out till' year. cliTiiate contimunisly waiiii and 

 wet. 



Needle-leaved evergreen or coniferous forest : 

 l'ore.st.>; of ])iiu's. s])riices. t'lrs. larciies, lieiu- 

 k.cks. ami tlie like. 



Forest-tundra: stunted o\w\\ k''"^^''' of conifer- 

 ous forests in cold climates. 



Tundra: extensive flat or gently rolling treeless 

 areas occurring in cold climates. 



Alpine tundra : treeless areas at liiglier elevations 

 of moinitains. 



Bog: wet areas in northern climates containing 

 sphagnum, heath plants, coniferous trees. 



Swamp : wet areas covered with trees or shrubs. 



Marsh: wet areas containing sedges, rushes, cat- 

 tails, and the like. 



Inasmuch as animals choose niches in response 

 primarily to the physical structure of the vegetation 

 regardless of its taxonomic composition, it is helpful 

 in describing biotic communities to show the vegeta- 

 tion structure in as much detail as possible. This 

 may be done by semi-realistic diagrams or by a sys- 

 tem of symbols (Dansereau 19.S1). 



The 50 



per 



cent rule 



If the primary basis for community recognition 

 is based on the life-form of the dominants, which on 

 land is expressed in the physiognomy of the vegeta- 

 tion and in some aquatic habitats on the life form of 

 the predominant animals, then the secondary break- 

 down of community units must be on the basis of tax- 

 onomic units. Here, the species unit is most useful, 

 as the species is the smallest taxon having objective 

 reality and precise interrelations with its environ- 

 ment. 



Two aggregations of species occurring naturally 

 in different areas or in the same area at different 

 times are to be considered as distinct communities 

 when at least 50 per cent of the predominant species 

 of each aggregation are if not exclusive at least char- 

 acteristic to the aggregation. This we may call the 

 50 per cent rule. The recognition of communities 

 should not be influenced by the presence of rare 

 species, for such are near the boundary of their habi- 

 tat or geographic range. It is important to have quanti- 

 tative information on the size of the populations to 

 evaluate the importance of each species before com- 

 munity classification is attempted (Sparck 1935). 



The distinctiveness of communities must work in 

 both directions ; that is to say, 50 per cent or more 

 of the important species of each aggregation must be 

 different from the other aggregation. This means 

 that the two aggregations are more different than 

 they are alike. If the species composition does not 



exliihit the 50 i)er cent distinction, the two aggrega- 

 tions are considered as belonging to the same com- 

 nuinity. If the difference ap|)ro;iches but does not 

 e(|iial 50 per cent, it is often worthwhile to designate 

 the two aggregations as jacifs if they are serai, or 

 farialions if they are climax, of the s.une community. 

 It is i^referable to u.se this criterion for differentiating 

 communities, in the light of present ecological knowl- 

 edge, rather than use more involved statistical cri- 

 teria (Hray 1956). The 50 per cent rule has been 

 earlier ap])lied for separating zoogcographic regions 

 (Mavr 1944). 



!\ anting lommunities 



Since comnnmities are distinguished by differ- 

 ences in life form and taxonomic composition of the 

 dominant or predominant organisms, these charac- 

 teristics are usually used also in naming the com- 

 munity. Where the habitat is well defined but vege- 

 tation is largely or wholly lacking, as in many acjuatic 

 communities, habitat may be used in the terminology. 

 .Since names are largely a matter of convenience, 

 they should be short and be derived from some easily 

 recognized feature of the community or habitat. Very 

 often the generic names of two, sometimes three, 

 conspicuous dominants are used to name plant com- 

 munities ; two or three predominant characteristic or 

 exclusive animal species, together with the prevail- 

 ing type of vegetation or habitat, are employed to 

 name animal communities. In case of some large 

 communities, geographic names are more convenient. 



Large geographic units, differentiated on the basis 

 of difference in the climax type of vegetation, are 

 called hiomes. They are specifically named by the 

 characteristic form of vegetation present : tundra 

 biome, or grassland biome, for instance. 



Secondary communities within the biome can be 

 distinguished as climax or serai, respectively, by the 

 suffixes -iation and -ies. An association is a climax 

 plant community identified by the combination of 

 dominant species present ; an associes is an equiva- 

 lent serai plant community. Thus we may speak, for 

 instances, of the Fagits-Acer association, which is a 

 climax deciduous forest community, and of the Ca- 

 lamoyrostis-Andropogon associes, which is a grass 

 stage in a sand sere (Clements and Shelford 1939). 



Animal communities on land are related to differ- 

 ent life-forms of plants or types of vegetation, but 

 only seldom to plant communities distingui.shed by 

 the taxonomic composition of the plant dominants. 

 Thus animal communities must be analyzed and 

 named independently of plant communities. A bi- 

 ociation is a climax animal or biotic community 

 identified by the distinctiveness of the predominant 

 animal species ; a hiocics is the serai equivalent. The 



The biotic community 29 



