THE BIOTIC ENVIRONMENT OF ORGANISMS 



579 



the soil and thus provides more soil nutrients and retains more soil mois- 

 ture. Shade is provided, and the extremes of light and heat are thereby 

 reduced. The sand becomes anchored by the roots of the pioneers and 

 ceases to drift with the wind. Now other plants and animals can invade 

 the area ; they were unfitted to exist under the extreme pioneer conditions 

 but once they can find a foothold are better adapted to the increasing 



Fig. 33.14. The climax association of northern Florida. A broad-leaved, evergreen ham- 

 mock dominated by magnolia, holly, red bay, and laurel oak. (Photo taken near Gainesville, 

 Fla., by J. Speed Rogers.) 



biotic competition. In time the newcomers will replace the members 

 of the pioneer community and, as they continue to occupy the area, pro- 

 duce a deeper, more organic, and moister soil, a denser shade and more 

 humid atmosphere. They, in turn, will be supplanted by another complex 

 of organisms still better fitted to survive and reproduce in the increas- 

 ingly intense biotic competition of the ameliorated physical environment. 

 These successive changes are not unending. In time the area will be- 

 come suitable for invasion by a group of plants and animals that, although 



