Macro- vs. Microhabitat 



Let us first differentiate between macro- and microhabitat features by examining a 

 river from its headwaters to its mouth. Numerous authors have reported the addition 

 or replacement of species as a function of stream order, stream size, gradient, or other 

 descriptions of longitudinal gradations on environmental considerations. ^5 An initial 

 viewpoint relates the "longitudinal succession" of species as a function of such 

 variables as mean depth, temperature, mean velocity, water quality, average 

 substrate composition, or other environmental conditions which exhibit gradational 

 change. These are the macro-features of the stream habitat. 



A second perspective is to examine local preference or response in regard to the 

 morphological, physiological, or behavioral adaptions of various species. Many 

 studies have shown that the spatial and temporal selection of certain microhabitat 

 conditions reduces interspecific competition. 2^,27,28 in fact, expansion into another 

 species' preferred microhabitat in the absence of that species (competitive release) 

 occurs less frequently in streams than one would expect. 



The geographic distribution of species in riverine systems is largely dictated by 

 those longitudinal characteristics which define the macrohabitat. In essence, the 

 characteristics of watershed and water quality establish the limits of distribution of a 

 species. These bounds are often discontinuous — subject to inversions in macro- 

 habitat gradients. 



If the macrohabitat conditions are sufficient for the growth and propagation of 

 fish, the distribution and abundance offish within the macrohabitat is a function of 

 the availability of proper microhabitat conditions. A microhabitat is then perceived 

 as a necessary subset of the macrohabitat. A macrohabitat might be adequate for 

 fishes to exist, but without the necessary microhabitat fish abundance will be limited. 

 The converse is also true. Therefore, the quantification of habitats must concern both 

 the longitudinal (macrohabitat) distribution of species and the three-dimensional 

 (microhabitat) distribution within the macrohabitat. 



Gorman and Karr^' concluded that four variables were significant in determining 

 the distribution and abundance of species in a river system. These are energy source 

 (watershed inputs), water quality, channel structure, and flow regime. From the 

 above discussion it can be argued that certain variables such as energy source and 

 water quality change longitudinally through a system and could logically be defined 

 as macrohabitat features. Channel structure and flow characteristics (hydraulic 

 structure) together determine the microhabitat, but these too change longitudinally 

 through the system. 



The approach taken by the IFG is to superimpose detailed microhabitat 

 characteristics onto more generally described, relatively homogeneous macrohabitat 

 based on changes in watershed characteristics, water quality, overall channel 

 geometry, and flow regime. Thus, a river system may be segmented into sections in 

 which the macrohabitat conditions are relatively homogeneous. Macrohabitat 

 gradations are illustrated by proceeding from one river segment to the next. 



Within each of these large, relatively homogeneous segments, small reaches are 

 randomly selected for detailed study of the relationship between microhabitat and 

 streamflow. Such reaches are called representative reaches. Variations in micro- 

 habitat, as determined by channel structure and streamflow, are described over the 

 length of macrohabitat as represented by these sample reaches. This approach allows 

 an investigator to describe not only the microhabitat conditions, but also how 

 microhabitat intergrades with macrohabitat throughout the entire river system. 

 Therefore, both the longitudinal succession perspective and the microhabitat 

 selection perspective of riverine ecology are incorporated in the approach. 



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