negatively with that of exchangeable Ca in soil. The richness of morphospecies of oribatid mites positively correlated with phosphorus concentration in litter, above-ground biomass, tree diversity and litterfall amount, and negatively correlated with elevation and Ca in soil. Morphospecies from families Galumnidae, Otocepheidae, Haplozetidae and Scheloribatidae were dominant in each plot. Canonical correspondence analysis (CCA) showed the importance of elevation for the community structure of oribatid mite. In conclusion, total density or morphospecies richness of oribatid communities was influenced by both geology and elevation, and morphospecies composition was strongly influenced by elevation.     

Changes in the community composition and trophic structure of microarthropods in sporocarps of the wood decaying fungus Fomitopsis pinicola along an altitudinal gradient

Soil microarthropods colonize a wide range of habitats including microhabitats such as earthworm burrows, ant nests, tree trunks, moss mats and wood decaying fungi. While many of these microhabitats have been investigated intensively, the role of wood decaying fungi as a habitat and food resource for microarthropods found little attention. We investigated the density, community structure, reproductive mode and trophic structure of microarthropods, in particular oribatid mites, in the wood decaying fungus Fomitopsis pinicola (Schwarts: Fr) Karst. along an altitudinal gradient in Germany spanning from 350 m to 1160 m. Microarthropods were extracted from sporocarps, and stable isotope ratios (¹⁵N/¹⁴N; ¹³C/¹²C) of the fungus and the microarthropods were measured. Densities of most microarthropod taxa were highest at lower altitudes and decreased with increasing altitude. Oribatid mites were the dominant animal taxon. Their community structure gradually changed with altitude. Stable isotope ratios indicated that oribatid mite and other arthropod species occupy distinct trophic niches but most do not feed on F. pinicola. Notably, species of the same genus, e.g. Carabodes, occupied different trophic niches. Most oribatid mite species in F. pinicola reproduced sexually which is similar to the bark of trees but in contrast to the soil where most species reproduce via parthenogenesis. The findings indicate that (1) at high altitudes microarthropod density in fungal fruiting bodies is limited by low temperatures reducing animal metabolism and reproduction, and this also affects oribatid mite community structure, (2) despite the uniform habitat trophic niches of oribatid mite species differ and this also applies to morphologically similar species of the same genus, and (3) feeding on F. pinicola or associated resources facilitates the dominance of sexual reproducing species.     

Molecular detection of nematode predation and scavenging in oribatid mites: Laboratory and field experiments

Recent stable isotope analyses indicate that a number of putative detritivorous soil microarthropods is not typical detritivores but rather live as predators or scavengers. Using molecular gut content analyses the present study investigates if nematodes indeed form part of the diet of oribatid mites. First, in a no-choice laboratory feeding experiment two nematode species (Phasmarhabditis hermaphrodita and Steinernema feltiae) were offered to eight species of oribatid mites and one gamasid mite. Second, after feeding for 4 and 48 h on each nematode species the detection time of prey DNA in the oribatid mite species Steganacarus magnus was investigated. Third, in a field experiment nematode prey (P. hermaphrodita and S. feltiae) in the diet of microarthropods was investigated distinguishing between scavenging and predation. In the no-choice laboratory experiment not only the gamasid mite but also several of the studied oribatid mite species consumed nematodes. After feeding on nematodes for 4 h prey DNA was detectable in S. magnus for only 4 h, but after feeding for 48 h prey DNA was detectable for 128 h, indicating that the duration of feeding on prey is an important determinant for prey DNA detection. The field experiment confirmed that oribatid mite species including Liacarus subterraneus, Platynothrus peltifer and S. magnus intensively prey on nematodes. Interestingly, DNA of dead P. hermaphrodita was detectable to a similar degree as that of living individuals indicating that scavenging is of significant importance in decomposer food webs. Results of our study indicate that predation and scavenging on nematodes by “detritivorous” microarthropods in soil food webs need to be reconsidered.     

Metal concentrations in soil and invertebrates in the vicinity of a metallurgical factory near Tula (Russia)

To investigate the effects of emissions from a large metal works near Tula in the Russian Federation, we measured concentrations of iron, manganese, zinc, copper, nickel, lead and cadmium in soil, litter and invertebrates at four sampling sites at different distances from the factory. The sites were located in woodlands in the bed of the Voronka river, near the town of Kosaya Gora in the district of Tula. Additional soil properties (organic matter content, clay content, water holding capacity, Ca, Mg, N, P, and pH) were measured that could explain differences in the bioavailability of the metal burdens. It appeared that the factory is a source of Fe, Mn, Zn, Cu, Ni and Pb. One of the sampling sites had a high nitrogen content in the litter due to emissions from a fertilizer plant in the area. Most of the metal contamination was limited to the immediate surroundings and did not extend beyond a distance of 5 km. Only the site close to the factory can be considered as polluted, however, background concentrations of metals in the Tula area seem to be significantly lower than in present Western European soils and a reference system still has to be developed. Exchangeable metal concentrations (0.01 M CaCl₂ extracts from soil) were very low and were not correlated with the total concentrations, indicating low bioavailability of the pollution. At the most polluted site, concentrations of all metals were positively correlated with each other; correlations decreased with increasing distance. Metal concentrations in soil were often negatively correlated with organic matter content, especially so for nickel. Metal concentrations in invertebrates showed considerable variation between individual species, however, some general patterns were obvious. Concentrations were high in earthworms, oribatid mites and carabid beetles, and low in springtails, centipedes and spiders. There was no relationship between the trophic position of a species and its metal accumulating ability. Iron concentrations in invertebrates at the polluted site were a factor of 2 to 4 higher than at the most remote (reference) site; for zinc and copper the internal concentrations were also elevated, but to a lesser extent than the soil concentrations. The data illustrate the extremely complicated relationship between metal residues in invertebrates and metal concentrations in soil. For most of the saprophageous and predatory arthropods studied total concentrations nor exchangeable concentrations in soil are good predictors; species-specific feeding mechanisms and metal physiologies seem to be the main determinants.     

Responses of oribatid mite communities to summer drought: The influence of litter type and quality

A litterbag experiment was used to study the impact of extended periods of summer drought on the structure of oribatid mite communities (Acari, Oribatida) developing in two litter types (beech, spruce) of two qualities (fresh, pre-incubated). Within each litter type, litter quality determined species composition and densities and, in turn, this determined the impact of drought upon the oribatid mite communities. In both litter types, drought had a greater impact on community development in the pre-incubated compared to the fresh litter. In the short-term perspective of the present study, oribatid mite communities in beech litter were less sensitive to summer drought than those in spruce litter. This was partly due to the presence of site-specific, drought-tolerant species but seemed also strongly related to differences in the decomposition patterns between the litter types. Marked changes in densities and composition of oribatid communities after only one period of summer-drought suggest that there is a potential for a significant alteration of oribatid community structure in both litter types if climatic changes persist.     

Stable isotopes revisited: Their use and limits for oribatid mite trophic ecology

In this review we summarize our knowledge of using stable isotopes (¹⁵N/¹⁴N, ¹³C/¹²C) to better understand the trophic ecology of oribatid mites. Our aims are (a) to recapitulate the history of stable isotope research in soil animals with a focus on oribatid mites, (b) to present new stable isotope data for oribatid mites and overview the current state of knowledge of oribatid mite trophic niche differentiation, (c) to compile problems and limitations of stable isotope based analyses of trophic relationships and (d) to suggest future challenges, questions and problems that may be solved using stable isotope analyses and other novel techniques for improving our understanding on the trophic ecology of soil invertebrates. We conclude that (1) in addition to ¹⁵N/¹⁴N ratios, ¹³C/¹²C ratios contribute to our understanding of the trophic ecology of oribatid mites, allowing, e.g. separation of lichen- and moss-feeding species, (2) there likely are many lichen but few moss feeding oribatid mite species, (3) oribatid mite species that are endophagous as juveniles are separated by their stable isotope signatures from all other oribatid mite species, (4) fungivorous oribatid mite species cannot be separated further, e.g. the fungal taxa they feed on cannot be delineated. A particular problem in using stable isotope data is the difficulty in determining signatures for basal food resources, since decomposing material, fungi and lichens comprise various components differing in stable isotope signatures; ¹³C/¹²C ratios and potentially other isotopes may help in identifying the role of these resources for decomposer animal nutrition.     

Successful analysis of gut contents in fungal-feeding oribatid mites by combining body-surface washing and PCR

The feeding habits of soil mesofauna have been a mystery for decades, and depending on the methods used, different degrees of feeding specialisation have been observed. A new way to study the almost unknown feeding habits of soil mesofauna e.g. oribatid mites is to use PCR-based techniques. When applying PCR on these small organisms, the low amount of ingested DNA can cause problems. Even more important is to certify that the amplified DNA does not originate from body-surface contamination. The aim of this study was to analyse if washing of the body surface combined with PCR can be a successful approach when identifying the food ingested by fungivorous mites. The method was developed in a laboratory system where we used the oribatid mite Archegozetes longisetosus as a model organism due to its relatively short life cycle and ease of laboratory culturing. The results demonstrated that surface contamination is a serious problem. Both washing and dissection was needed to remove surface contamination on such small organisms. To get a reliable result the samples also had to contain at least five pooled guts, but preferably ten. This is the first step towards a successful use of PCR-based methods to study natural feeding habits of species most likely contaminated on the body surface. When modified for field conditions, the results obtained by this method have a high potential to answer many questions about the animals feeding habits, and their functional role in the soil.     

Effects of microhabitat diversity and geographical isolation on oribatid mite (Acari: Oribatida) communities in mangrove forests

The effects of microhabitat diversity and geographical isolation on the structure of oribatid communities were studied in mangrove forests on the Ryukyu Islands of Japan. The study took place at three sites on two islands 470 km apart. Oribatid mites (Oribatida) were extracted from leaves, branches, bark of trunks (0-50, 50-100, and 100-150 cm high) and of knee roots, and from forest-floor soil and littoral algae, each defined as a microhabitat of oribatid mites. At the 0-50 cm height, the species composition of the oribatid communities on the knee-root bark and the bark of trunks of Bruguiera gymnorrhiza differed significantly from that on the other microhabitats. This difference was attributed to tidal flooding of the mangrove forests. Cluster analysis showed that oribatid communities in the same microhabitat at different sites tended to be more similar than those on different microhabitats at the same site. This implies that the species composition of oribatid communities in mangrove forests is more likely to be affected by factors responsible for microhabitat diversity (characterized specifically by the flooded trunks) than by geographical distance between the islands.     

Effects of forest stand type on oribatid mite (Acari: Oribatida) assemblages in a southwestern Quebec forest

Despite the ubiquity of oribatid mites in soil and litter systems, and their importance in decomposition and nutrient cycling processes, little is known of the factors underlying the composition of their assemblages. Our objective was to address this by determining how oribatid assemblage composition changes by forest stand type. This work was done in and near a hardwood forest in southwestern Quebec, Canada. We sampled mites by collecting 1 L of litter and 170 cm³ of soil from four sites in each of four distinct habitat types: American beech stands, sugar maple stands, mixed deciduous stands and mixed conifer plantations. Samples were collected in July and September 2005, and June 2006, and over 6500 oribatid mites were collected and identified to species. Abundance and species richness differed between forest types: for abundance conifer>beech>maple>mixed deciduous while for species richness beech and conifer>maple>mixed deciduous. Ordination analyses revealed that conifer plantations and beech stands supported distinct assemblages, while there were some overlap in the assemblages found in maple stands and mixed deciduous stands. These data support the importance of aboveground plant communities in affecting the composition of oribatid assemblages even at local scales and provide insight into additional impacts that may be caused by shifts in plant species ranges due to global changes.     

Trophic stability of soil oribatid mites in the face of environmental change

A key issue in ecology is the degree to which trophic structure within communities responds to environmental change. Organisms with generalist diets are more flexible in their feeding habits than are specialists, and may be affected less in a changing environment. Soil fauna fulfill crucial ecosystem functions in terrestrial ecosystems and many are thought to have generalized diets. They may therefore be buffered from negative effects of environmental change. Here, we used ¹⁵N isotope analysis to study trophic differentiation among 91 species of oribatid mites and their responses to chronic atmospheric N deposition. Combining our own measurements with published data, we established that the trophic positions of mite species were remarkably stable within and among forests, as well as between ambient and experimental N deposition. Trophic stability indicates a higher than expected level of feeding specialization, which may foster diversity, but limit the ability to switch food resources in a changing environment.     

Oribatid communities (Acari: Oribatida) inhabiting saxicolous mosses and lichens in the Krkonoše Mts. (Czech Republic)

Composition, structure and diversity of oribatid communities inhabiting saxicolous mosses and lichens were studied in the Krkonoše Mts. (Czech Republic) along an altitudinal gradient reaching from submontane to alpine belt. Samples of various saxicolous mosses and lichens from 197 stands were collected. Impact of altitude and dominant moss or lichen species on community structure were analysed. Data were evaluated using divisive cluster analysis and direct ordination analysis. Altogether 104 oribatid species were recorded. Four groups of saxicolous habitats, which differ in the composition of their oribatid communities, were distinguished:1. Mosses below the upper forest line with accessory higher plants growing on their surface. Higher plants indicate favourable moisture conditions and an appreciable degree of humus layer development. Their oribatid mite community is rich in number of frequent species and consists of ubiquitous species, ubiquitous species with higher requirements for moisture and amount of decaying organic matter, a high number of soil dwelling species and several hygrophilous species.2. Mosses below the upper forest line without accessory higher plants. They predominantly include mosses with no or only a weakly developed humus layer. Their oribatid mite community is composed mainly of ubiquitous species and a few soil dwelling species.3. Mosses in open areas above the upper forest line. Their humus layer is not developed at all or only weakly. Humidity and temperature fluctuations are here much higher compared with mosses below the upper forest line, which are protected by the specific forest microclimate. Their oribatid mite community is poor in number of frequent and dominant species and consists predominantly of Oribatula cf. pallida and two specialised species living exclusively in mosses and lichens (Mycobates tridactylus and Trichoribates monticola).4. Saxicolous lichens. Their oribatid community comprises ubiquitous species, species frequent both in mosses and lichens and several species with a strong affinity to lichens (Mycobates carli and five species of the genus Carabodes).