Rock-solid differences in macro communities

Posted on March 27, 2012

To understand why aquatic communities are structured the way they are, it’s necessary to assess the importance of broadscale regional factors relative to more local influences.   Landscape geology can have profound effects on the physical and chemical environments of aquatic systems, and thus the species composition of biological communities.  The PreCambrian Shield covers most of northern North America and takes the form of ancient metamorphic or igneous rock close to the surface, below a thin layer of low-nutrient soil.  In southern Ontario there’s a sharp boundary between Shield and off-Shield regions, and Canadian scientists collected data from 125 sites spanning both zones to examine the distribution of macroinvertebrate species relative to the underlying geology.  Relationships between environmental variables measured at a subset of 58 sites were clarified  using multivariate (correspondence) analysis.   The results showed that the location of sites (i.e, Shield or off-Shield) could be accurately predicted based either on environmental features alone or on macroinvertebrate community composition alone.  Aquatic systems in the Shield region tended to have  relatively steep channel slopes and depths, higher flow velocities, and more erosion-adapted macroinvertebrate species, such as caddis, stoneflies and mayflies.   In contrast, waters in non-Shield areas, where limestone soils are more typical, were richer in elements released by weathering, had higher levels of alkalinity, conductivity, pH, productivity and turbidity, and greater numbers of depositional, pool-adapted species such as dragonflies, bugs, beetles and snails.  In distinguishing between Shield and off-Shield sites, conductivity and alkalinity were the most important  factors.   The strong influence of geology on invertebrate communities suggests that constraints set by water chemistry may limit the capacity of particular species to modify their distributional range in response to climate change.

Reference:   Neff, M.R. & Jackson, D.A.  2011.  Effects of broad-scale geological changes on patterns in macroinvertebrate assemblages.  Journal of the North American Benthological Society 30(2), 459–473.