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Mitchell, Edward

Nom
Mitchell, Edward
Affiliation principale
Site web
Fonction
Professeur ordinaire
Email
edward.mitchell@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/357
_
0000-0003-0358-506X

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  • Publication
    Accès libre
    Community development along a proglacial chronosequence: are above-ground and below-ground community structure controlled more by biotic than abiotic factors?
    (2010)
    Carlson, Matthew L.
    ;
    Flagstad, Lindsey A.
    ;
    Gillet, François 
    ;
    Mitchell, Edward 
    1. We studied vascular plant and soil-dwelling testate amoeba communities in deglaciated sites across a range of substrate ages in Kenai Fjords, Alaska, USA to test four hypotheses. (i) Patterns of community assembly are similar for vascular plants and testate amoebae. (ii) Vascular plant and testate amoeba communities are more strongly correlated to abiotic variables than to each other, since these communities are not directly linked trophically. (iii) Plant community structure becomes less associated with abiotic condition in succession relative to testate amoebae, as species replacement is believed to be more common for plants than testate amoebae. (iv) Above- and below-ground communities become more strongly linked over the succession, due a shift from predominantly allogenic to autogenic forces.
    2. We assessed relationships among biotic communities and abiotic site variables across the chronosequence using multiple factor analysis, redundancy analysis (RDA) and a moving-window analysis. 3. The diversity patterns and the communities’ response to site and soil variables differed between groups. The composition of both communities was significantly explained by bedrock type and moisture regime. The vascular plant community, however, was more influenced by distance from the glacier.
    4. Testate amoeba and vascular plant community patterns were significantly linked to each other and to location and physical conditions. The moving-window RDA indicates the variation explained by the physical and chemical environment tended to slightly decrease through the chronosequence for testate amoebae, while a bell-shape response was evidenced for vascular plants. The variation of the microbial community explained by the plant community was very low in the early stages of the succession and became higher than the variation explained by the environmental variables later in the chronosequence.
    5. Synthesis. These results suggest that vascular plants and testate amoebae are as linked or more in ecosystem development than either community is to changes in site condition. Furthermore, the strength of interactions varies along the succession. Thus, ecological links may be more important than macro-scale abiotic site condition is to community development, even between communities without direct trophic interactions.
  • Publication
    Accès libre
    Ecology of testate amoebae (Protista) in south-central Alaska peatlands: building transfer-function models for palaeoenvironmental studies
    (2006)
    Payne, Richard J.
    ;
    Kishaba, Keiko
    ;
    Blackford, Jeff J.
    ;
    Mitchell, Edward 
    Testate amoebae are valuable indicators of peatland hydrology and have been used in many palaeoclimatic studies in peatlands. Because the species' ecological optima may vary around the globe, the development of transfer function models is an essential prerequisite for regional palaeoclimatic studies using testate amoebae. We investigated testate amoebae ecology in nine peatlands covering a 250-km north-south transect in south-central Alaska. Redundancy analysis and Mantel tests were used to establish the relationship between the measured environmental variables (water-table depth and pH) and testate amoebae communities. Transfer-function models were developed using weighted averaging, weighted average partial least squares and maximum likelihood techniques. Model prediction error was initially 15.8 cm for water-table depth and 0.3 for pH but this was reduced to 9.7 cm and 0.2 by selective data exclusion. The relatively poor model performance compared with previous studies may be explained by the limitations of one-off water-table measurements, the very large environmental gradients covered and by recent climatic change in the study area. The environmental preferences of testate amoebae species agree well with previous studies in other regions. This study supports the use of testate amoebae in palaeoclimate studies and provides the first testate amoebae transfer function from Alaska.