Sandrine Salmon

Earthworm excreta attract soil springtails: laboratory experiments on Heteromurus nitidus (Collembola : Entomobryidae)

Abstract Microarthropods are often found more abundantly in soils with earthworms than in soils without. Earthworms probably create a favourable environment for microarthropods but few studies have aimed to explain this earthworm effect. The soil collembolan (Hexapoda) Heteromurus nitidus , living in soils at pH>5 only and thus rich in earthworms, is particularly attracted by earthworms in humus cores. The effect of earthworms on the distribution of H. nitidus can be mediated either by direct contact or by odour perception. Two experimental designs were used to determine the pathway of attraction. The first set of experiments studied the effect of direct contact with earthworm excreta on the distribution of H. nitidus . The mixture of urine and mucus of the lumbricid earthworms Aporrectodea giardi and Alollobophora chlorotica significantly attracted H. nitidus as compared to deionized water while fresh earthworm casts were not preferred to calcic mull made of older casts. The same experiment involving direct contact with mucus and methyl blue showed that Collembola sucked on mucus/urine, indicating that the interaction of Collembola and earthworms was at least partly trophic. The second experiment demonstrated that H. nitidus was attracted by the odour of Aporrectodea giardi at short distance. The odour of excreta (mucus, urine and casts) of Aporrectodea giardi also attracted H. nitidus but this attraction was weaker and did not occur constantly, possibly due to interactions with light and aggregation pheromones. We conclude that the prominent pathway by which earthworms could attract H. nitidus in the field is through direct contact with earthworm mucus and urine. The acid-intolerant distribution of this species in the field could be partly explained by a trophic interaction with some earthworm species.

Distribution of Heteromurus nitidus (Hexapoda, Collembola) according to soil acidity : interactions with earthworms and predator pressure

Abstract Culture (8 weeks, in sieved fresh humus) and choice (16 weeks in compartmented boxes containing fresh or defaunated humus, or 5 days on compacted humus) experiments at varying pH values demonstrated that the soil-dwelling Collembolan Heteromurus nitidus (Entomobryomorpha) can live and even prefer humus with pH

Does the invasive species Reynoutria japonica have an impact on soil and flora in urban wastelands

Invasive plants are recognised as a major threat to biodiversity. Although they are well-established in natural areas, the supposed negative impacts of invasive plants upon communities and ecosystems have so far been poorly investigated in urban areas, where invasions are a main issue for ecologists and for urban planners and managers. We propose to assess the effects of an invasive species along an invasion gradient in a typical urban habitat. We focused on the Japanese knotweed (Reynoutria japonica Houtt.), a widespread invasive species in Europe and North America. We considered eight urban wastelands invaded by this species in the heart of the Greater Paris Area, France. On each site, we ran four transects from the centre of the Japanese knotweed patch towards the uninvaded peripheral vegetation. We recorded the flora using the line intercept method, and several soil parameters (thickness of A horizon, abundance of earthworm casts, topsoil Munsell value, pH) every metre along each transect. The A horizon was thicker and the topsoil darker under R. japonica canopy. Thus, this invasive plant species seemed to influence soil organic matter pool. However, our results also steadily showed that R. japonica locally excluded and/or severely reduced the cover of many plant species through competition. Our study clarified the local effects of R. japonica: an influence on the soil organic matter, and a severe negative impact on wasteland plant communities. We suggest implications in both conservation and restoration ecology.

Study of sucrose and mannitol transport in plasma-membrane vesicles from phloem and non-phloem tissues of celery (Apium graveolens L.) petioles

The mature petiole of celery is an organ with versatile sink/source capacities where sucrose and mannitol are unloaded from and reloaded into the phloem cells. Plasma-membrane vesicles were purified by twophase partitioning either from phloem strands isolated from mature petioles of celery (Apium graveolens L.) or from mature petioles devoid of vascular bundles. Both types of vesicle were comparable in purity (more than 86% of plasma-membrane origin), size (135 nm diameter) and orientation (72% right-side-out). Plasma-membrane vesicles from phloem tissues had a higher vanadate-sensitive ATPase activity than plasma-membrane vesicles from petioles. Plasma-membrane vesicles from phloem tissues accumulated mannitol and sucrose in response to an artificial proton-motive force, in agreement with the existence of proton/substrate carriers. Plasma-membrane vesicles from petioles devoid of vascular bundles accumulated only mannitol following application of an artificial proton-motive force. The data suggest the volvement of apoplasmic transport events. The pathway for sucrose uptake in storage parenchyma cells is discussed in the light of the available physiological data.

The impact of earthworms on the abundance of Collembola: improvement of food resources or of habitat?

I assessed the direct influence of earthworm excretions, and the impact of earthworms through their action on the soil structure (increased macroporosity), on the population dynamics of the collembolan species Heteromurus nitidus. The intestinal content of Collembola arising from cultures on different soil types was observed, and two experimental cultures of H. nitidus were run: (1) a culture performed on an inert substrate supplied either with earthworm casts or with soil as food resource, (2) an experiment using microcosms with cores of two humus forms (moder and calcic mull), in the presence or absence of earthworms. The observation of gut contents revealed that H. nitidus feeds on excrements, the composition of which (ratio organic matter/mineral matter) varies according to the humus form where it lived. Slightly aged (10–15 days) organo-mineral casts of earthworms appeared to be a better food than calcic mull aggregates or organic material from moder. Densities of H. nitidus cultured in cores of calcic mull were higher than in moder, except when cores of moder were inhabited by an anecic earthworm for 2 months. The humus form strongly influenced populations of H. nitidus, firstly because densities of predators were higher in moder than in calcic mull, and probably also because of soil macroporosity. It was concluded that earthworms would affect predation on H. nitidus by creating a network of interconnected macropores in which Collembola can move and find shelter.

A Thesaurus for Soil Invertebrate Trait-Based Approaches

Soil invertebrates are known to be much involved in soil behaviour and therefore in the provision of ecosystem services. Functional trait-based approaches are methodologies which can be used to understand soil invertebrates’ responses to their environment. They (i) improve the predictions and (ii) are less dependent on space and time. The way traits have been used recently has led to misunderstandings in the integration and interpretation of data. Trait semantics are especially concerned. The aim of this paper is to propose a thesaurus for soil invertebrate trait-based approaches. T-SITA, an Internet platform, is the first initiative to deal with the semantics of traits and ecological preferences for soil invertebrates. It reflects the agreement of a scientific expert community to fix semantic properties (e.g. definition) of approximately 100 traits and ecological preferences. In addition, T-SITA has been successfully linked with a fully operational database of soil invertebrate traits. Such a link enhances data integration and improves the scientific integrity of data.

Responses to light in a soil-dwelling springtail

It has been widely assumed that Collembola respond to light, but until now there has been very little experimental proof of this. Field observations allowed to distinguish soil-dwelling species that would escape from light from surface-dwelling species that would be attracted to light. However, the supposed effect of light could be due to other factors such as temperature or dryness. We demonstrated that individuals of the collembolan species Heteromurus nitidus (Entomobryidae), when placed in a light gradient (temperature and moisture being homogeneous), clustered in the darker area. This effect occurred rapidly and changes in the distri- bution of animals, persisted after illumination ceased. This shows light to act as a strong repellent for this soil-dwelling collembolan species. 0 1999 Editions scientifiques et mkdicales Elsevier SAS

Ionic identity of pore water influences pH preference in Collembola.

A test system described by Van Straalen and Verhoef [J Appl Ecol 34 (1997) 217] was used in order to check whether the endogeic Collembolan Heteromurus nitidus was repelled by acid pH. In each of the eight experimental runs 16 naive animals were allowed to select sectors in a circular pH gradient made of pure quartz sand impregnated with McIlvaines buffer solutions at constant osmolarity. Di-sodium or di-potassium hydrogen phosphate was mixed with citric acid in varying proportions, giving rise to acidity levels ranging from pH 2 to 9. The animals reacted quite differently according to whether Na or K was used as the metallic cation. With potassium, a strong variation was observed from one experimental box to another, H. nitidus aggregating at pH levels varying from 4 to 8, the most frequent aggregation being observed at pH 6. With sodium, aggregation occurred over a more restricted range, from pH 7 to 9, most frequent aggregation being at pH 8. It was concluded that the most acidic pH range (2–3) was avoided by the animals and that the chemical composition of buffer solutions strongly influenced results of pH-preference tests. If we consider that (i) the absence of H. nitidus from acid soils (pH 5–3.5) can be explained by biological rather than by chemical effects, (ii) this species can be cultured in acid as well as alkaline soils in the absence of predators, it ensures that potassium gives results that are more representative of the distribution of H. nitidus according to soil acidity than sodium.

Influence of the spatial variability of soil type and tree colonization on the dynamics of Molinia caerulea (L.) Moench in managed heathland

European heathland communities on acid, nutrient-poor soils have a high ecological value due to their special environmental conditions. Natural succession (tree colonization and the emergence of grasses) poses a threat to this type of habitat and different types of management strategy must be considered if it is to be maintained. A previous study on a dry heathland area located in the Fontainebleau forest (France) showed a gradual shift from a pure ericaceous stand to a mosaic of grasses and Ericaceae, despite the application of measures such as removal of woodland species to sustain the habitat. Habitat change was due to local expansion of a grass, Molinia caerulea (L.) Moench. The present paper aimed to identify factors responsible for the expansion of M. caerulea and the subsequent decrease in ericaceous heath. We focused our study on spatial variability of soil properties (soil horizons, pH, water content) and reforestation (density of birch individuals and proximity to woodland) as a suite of possible factors promoting the expansion of M. caerulea. We show that the development of grasses was correlated with thin soil E horizon and spatial distribution of old shoots of birch, Betula pendula Roth, which are regularly cut and then resprout. These results suggest that new methods to avoid tree colonization must be introduced if typical heathland is to be maintained.

Shifts and linkages of functional diversity between above‐ and below‐ground compartments along a flooding gradient

Summary Trait-based approaches have the potential to reveal general and predictive relationships between organisms and ecosystem functioning. However, the mechanisms underlying the functional structure of communities are still unclear. Within terrestrial ecosystems, several studies have shown that many ecological processes are controlled by the interacting above- and below-ground compartments. However, few studies have used traits to reveal the functional relationships between plants and soil fauna. Mostly, research combining plants and soil fauna solely used the traits of one assemblage in predictive studies. Above-ground (plants) and below-ground (Collembola) compartments were sampled over a flooding gradient in northern France along the Seine River. First, we measured the effect of flooding on functional and taxonomic assembly within both communities. We then considered the linkages between plant and Collembolan species richness, community traits and assessed whether traits of both compartments converged at high flooding intensity (abiotic filtering) and diverged when this constraint is released (biotic filtering). Species richness of both taxa followed the same bell-shaped pattern along the gradient, while a similar significant pattern of functional richness was only observed for plants. Further analyses revealed a progressive shift from trait convergence to divergence for plants, but not for Collembola, as constraints intensity decreased. Instead, our results highlighted that Collembola traits were mainly linked to the variations in plant traits. This leads, within Collembola assemblages, to convergence of a subset of perception and habitat-related traits for which the relationship with plant traits was assessed. Synthesis. Using a trait-based approach, our study highlighted that functional relationships occur between above- and below-ground compartments. We underlined that functional composition of plant communities plays a key role in structuring Collembola assemblages in addition to the role of abiotic variables. Our study clearly shows that functional diversity provides a new approach to link the above- and below-ground compartments and might, therefore, be further considered when studying ecological processes at the interface between both compartments.