406 The Community 



found in situations in which the chmatic or edaphic conditions appear 

 to change only slowly. Such a result occurs when alterations in the 

 environment favor the survival of a new controlling^ species; it is seen 

 particularly among stands of rooted land plants but is also apparent 

 in the zonation of sessile plants and animals in the tidal zone. 

 Among terrestrial plants, for example, the competition for light, water, 

 and nutrients may be so keen that, at the point in the environmental 



Fig. 11.2. Cabbage palm hauunocks in the everglades of central Florida sharply 

 delineated from the surrounding sawgrass. The sawgrass community is abruptly 

 separated from the water community by a narrow ecotone in wliich a red man- 

 grove is developing. 



gradient at which the conditions favor a different dominant species, 

 the new species will take over completely, choking out the first species 

 and producing a rather distinct line between the areas dominated by 

 the two species. In approaching a pond in an oak forest, for example, 

 the trees often give way rather abruptly to a zone of high shrubs. 

 Beyond this zone are clearly marked zones of low shrubs, sedges, or 

 grasses, and the last of these gives place to the reeds and other emer- 

 gent plants at the pond margin. This distinct zonation exists in spite 

 of the fact that a quite imiform gradation in soil moisture may be 

 found from the dry ground on which the oaks stand to the completely 

 saturated soil near the water's edge. An instance of the zonation of 

 dominant plants around a shallow pool near the seashore is shown in 

 Fig. 11.3. 



