The Enigma of Soil Animal Species Diversity Revisited: The Role of Small-Scale Heterogeneity

Peer reviewedPublisher PD

Spatial microdistribution of mites and organic matter in soils and caves

Spatial variability is a key to understanding the structure and function of soil biodiversity. This research addressed the question of whether a mite centimetric spatial pattern corresponds to that of organic matter in two caves and in the mineral horizon of three forest soils. Therefore, different statistical tools were used and compared: classic aggregation analysis through variance-to-mean ratio and Morisita's index, nested hierarchical ANOVA and Moran's I autocorrelation coefficient. The spatial pattern of organic matter was generally found to be of the gradient or patch type, influenced by slope and trees. Cave sediments far from the entrance contained smaller patches of organic matter than deep soil and cave entrances. Mite density spatial patterns sometimes corresponded to that of organic matter in soils but never in caves. Correlograms of mite species composition were significant in soils but not in caves. The difference between the space available in caves compared with that in the tortuous pores of soil is thought to be a major structuring factor for mites, by influencing their mobility. Correlograms were the most dependable spatial analysis tools, as they could be statistically tested and used to clarify the patterns detected by the nested hierarchical ANOVA method

New Ereynetid mites (Acari : Tydeoidea) from karstic areas: True association or sampling bias?

A new genus and two new species of ereynetid mites, one edaphobitic, the other troglobitic, are described from three European karst areas. Free-living species of the Riccardoella-complex exhibit ramified barbules in guard setae associated with tarsal solenidia, whereas parasitic species lack these characters. Ramified barbules in guard setae are thus considered specific adaptations to soil habitat. Free-living species of the Riccardoella-complex are seemingly restricted to karstic and other calcareous-rich areas while parasitic species live exclusively on slugs and snails. The relationship between calcium and Riccardoella-complex mites is discussed. A key is provided for the genera of Ereynetinae

Soil biodiversity: myth, reality or conning?

The study of soil microarthropod biodiversity is illustrative of problems that are related to other soil organisms (fungi, for instance) or that can be found in other environments (canopy, oceanic sediments, hosts accommodating parasites, etc.). Indeed, the contribution of the soil fauna to global biodiversity remains an enigma even though, in recent years, it has received considerable attention. Our contention is that the debate on soil biodiversity will remain open - and even sterile - as long as adequate sampling methodologies are not set up, critically evaluated and largely used. First, a critical review of the sampling strategies used for soil microarthropods is presented. In addition to an extensive compilation of publications on extraction method efficiency, articles from two journals devoted to soil biology are compared for two five-year periods (before and after Erwin's papers and before and after Rio). The most frequently used extraction methods (over 90% of studies) have a poor numerical efficiency (e.g. 7-26% for the Berlese-Tullgren funnels) and also are selective with respect to their efficiency for certain taxa (variable taxonomic and functional efficiency), 75% of studies are restricted to the upper 10 cm of soil and therefore overlook largely the microarthropod populations. some groups are often neglected, however diversified they are, and the taxonomic resolution tends to become impoverished in recent years. In the second part of our study, the importance of bias induced by inadequate or restricted sampling strategies on biodiversity estimates is evaluated: densities are dramatically underestimated (down to 14 times less); conversely species aggregation, a factor advocated to explain the existence of numerous soil species, is overestimated; some functional groups may be quite overlooked; the species distribution along a gradient deduced from the sampling may be rather different from that really existing in the soil and interfere with the evaluation of beta-diversity; species richness is often crudely underestimated (down to 50%). Overall, at most 10% of soil microarthropod populations have been explored and 10% of species described. Obviously, much has still to be done to evaluate soil microarthropod biodiversity and a fortiori understand the mechanisms underlying it. Improving and renewing the soil sampling strategy is thus a prerequisite to any real advance in our knowledge of this fascinating and obscure domain

Extracting endogeous microarthropods: A new flotation method using 1,2-dibromoethane

Many methods have been proposed for extracting soil microarthropods. They are generally well adapted for specific substrates and taxonomical groups. Flotation in saturated aqueous salt solutions was tested and should not be used. Carbon tetrachloride flotation and LUDOX(TM) methods were found to be unable to extract microarthropods from loamy soil samples. Therefore, a new flotation method using 1,2-dibromoethane (DBE) was set up. It was especially efficient for soft-bodied animals and deep soil samples, which usually are very difficult to process. The method applies to any soil with low organic content, including most cultured soils. It was found to extract five times more microarthropods and 1.5 more taxa than a classic dry funnel method. Its absolute efficiency varied from 41 to 100 % depending on the taxa and stase. As DBE is hazardous, special precautions have to be taken when handling the product. (C) Elsevier, Paris

Diversity of soil oribatid mites (Acari : Oribatida) from High Katanga (Democratic Republic of Congo): a multiscale and multifactor approach

Although the soil is a major reservoir of biodiversity, our knowledge of its mesofauna remains scanty, especially in the tropics. The diversity of oribatids (149 adult oribatid mite species) is analyzed for the first time in an African soil and studied in three ecosystems of a regressive sere: forest, woodland and savanna. Savanna is the richest ecosystem overall, with 105 collected species, whereas the mean number of species per releve (alpha diversity) is highest in forest. In barren soils, the number of species observed along the sere drops regularly from the typical forest to the savanna. However, this pattern is complicated by other factors acting at different scales. The increase of oribatid richness parallels that of habitat complexity, from barren soil to termitaria colonized by grasses and trees. On a finer scale, soil properties also influence species richness, either indirectly through density (water content) or directly (total nitrogen, C/N ratio, organic matter), but their importance varies in relation to seasons. Most exclusive species (nearly 90%) are housed in the two extreme types of vegetation, forest and savanna. On a finer scale, two habitats, the typical forest and the termitaria in the savanna, are remarkable by the number of exclusive species and are worth protecting through effective conservation measures

Are there real endogeic species in temperate forest mites?

The determinants of mite diversity in soil and the reasons why so many species coexist are poorly understood. There is evidence that niche differentiation (i.e. microhabitat complexity) in the titter layers of forest floors is important, however, little is known for deeper horizons since mite density and diversity in deeper soil layers have been rarely studied. In order to address this dearth of information, we collected microarthropods from both the forest floor and the mineral soil to a depth of 1 m in two deciduous forest locations. The density exceeded 8 x 105 microarthropods M-2 in one location, and a number of individuals were collected from deep in the soil. No species was exclusively living in mineral horizons. Measurements of porosity spectrum, pH, water content, total C and total N were made at each depth and related to mite diversity and species richness. Meso- and microporosity were strongly correlated with species distribution while macroporosity and pH were correlated to density and species richness. (C) 2004 Elsevier GmbH. All rights reserved

Survey of mites in caves and deep soil and evolution of mites in these habitats

We studied adaptations to subterranean environments in Acari. Mite populations of two caves and of the mineral horizon of soil from three forest sites were analysed. In the soil, body length was limited by the pore size. The proportions of (i) predators, (ii) phoretic mites, and (iii) primitive oribatids were higher in caves. This was potentially explained by (i) polyphagy or predation on other groups and reduced interspecific competition, (ii) the fragmented structure and liability to flooding of the cave habitat, and (iii) the higher humidity in caves. Adaptations to dry habitats probably greatly influenced the evolution in Oribatida. It is proposed that numerous mites colonized caves directly from deep soil without a surface stage

Comparison of endogeic and cave communities: microarthropod density and mite species richness

Quantitative studies of mite communities in endogeic and cave ecosystems are scarce. In this paper, we tested and validated the hypotheses that (1) deep soil and cave mite communities are distinct and (2) that species composition is more variable in caves than in deep soils. Mites were sampled in May, November, and January at 15-20 cm depth in three temperate forest soils and at the surface of sediments in two caves situated directly below two of these soils. Endogeic mite densities ranged from 77 to 225 individuals/dm(3) vs. 9 to 43 in caves. Organic matter was found to be the main factor correlated to density in soils while flooding are thought to profoundly affect cave communities: it introduce accidental species that eventually die without breeding, inducing a sharp seasonal variation in mite density. Mite richness estimates amounted to about 80 species in most locations. (1) Cave populations were distinct from endogeic ones and migration between those habitat compartments is thus supposed to be infrequent. (2) Both the fragmented structure and flooding that lead to the addition of accidental species, are invoked to explain the larger variability of cave communities compared to soil communities. (C) 2005 Elsevier SAS. All rights reserved