410 The Community 



the light with Httle difference whether the interfering neighbors are 

 of the same or of different species. 



In many commmiities one species is particularly conspicuous 

 because it is the largest or the most numerous. This species is often 

 called the dominant, and its name is usually given to the community, 

 as in a spruce forest community. Occasionally two or more species 

 share the honors in respect to dominance, as in an oak-hickory forest. 

 Very frequently the species that is dominant in conspicuousness also 

 appears to exert a controlling inffuence over the other members of the 

 community, but sometimes a less conspicuous species may have a pre- 

 ponderant inffuence on the inhabitants of the biotope. In a sphag- 

 num-heath community, for example, the heath plants are the most 

 conspicuous, but the presence of the sphagnum controls the nature of 

 the community as a whole. In some biocenoses microorganisms may 

 play the critical part although they are among the least conspicuous 

 of the species present. The seedlings of pine and some other trees 

 do not grow properly unless suitable fungi are present to form mycor- 

 rhizae on their roots. Thus, although the species designated as the 

 dominant frequently does exert the major controlling influence on 

 the biocenose, this is not necessarily true even in those communities 

 in which a high degree of interspecies dependence exists. 



In the second general type of situation, in which the members of 

 the community owe their presence primarily to a more or less direct 

 and independent control by physical features of the environment, 

 a species designated as dominant because of prominence or abun- 

 dance clearly does not exert a critical control over the other inhabit- 

 ants. For this reason some ecologists refer to such a species as "pri- 

 mary" rather than dominant. Dominance based on controlling in- 

 ffuence must be clearly distinguished from dominance based on mere 

 conspicuousness. In communities of low interspecies dependence the 

 relative numbers and prominence of the various species are likely to 

 vary from place to place. Great care must be taken in applying 

 names to such communities to avoid ambiguity and the implication 

 that conspicuous species are necessarily controlling. 



Ecotone 



The transition zone of tension between communities presents a 

 situation of special ecological interest and is known as an ecotone. 

 The border between forest and grassland, the bank of a stream run- 

 ning through a meadow, or the boundaries between any other com- 

 munities on land or in the water furnish illustrations of ecotones 



