Résultat de la recherche
Patterns of earthworm, enchytraeid and nematode diversity and community structure in urban soils of different ages
Annelids (Lumbricidae and Enchytraeidae) and nematodes are common soil organisms and play important roles in organic matter decomposition, nutrient cycling and creation of soil structure and porosity. However, these three groups have rarely been studied together and only few studies exist for urban soils. We studied the diversity and community composition of annelids and nematodes in soils spanning more than two centuries of urban soil development in Neuch^atel (Switzerland) and assessed the relationships 1) among these three groups and 2) between each group and environmental (physical, chemical and functional) characteristics of soils and soil age. While the groups of environmental variables were correlated (Mantel tests) no correlation was found between pairs of soil fauna groups and between each soil fauna group and environmental variables. More specifically, redundancy analyses showed that earthworm assemblages were best correlated with soil bulk density and with soil depth, the latter being positively correlated with soil age. Enchytraeid assemblages and the proportion of enchytraeid r-strategists were respectively best correlated with soil carbonate content and negatively correlated with soil age. Nematodes assemblages were best correlated with soil water content. Moreover, relationships between pairs of soil biota groups, and between each group and environmental (physical, chemical and functional) variables, varied along the soil age gradient (moving window analysis). This study provides new knowledge on urban soil biodiversity and how environmental conditions can influence soil diversity and community patterns in the urban context. The contrasted community patterns of earthworms, enchytraeids and nematodes in urban soils of different ages and their different ecological roles suggest that they represent potential complementary indicators of soil quality and functioning such as soil formation and organic matter dynamics.
Above- and below-ground aspects of floodplain restoration: from biodiversity to ecosystem functions
La biodiversité est le moteur du fonctionnement des écosystèmes. Elle est directement responsable des services fournis par les écosystèmes aux sociétés humaines. Cependant, la diversité de certains taxa dont beaucoup d'organismes du sol reste peu connue; et les processus qui créent, maintiennent et détruisent cette biodiversité ne sont que très partiellement compris. De ce fait, il est extrêmement difficile de restaurer la biodiversité dans les écosystèmes perturbés.
Les zones alluviales cristallisent ces problèmes. Elles accueillent une extraordinaire biodiversité qui est souvent mal caractèrisée; elles sont mondialement menacées; et il n'y a que peu de consensus en ce qui concerne les méthodes pour revitaliser leur biodiversité
Dans le but d'améliorer notre compréhension des écosystèmes dynamiques et de la multitude d'interactions écologiques qu'ils hébergent, cette thèse se concentre sur trois aspects des zones alluviales: les sols alluviaux, la biodiversité hébergée par ces sols et les changements de biodiversité entre les différents groupes étudiés le long des gradients environnementaux.
Les résultats obtenus lors de cette thèse comblent plusieurs lacunes dans la connaissance écosystémique des zones alluviales et fournissent des perspectives pour améliorer la gestion de ces écosystèmes. En effet, l'hétérogénéité spatio-temporelle de la morphologie des sols fournit des informations structurelles et fonctionnelles sur les zones alluviales qui peuvent être inclues dans les protocoles des projets de revitalisation. De plus, les conditions environnementales influencent fortement les caractéristiques fonctionnelles des communautés. Ces caractéristiques peuvent donc être exploitées comme outil de bioindication. Finalement, les résultats du présent travail ont démontré le rôle de la diversité en tant qu'assurance pour le maintient du fonctionnement des communautés dans les écosystèmes récemment revitalisés.
Les prochains défis à relever consistent à adopter une approche holistique des réseaux trophiques du sol, et définir l'importance des processus écologiques dans la structuration de la biodiversité des écosystèmes dynamiques., Biodiversity sustains ecosytem functioning and services to human societies. However, the diversity of certain taxa including many soil organisms remains poorly known and the processes creating, sustaining, and destroying biodiversity are only partially understood. As a result, restoring diversity in disturbed ecosystem remains a challenging task.
Floodplains cristallize these issues. They host an extraordinary biodiversity that is often poorly charaterized, they are threathen worldwide, and there is little agreement about the ways to restore their biodiversity.
Aiming at improving our understanding of disturbed ecosystems and the many ecological interactions they host, this PhD thesis focuses on three aspects of floodplain ecosystems: floodplain soils, below-ground biodiversity, and the changes in biodiversity among taxonomic groups and along environmental gradients.
With this respect, the results obtained during this PhD thesis fulfil several gaps in the knowledge of riparian ecosystems and provide perspectives for improved management of floodplains and riverine ecosystems. For instance, the spatio-temporal heterogeneity of soil morphology provides structural and functional information on floodplain ecosystems that can be included into restoration project protocols. Moreover, the strong impacts of environmental conditions on community functional characteristics can be developed into bioindication tools. Finally, the results of this PhD thesis revealed the role of diversity as an insurance to community functioning in recently restored ecosystems.
Future challenges include using holistic approaches for the study of the diversity and the functioning of the soil foodwebs, and disentangling the importance of assembly processes in structuring biodiversity patterns in disturbed ecosystems.
Heterogeneity of soil carbon pools and fluxes in a channelized and a restored floodplain section (Thur River, Switzerland)
Due to their spatial complexity and dynamic nature, floodplains provide a wide range of ecosystem functions. However, because of flow regulation, many riverine floodplains have lost their characteristic heterogeneity. Restoration of floodplain habitats and the rehabilitation of key ecosystem functions, many of them linked to organic carbon (C) dynamics in riparian soils, has therefore become a major goal of environmental policy. The fundamental understanding of the factors that drive the processes involved in C cycling in heterogeneous and dynamic systems such as floodplains is however only fragmentary.
We quantified soil organic C pools (microbial C and water extractable organic C) and fluxes (soil respiration and net methane production) in functional process zones of adjacent channelized and widened sections of the Thur River, NE Switzerland, on a seasonal basis. The objective was to assess how spatial heterogeneity and temporal variability of these pools and fluxes relate to physicochemical soil properties on one hand, and to soil environmental conditions and flood disturbance on the other hand.
Overall, factors related to seasonality and flooding (temperature, water content, organic matter input) affected soil C dynamics more than soil properties did. Coarse-textured soils on gravel bars in the restored section were characterized by low base-levels of organic C pools due to low TOC contents. However, frequent disturbance by flood pulses led to high heterogeneity with temporarily and locally increased C pools and soil respiration. By contrast, in stable riparian forests, the finer texture of the soils and corresponding higher TOC contents and water retention capacity led to high base-levels of C pools. Spatial heterogeneity was low, but major floods and seasonal differences in temperature had additional impacts on both pools and fluxes. Soil properties and base levels of C pools in the dam foreland of the channelized section were similar to the gravel bars of the restored section. By contrast, spatial heterogeneity, seasonal effects and flood disturbance were similar to the forests, except for indications of high CH4 production that are explained by long travel times of infiltrating water favoring reducing conditions. Overall, the restored section exhibited both a larger range and a higher heterogeneity of organic C pools and fluxes as well as a higher plant biodiversity than the channelized section. This suggests that restoration has indeed led to an increase in functional diversity.
Soil protistology rebooted: 30 fundamental questions to start with
Protists are the most diverse eukaryotes. These microbes are keystone organisms of soil ecosystems and regulate essential processes of soil fertility such as nutrient cycling and plant growth. Despite this, protists have received little scientific attention, especially compared to bacteria, fungi and nematodes in soil studies. Recent methodological advances, particularly in molecular biology techniques, have made the study of soil protists more accessible, and have created a resurgence of interest in soil protistology. This ongoing revolution now enables comprehensive investigations of the structure and functioning of soil protist communities, paving the way to a new era in soil biology. Instead of providing an exhaustive review, we provide a synthesis of research gaps that should be prioritized in future studies of soil protistology to guide this rapidly developing research area. Based on a synthesis of expert opinion we propose 30 key questions covering a broad range of topics including evolution, phylogenetics, functional ecology, macroecology, paleoecology, and methodologies. These questions highlight a diversity of topics that will establish soil protistology as a hub discipline connecting different fundamental and applied fields such as ecology, biogeography, evolution, plant-microbe interactions, agronomy, and conservation biology. We are convinced that soil protistology has the potential to be one of the most exciting frontiers in biology.
Functional responses of multi-taxa communities to disturbance and stress gradients in a restored floodplain
1. Trait-based approaches can reveal the mechanisms through which disturbances or stress impact communities, allowing comparisons of the role of different mechanisms in shaping communities among taxonomic groups. Such information can lead to higher comparability, transferability and predictability of the outcome of restoration projects. However, multitaxa trait-based approaches were rarely used in the context of ecosystem restoration. 2. We investigated the responses to environmental gradients of seven taxa (vascular plants, staphylinid and carabid beetles, spiders, isopods, diplopods and earthworms) in a restored floodplain using a species traits approach. We assessed the impact of flood disturbances and soil hydric stress on the functional diversity (FD) and community-weighted mean (CWM) response of traits for each taxon. 3. Ordination of hydrological variables revealed two main gradients. The first was related to the spatiotemporal dynamics of flood disturbances and the second to the average changes in soil hydric conditions. 4. The analysis of CWM revealed that larger, poorly mobile species with narrow ecological tolerances were filtered by regular floods and/or changes in soil hydric conditions. 5. Functional diversity patterns differed between the two gradients: decreasing with increasing flood disturbance, but increasing along the soil hydric stress gradient. This suggests that the mechanisms shaping community composition differ between the two gradients with environmental filtering being dominant with increasing flood disturbances and competition decreasing with more soil hydric stress. 6. Synthesis and applications. Our study shows that the impact of restored flood disturbances and soil hydric stress on plant and invertebrate functional diversity and community- weighted mean can be positive, negative or more complex depending on the taxonomic group and environmental gradient considered. The patterns can to some extent be explained by the specific characteristics of each group. Larger, poorly mobile species with narrow ecological tolerances were particularly vulnerable to changes in disturbance and stress regime following floodplain restoration. These species may therefore be lost in the initial phases of restoration projects, but other more characteristic species of dynamic floodplains will be favoured. Understanding the consequences of these contrasted responses for biodiversity conservation and ecosystem functioning constitutes the next challenge for ecosystem restoration.
Patterns of earthworm communities and species traits in relation to the perturbation gradient of a restored floodplain
Little is known about the diversity and ecology of earthworms in floodplains, as well as their response to natural and anthropic perturbations (e.g. floods, river channelisation, floodplain restoration). We characterised the patterns of earthworm communities and species traits in the different habitats of a lowland restored floodplain in Switzerland. In addition to classical species-based metrics, such as species richness and Shannon diversity, species traits were used to calculate the community weighted means (CWMs) of traits and functional dispersion (FDis). We hypothesised that trait-based metrics would reveal clearer patterns than classical approaches. The distribution of earthworm traits varied among habitats in relation to changes in flooding frequency: poorly developed gravel bar soils most exposed to flooding were characterised by high abundance of small epigeic species and low abundance of large anecic species. Differences in anecic and endogeic earthworm community structure matched flood frequency. In agreement with our hypothesis, CWMs were more strongly correlated to environmental variables than species composition, diversity, or functional diversity. Based on these results, the ratio of the relative abundances of epigeic and anecic species, and the differences in species composition within anecic and endogeic ecological types of earthworms were identified as indicators of soil development in floodplains.
Patterns of earthworm, enchytraeid and nematode diversity and community structure in urban soils of different ages
Annelids (Lumbricidae and Enchytraeidae) and nematodes are common soil organisms and play important roles in organic matter decomposition, nutrient cycling and creation of soil structure and porosity. However, these three groups have rarely been studied together and only few studies exist for urban soils. We studied the diversity and community composition of annelids and nematodes in soils spanning more than two centuries of urban soil development in Neuchâtel (Switzerland) and assessed the relationships 1) among these three groups and 2) between each group and environmental (physical, chemical and functional) characteristics of soils and soil age.
While the groups of environmental variables were correlated (Mantel tests) no correlation was found between pairs of soil fauna groups and between each soil fauna group and environmental variables. More specifically, redundancy analyses showed that earthworm assemblages were best correlated with soil bulk density and with soil depth, the latter being positively correlated with soil age. Enchytraeid assemblages and the proportion of enchytraeid r-strategists were respectively best correlated with soil carbonate content and negatively correlated with soil age. Nematodes assemblages were best correlated with soil water content. Moreover, relationships between pairs of soil biota groups, and between each group and environmental (physical, chemical and functional) variables, varied along the soil age gradient (moving window analysis).
This study provides new knowledge on urban soil biodiversity and how environmental conditions can influence soil diversity and community patterns in the urban context. The contrasted community patterns of earthworms, enchytraeids and nematodes in urban soils of different ages and their different ecological roles suggest that they represent potential complementary indicators of soil quality and functioning such as soil formation and organic matter dynamics.
Response of Sphagnum Peatland Testate Amoebae to a 1-Year Transplantation Experiment Along an Artificial Hydrological Gradient
Peatland testate amoebae (TA) are well-established bioindicators for depth to water table (DWT), but effects of hydrological changes on TA communities have never been tested experimentally. We tested this in a field experiment by placing Sphagnum carpets (15 cm diameter) collected in hummock, lawn and pool microsites (origin) at three local conditions (dry, moist and wet) using trenches dug in a peatland. One series of samples was seeded with microorganism extract from all microsites. TA community were analysed at T0: 8–2008, T1: 5–2009 and T2: 8–2009. We analysed the data using conditional inference trees, principal response curves (PRC) and DWT inferred from TA communities using a transfer function used for paleoecological reconstruction. Density declined from T0 to T1 and then increased sharply by T2. Species richness, Simpson diversity and Simpson evenness were lower at T2 than at T0 and T1. Seeded communities had higher species richness in pool samples at T0. Pool samples tended to have higher density, lower species richness, Simpson diversity and Simpson Evenness than hummock and/or lawn samples until T1. In the PRC, the effect of origin was significant at T0 and T1, but the effect faded away by T2. Seeding effect was strongest at T1 and lowest vanished by T2. Local condition effect was strong but not in line with the wetness gradient at T1 but started to reflect it by T2. Likewise, TA-inferredDWTstarted to match the experimental conditions by T2, but more so in hummock and lawn samples than in pool samples. This study confirmed that TA responds to hydrological changes over a 1-year period. However, sensitivity of TA to hydrological fluctuations, and thus the accuracy of inferred DWT changes, was habitat specific, pool TA communities being least responsive to environmental changes. Lawns and hummocks may be thus better suited than pools for paleoecological reconstructions. This, however, contrasts with the higher prediction error and species’ tolerance for DWT with increasing dryness observed in transfer function models.
Toward the use of testate amoeba functional traits as indicator of floodplain restoration success
Functional traits (FT) offer a new framework to understand the ecology of organisms and overcome taxonomic difficulties that currently limit the study of minute soil taxa. FT are likely to be selected by environmental filters and hence they may provide more direct information on ecosystem characteristics than the species composition of a community. We tested the potential of testate amoeba (TA) functional traits as bioindicators of selected ecosystem processes in the context of a restored floodplain in north-western Switzerland. The floodplain was divided into six functional process zones (FPZs) associated to distinct post-restoration successional stages. We selected TA FT and computed three functional indices: functional richness (FRic), divergence (FDiv), evenness (FEve), and dispersion (FDis). We then compared the patterns of functional indices and classical diversity indices such as species richness, diversity and evenness. We assessed whether traits converged or were over-dispersed in the different FPZs using a randomization procedure. Finally, we related environmental variables and functional traits using the "Fourth Corner" statistic. This procedure enabled us to highlight relations that can potentially be used for bioindication. Promising candidates include the relationships between shell biovolume and vegetation structure and between shell compression and plant litter input variables. (C) 2011 Elsevier Masson SAS. All rights reserved.
Dispersal limitations and historical factors determine the biogeography of specialized terrestrial protists
Recent studies show that soil eukaryotic diversity is immense and dominated by micro‐organisms. However, it is unclear to what extent the processes that shape the distribution of diversity in plants and animals also apply to micro‐organisms. Major diversification events in multicellular organisms have often been attributed to long‐term climatic and geological processes, but the impact of such processes on protist diversity has received much less attention as their distribution has often been believed to be largely cosmopolitan. Here, we quantified phylogeographical patterns in Hyalosphenia papilio, a large testate amoeba restricted to Holarctic Sphagnum‐dominated peatlands, to test if the current distribution of its genetic diversity can be explained by historical factors or by the current distribution of suitable habitats. Phylogenetic diversity was higher in Western North America, corresponding to the inferred geographical origin of the H. papilio complex, and was lower in Eurasia despite extensive suitable habitats. These results suggest that patterns of phylogenetic diversity and distribution can be explained by the history of Holarctic Sphagnum peatland range expansions and contractions in response to Quaternary glaciations that promoted cladogenetic range evolution, rather than the contemporary distribution of suitable habitats. Species distributions were positively correlated with climatic niche breadth, suggesting that climatic tolerance is key to dispersal ability in H. papilio. This implies that, at least for large and specialized terrestrial micro‐organisms, propagule dispersal is slow enough that historical processes may contribute to their diversification and phylogeographical patterns and may partly explain their very high overall diversity.