412 The Community ^ 



area is an ecotone only if tension exists between the bordering com- 

 munities, and this is often difficult to demonstrate, especially for 

 large areas. Accordingly, the decision as to what is to be considered 

 an ecotone depends upon the scope of the biocenose as recognized by 

 the individual ecologist. 



In the ecotone area the conditions of temperature, moisture, light, 

 wind, and other physical influences are different from, and usually 

 intermediate between, those existing well within either of the border- 

 ing communities. These or other conditions, such as food or shelter, 

 may be superior in the region of the ecotone for certain species. 

 These special influences of the ecotone area prevail below ground and 

 high in the vegetation as well as on the surface of the soil. As a 

 consequence various kinds of plants and animals not occurring, or rela- 

 tively rare, in the bordering communities may become abundant in 

 the ecotone. Shrubs typically grow in profusion at the forest edge 

 and harbor a distinct fauna. At the margin of a pond willows and 

 cattails thrive in the transition between land and water, and here 

 are found such animals as turtles, frogs, herons, red-winged black- 

 birds, muskrats, and a host of invertebrates that are entirely absent 

 or much less abundant in the center of the pond or in the terrain far 

 removed from the water. 



These species favored by the special conditions of the ecotone join 

 with the outpost representatives of the principal inhabitants of the 

 bordering communities in populating the ecotone. The species oc- 

 curring in the ecotone may thus form a distinct functional community 

 of their own. It must be remembered, however, that the ecotone 

 inhabitants owe their existence to the presence of the particular con- 

 ditions on each side and that the ecotone assemblage would disappear, 

 or be considerably modified, if the bordering communities or condi- 

 tions were removed or seriously changed. 



As a rule the ecotone contains more species and often a denser 

 population than either of the neighboring communities, and this gen- 

 erality is known as the principle of edges. The greater variety of 

 plants in the ecotone provides more cover and food, and thus a greater 

 number of animals can be supported. In measurements of bird 

 populations in a variety of areas in the central part of the United 

 States Kendeigh ( 1944 ) and Johnson ( 1947 ) distinguished between 

 "forest-edge" species and species which confine their territory, nest- 

 ing, feeding, and roosting to the interior of the forest. 



As conservationists and wildlife managers have become aware of 

 the principle of edges, they have bent their efforts toward increasing 

 the amount of available ecotone area in each region since this usually 



I 



