DYNAMIC NATURE OF CLIMAX 249 



jacent climaxes, and these constitute the preclimaxes and postclimaxes 

 that form the most ilhiminating pieces of the mosaic. 



Next in significance arc the topographic processes that initiate suc- 

 cession and bring about ontogenetic (short successional) changes in 

 the chmax by contrast with the phylogenetic (chmatic) ones just men- 

 tioned. Lake, pond and stream, lava flow, rock fields, dunes and sand 

 hills all break the continuity of the biome and serve as foci for the 

 development of hydrosere and xerosere. Though these finally cul- 

 minate in the climax, the opportunity for their initiation is constantly 

 renewed from time to time and place to place, and this taken with 

 varying rates of progression explains why immature stages of the cli- 

 max are to be found everywhere through it. 



To the A^ariety wrought in the climax picture by these primary suc- 

 cessions are added, in most communities available for scientific study, 

 the modifications intensified and initiated by man. These are less 

 deep seated but much more frequent and result from disturbances of 

 all kinds, notably fire, trapping, hunting, animal control, clearing, cul- 

 tivation, and grazing. Fire, lumbering, clearing, hunting and trapping 

 have destroyed or modified the climax in practically all forest regions. 

 Cultivation, together with hunting, trapping, and mammal control, 

 has left but scattered and incomplete fragments of the humid prairies, 

 and grazing and mammal control have changed the composition of 

 semi-arid ones by favoring certain dominants and influents at the ex- 

 pense of others. 



Under natural conditions, the numbers of animals fluctuate greatly, 

 in more or less definite response to climatic cycles, as already indi- 

 cated in Chapter 5. It is true also that the dominants and subdomi- 

 nants of the plant matrix of grassland, for example, undergo gi'eat 

 variations in growth and number of individuals from year to year. 

 This process of annuation often produces striking differences in the 

 composition and appearance of the climax at the wet and dry extremes 

 of the sunspot cycle. The greatest fluctuation in forest is in seed crop 

 and herbaceous growth. Annuations and variations in abundance also 

 operate in fresh-water and marine climaxes (Blegvad, 1925). Quite 

 as pronounced in many instances is the orderly procession of aspects 

 through the biome from spring to fall and winter. In this, the changes 

 in the plant matrix are concerned wtih dominance and pattern rather 

 than with composition, but with the subinfluent and veinfluent animals 

 marked changes in population occur from aspect to aspect. 



In preparing this book two courses were open to the authors: either 

 to present a description of examples of the natural phenomena with 

 which the book is concerned as a basis for general discussion to fol- 



