environmental conditions of a specific habitat and because the 

 relations between these species permit them to be associated (see 

 Charts 1 and 2, pp. 18 and 19). In other words, the plants and 

 animals in the grouping are able to coexist in the environment 

 of the habitat. In addition, the evolutionary history of the asso- 

 ciated species, the environments of the past, and the relict popu- 

 lations of earlier stages of succession influence the association. 

 Plant groupings are not merely random aggregations resulting 

 from the first seeds or other propagules reaching an area. The en- 

 vironmental conditions play a most important role in determin- 

 ing the kinds of plants that become associated. For example, the 

 occurrence of each species in each stage of succession in southern 

 New Mexico-^"* and groupings within the extensive grassland of 

 California^ ^ appear to be determined by the kind of soil in the 

 various areas. Pronounced relationships have been found between 

 the grouping of plants and the soil characteristics in many places^^ 

 as, for example, on well-drained soils in the arctic tundra of 

 northern Alaska, where the correspondence is so close that the 

 vegetation type can be predicted when the soil profile and gen- 

 eral location on the Arctic slope are known.* ^^ In many com- 

 munities it appears that a state of interaction and coordination 

 between species and the physical environment is gradually pro- 

 duced. ^ This takes a long time, involving the genetic continuity 

 of species. Invaders cannot intrude, for the community develops 

 a characteristic environment which determines the kinds of plants 

 that can grow there. 



Some groups contain many species, others few. The number 

 depends largely upon (1) the nature of the habitat, including the 

 length of growing season, availability of moisture and mineral 

 nutrients, and absence of disturbing influences; (2) the degree of 

 overlapping in ranges of ecological amplitude of the species; and 

 (3) suflficient associative capacity so species can live together (see 

 Figures 1-7, 1-17, and 1-27). Each species has its own individ- 

 uality, but when it becomes a member of a group it is no longer 

 independent — there are too many interacting organisms. The in- 

 vasion of many plants is often restricted by the strong competitive 

 capacity of various dominant grasses or other plants so that the 



CSrovrnpismg o£ Species • 61 



