Scale-related patterns in the spatial and environmental components of stream macroinvertebrate assemblage variation

Authors

  • Heikki Mykrä,

    Corresponding author
    1. University of Jyväskylä, Department of Biological and Environmental Science, PO Box 35, FIN-40014 University of Jyväskylä, Finland,
    2. University of Oulu, Department of Biology, PO Box 3000, FIN-90014 University of Oulu, Finland
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  • Jani Heino,

    1. Finnish Environment Institute, Research Programme for Biodiversity, PO Box 413, FIN-90014 University of Oulu, Finland,
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  • Timo Muotka

    1. Finnish Environment Institute, Research Programme for Biodiversity, PO Box 140, FIN-00251 Helsinki, Finland and
    2. University of Oulu, Department of Biology, PO Box 3000, FIN-90014 University of Oulu, Finland
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*Correspondence: Heikki Mykrä, University of Jyväskylä, Department of Biological and Environmental Science, PO Box 35, FIN-40014 University of Jyväskylä, Finland. E-mail: heikki.mykra@bytl.jyu.fi

ABSTRACT

Aim  We examined the relative contributions of spatial gradients and local environmental conditions to macroinvertebrate assemblages of boreal headwater streams at three hierarchical extents: bioregion, ecoregion and drainage system. We also aimed to identify the environmental variables most strongly related to assemblage structure at each study scale, and to assess how the importance of these variables is related to regional context and spatial structuring at different scales.

Location  Northern Finland (6268° N, 25–32° E).

Methods  Variation in macroinvertebrate data was partitioned using partial canonical correspondence analysis into components explained by spatial variables (nine terms from the cubic trend surface regression), local environmental variables (15 variables) and spatially structured environmental variation.

Results  The strength of the relationship between assemblage structure and local environmental variables increased with decreasing spatial extent, whereas assemblage variation related to spatial variables and spatially structured environmental variation showed the opposite pattern. At the largest extents, spatial variation was related to latitudinal gradients, whereas spatial autocorrelation among neighbouring streams was the likely mechanism creating spatial structure within drainage systems. Only stream size and water acidity were consistently important in explaining assemblage structure at all study scales, while the importance of other environmental variables was more context-dependent.

Main conclusions  The importance of local environmental factors in explaining macroinvertebrate assemblage structure increases with decreasing spatial extent. This scale-related pattern is not caused solely by changes in study extent, however, but also by variable sample sizes at different regional extents. The importance of environmental gradients is context-dependent and few factors are likely to be universally important correlates of macroinvertebrate assemblage structure. Finally, our results suggest that bioassessment should give due attention to spatial structuring of stream assemblages, because important assemblage gradients may not only be related to local factors but also to biogeographical constraints and neighbourhood dispersal processes.

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