ECOLOGY 283 



conditions are effective, such as light, temperature, soil, and depth of 

 water, and that intensities of the conditions are grouped in a very general 

 way in nature. Thus in lakes there are different conditions of tempera- 

 ture, gases, bottom, light, etc., at different depths; and on land the soil 

 conditions, temperature, moisture, and other factors are different in 

 swamps, grass-land areas and forests. A study of the conditions in any 

 one place soon reveals that there are not only freshwater, marine and 

 terrestrial animals but that these general habitats may be divided. Thus 

 there are lake, pond and stream forms, prairie and forest animals. Fur- 

 thermore, detailed investigations show that each one of these groups or 

 associations may be divided into numerous subdivisions, because certain 

 forms prefer specific depths, temperatures, or chemical conditions in the 

 water, or temperature, moisture, and soil conditions on the land. Along 

 with these forms occur other more tolerant species which live in two or 

 more associations, but these are not so numerous as to obscure the smaller 

 groups. The following list of the animal communities of the streams and 

 forests 'of a region on the south shore of Lake Michigan, adapted from 

 Shelf ord, will illustrate the associations into which the animals of a region 

 may be grouped according to physical conditions. The associations are 

 named either from the nature of the habitat, or from the dominant plants. 



I. Stream Communities. 



1. Intermittent stream communities. 



(a) Intermittent rapids associations. 

 {h) Intermittent pool associations. 

 (c) Permanent pool associations. 



2. Permanent stream communities. 



(o) Spring dominated stages. 



(1) Spring associations. 



(2) Spring brook associations. 

 (6) Creek and river communities. 



(1) Pelagic associations. 



(2) Rapids associations. 



(3) Sand or gravel-bottom associations. 



(4) Sandy-bottomed stream associations. 



(5) Silt or sluggish-stream communities. 



(1) Sluggish-creek associations. 



(2) Pelagic associations. 



(3) Silt-bottom associations. 



(4) Vegetation associations. 



II. Forest Communities. 



1. Elm-ash series. 



(a) Low prairie associations. 



(b) Marsh-margin thicket associations. 



(c) Elm-ash association. 



2. Tamarack or floating bog series. \ 



(a) Low prairie or floating l:)og associations. 

 (6) Marsh-margin thicket associations, 

 (c) Tamarack forest associations. 



