Alexander Bruckner

The role of microarthropods in terrestrial decomposition: a meta-analysis of 40 years of litterbag studies

Litterbags have been utilized in soil ecology for about 50 years. They are useful because they confine organic material and thus enable the study of decomposition dynamics (mass loss and/or nutrient loss through time, colonization by soil biota) in situ, i.e. under field conditions. Researchers can easily restrict or permit access to certain size classes of soil fauna to determine their contribution to litter mass loss by choosing adequate mesh size or applying specific biocides. In particular, the mesofauna has received much attention since it comprises two very abundant and diverse microarthropod groups, the Collembola (springtails) and Acari (mites). We comprehensively searched the literature from the mid‐1960s to the end of 2005 for reports on litterbag experiments investigating the role of microarthropods in terrestrial decomposition. Thirty papers reporting 101 experiments satisfied our selection criteria and were included in the database. Our meta‐analysis revealed that microarthropods have a moderate but significant effect on mass loss. We discuss in detail the interactions of the microarthropod effect with study characteristics such as experimental design (e.g. number of bags, duration of experiment), type of exposed organic matter, climatic zone and land use of the study site. No publication bias was detected; however, we noticed a significant decrease in the microarthropod effect with publication year, indicating that, in the first decades of litterbag use, soil zoologists may have studied “promising” sites with a higher a priori probability of positive microarthropod effects on litter mass loss.

Plot-scale spatial patterns of soil water content, pH, substrate-induced respiration and N mineralization in a temperate coniferous forest

Samples of two perpendicular transects from an even-aged Norway spruce Picea abies forest . were used for geostatistical analysis of forest floor water content WC , pH, substrate-induced . . respiration SIR, a measure of microbial biomass , and N mineralization Nmin . Nmin data did not fulfil the stationarity assumptions of geostatistics and had to be detrended prior to analysis. All variables exhibited spatial structure. The degree of spatial dependence was generally high .60%-95% . pH and SIR were isotropically distributed; WC and Nmin were anisotropic. Three . different scales of spatial variability were detected at the site. 1 A fine-scale pattern with ranges - 1 m that was attributed to retarded decomposition, poor chemical and structural diversity of P. . abies litter, and lacking bioturbatic activity of earthworms. 2 A mesoscale pattern was indicated by sinoidal periodicity of most variograms, with inflection points every 1.0-1.5 m. This

Impact of faunal complexity on microbial biomass and N turnover in field mesocosms from a spruce forest soil

In a field study using soil mesocosms in an acid spruce forest soil we investigated the effects of mesofauna and macrofauna on microbial biomass, dissolved organic matter, and N cycling. Intact soil monoliths were taken from the ground, defaunated by deep-freezing, and wrapped in nets of various mesh-sizes to control re-immigration of different faunal size-classes. The monoliths were then replanted in the field. Three treatments of mesocosms were prepared: (1) with only microbiota, (2) microbiota and mesofauna, and (3) microbiota, mesofauna, and macrofauna (= complex fauna). After 8 months of exposure the mesocosms and the unmanipulated control plots (treatment 4) were destructively sampled. We estimated microbial biomass by substrate-induced respiration and the chloroform fumigation-extraction method. N cycling was measured by monitoring microbial N mineralization, the NHinf4sup+content, and selected amino acids and the activities of protease, urease, and deaminase. The results from the L/F layer showed that the pool of the microbial biomass was not changed by the activity of the mesofauna. However, the mesofauna and macrofauna together enhanced SIR. An increase in microbial N mineralization was only observed in treatment 3 (microbiota + complex fauna). Protease activity and NHinf4sup+content increased in treatments 2 (microbiota + mesofauna) and 3 (microbiota + complex fauna). The complex fauna induced a soil pH increase in treatment 3 as opposed to treatment 1 and the control. This increase was presumably due to excretory NHinf4sup+. Principal component analysis revealed that the complex fauna in treatment 3 caused a significantly higher N turnover per unit of microbial biomass.

Use of enclosed model ecosystems in soil ecology: a bias towards laboratory research

Enclosed model ecosystems, or microcosms, have become a major research tool in soil ecology. Due to the speed, statistical power and mechanistic insights attainable with laboratory-based microcosm experiments, these have added considerably to our ecological knowledge. However, soil ecologists agree that, due to problems of scale and artificiality, microcosm research should be carried out in the context of appropriately scaled field model ecosystems (e.g. mesocosms). This paper aims at clarifying the terminology of enclosed model ecosystems as well as determining and discussing the frequency with which laboratory and field model ecosystems are used in current soil-ecological research. Among 92 model ecosystem studies published from 1993 to 1998 in soil biological journals, only 19 were performed in the field. Laboratory microcosms are, on average, significantly smaller and experiment duration is significantly shorter than in field model ecosystem studies. They are easier to maintain and allow for a larger number of experiments in a unit of time. We argue that the bias towards laboratory research is mainly caused by the growing demand for publications with high-impact ratings in an increasingly competitive scientific world and by the fact that an increasing emphasis is being placed on subjects where research can be carried out very quickly.

Assessing ant assemblages: pitfall trapping versus nest counting (Hymenoptera, Formicidae)

Abstract.Pitfall trapping and nest counting are the most common census methods for ant assemblages. We examined the concordance between pitfall catches and nest counts on dry grassland. Spearman rank correlations and non-metric multidimensional scaling of the Bray Curtis similarity index revealed moderate concordance between the data collated by the two methods, but overall method-related differences were considerable. The dissimilarity was influenced by the type of land management, but not by trapping period or plot shape. Trapping success depended on nest density, ground vegetation cover and species-specific traits (inhabited stratum, colony size, foraging distance). Even when these factors were taken into account, the convertibility of pitfall trap and nest density values was unsatisfactory: the census method proved to be crucial in designing ant-ecological studies and interpreting literature data.

A method of preparing mesocosms for assessing complex biotic processes in soils

Equipment and handling methods for the preparation of soil mesocosms were developed. The mesocosms were used to investigate the interrelationships between mesofauna and microflora in a coniferous forest soil. Soil monoliths were taken from the ground, defaunated by deep-freezing, wrapped in nets to control reimmigration of different faunal size-classes, and replanted in the field for 8 months. in a practical test the technique described here proved to be an inexpensive field method for producing a replicated series of mesocosm in a short time. Deep-freezing is appropriate for defaunating soil monoliths. The fine nets effectively exluded meso-and macrofauna. No significant differences were found in the abundance of Enchytraeids and Collembola between recolonized mesocosms and the undisturbed control at the end of the study period. In contrast, oribatid mite abundace was still greatly reduced in the recolonized esocosms. Dominance structure and species composition of the more dominant oribatid species in the different treatments were apparently similar. To compensate for the low colonization ability of oribatids, a reintroduction of selected animal size-classes to defaunated monoliths is recommended.

Soil microhabitat diversity of a temperate Norway spruce (Picea abies) forest does not influence the community composition of gamasid mites (Gamasida, Acari)

The aim of this study is to test if the community of gamasid mites (Gamasina and Uropodina) prefers some natural microhabitats (decayed stumps, moss cushions, spruce needles, etc.) of the spruce forest floor, and if there is a characteristic assemblage of gamasid species in particular habitat patches. Some microhabitats like moss cushions, branches, decayed stumps, moss on decayed stumps, spruce needles and a soil horizon show higher gamasid abundance and number of species than, e.g. decaying cones or humus layer material. However, distinct communities, which inhabit specific microhabitats, are not found. The explanation is suggested to lie in the predatory mite life strategy of dominant species: they are highly mobile, hardly ever observed in aggregations and as predators, not directly dependant on patchily distributed resources.

Impact of faunal complexity on nutrient supply in field mesocosms from a spruce forest soil

A field study in an acidic spruce forest soil using soil mesocosms was conducted to investigate the effects of mesofauna and macrofauna on exchangeable cations, organic matter content, base saturation, and Ca-lactate extractable nutrients. In the field, intact soil monoliths were taken from the ground, defaunated by deep-freezing and wrapped in nets of various mesh-sizes to control immigration of different faunal size classes. The monoliths were then replanted in the field. Three types of treatments for the mesocosms were prepared: (1) microbiota only, (2) microbiota and mesofauna, (3) microbiota, mesofauna, and macrofauna (=complex fauna). After eight months the mesocosms and unmanipulated control plots (treatment 4) were destructively sampled and submitted to chemical analysis. Generally, the exchangeable base cations and Mn2+ showed higher contents with increasing faunal complexity, whereas the exchangeable acidic cations of Fe3+ and Al3+ decreased in the monoliths with complex fauna. These effects were significant for K+, Mg2+ and Mn2+ in the L/F-layer and for Ca2+, Mn2+, Al3+ and Fe3+ in the H-layer. As a possible explanation a rise of ion-binding sites in the course of enhanced humification processes is discussed.In the L/F-layer base cations showed higher concentrations in the monoliths with complex fauna as compared to the control plot, which contained intact roots. This might be due to nutrient uptake by roots in the control plot or enhanced mineralization in the monoliths with complex fauna, where roots had been cut.

Operator bias in software-aided bat call identification.

Software-aided identification facilitates the handling of large sets of bat call recordings, which is particularly useful in extensive acoustic surveys with several collaborators. Species lists are generated by “objective” automated classification. Subsequent validation consists of removing any species not believed to be present. So far, very little is known about the identification bias introduced by individual validation of operators with varying degrees of experience. Effects on the quality of the resulting data may be considerable, especially for bat species that are difficult to identify acoustically. Using the batcorder system as an example, we compared validation results from 21 volunteer operators with 1–26 years of experience of working on bats. All of them validated identical recordings of bats from eastern Austria. The final outcomes were individual validated lists of plausible species. A questionnaire was used to enquire about individual experience and validation procedures. In the course of species validation, the operators reduced the softwares estimate of species richness. The most experienced operators accepted the smallest percentage of species from the softwares output and validated conservatively with low interoperator variability. Operators with intermediate experience accepted the largest percentage, with larger variability. Sixty-six percent of the operators, mainly with intermediate and low levels of experience, reintroduced species to their validated lists which had been identified by the automated classification, but were finally excluded from the unvalidated lists. These were, in many cases, rare and infrequently recorded species. The average dissimilarity of the validated species lists dropped with increasing numbers of recordings, tending toward a level of ˜20%. Our results suggest that the operators succeeded in removing false positives and that they detected species that had been wrongly excluded during automated classification. Thus, manual validation of the softwares unvalidated output is indispensable for reasonable results. However, although application seems easy, software-aided bat call identification requires an advanced level of operator experience. Identification bias during validation is a major issue, particularly in studies with more than one participant. Measures should be taken to standardize the validation process and harmonize the results of different operators.

Composite sampling enhances the confidence of soil microarthropod abundance and species richness estimates

Summary The quantification of abundance and species richness of soil microarthropods is most often severely hindered by extraordinary data variability, highly skewed frequency distributions, many extreme and zero counts, and small sample sizes. We developed a composite sampling technique to enhance the confidence of abundance and species richness estimates. Many soil cores (n ♢ 100) are sampled, animals extracted, the extracts pooled, mixed, and subsamples (aliquots) taken. Compared to the standard (separate sampling units), no microarthropods were lost or mechanically damaged during the compositing procedure. The confidence of abundance estimates was substantially greater in the composite than in the standard, although not for taxa of low abundance (< ∪ 10 3 ind.m -2 ). Moreover, compositing was the superior technique in estimating species richness. The number of sampling units needed to recover a certain number of species with the composite was 70 % of the standard method. We conclude that composite sampling is a promising alternative to the standard technique and may help to increase the generally low confidence of microarthropod field data. Finally, potential limitations of composite plans are discussed: a great number of field cores from an unbiased sampling plan have to be composited; comparisons between composites of unequal size should be avoided; all information on the variation among field cores is lost by compositing; parallel measurements of fauna and other variables in the same cores are not possible.