COMPETITION. 



73 



Competition in air and in soil. — The competition between pioneers is usually 

 restricted to the soil, where the roots compete with each other for water. It is 

 often also the simple competition typical of famiUes, in which all the indi- 

 viduals make identical demands because they belong to one species. As the 

 families become communities by extension or by migration, the competition 

 becomes more complex and the outcome in many cases is dominance. This 

 is particularly true as the bare area becomes covered, and success in ecesis 

 comes to depend upon the ability to overshadow other plants. The taller 

 plant gradually gains the upper hand, partly because it receives more light and 

 makes more growth, and partly because its demands are increased by greater 

 transpiration. At the same time the shorter plant receives less light, grows 

 and transpires less, and its needs for water diminish. This interplay of com- 

 petition and reaction occurs in all communities with individuals of different 

 height and extent, but in varying degrees. In pure grassland, competition of 

 the roots for water is controlling, and the aerial shoots compete slightly or not 

 at all. Where broad-leaved herbs play an important or characteristic part 

 shoots compete with each other for Ught. This is true of typical prairie to such 

 a degree that actual layers come to be developed, as occurs also in other grass- 

 land. From the competition in the prairie to that of the scrub and the forest 

 is but a change of degree. The dominance of the trees is only the outcome of a 

 competition in which position means the control of light, and thus of water. 

 Competition of shoots alone may occur when the water-supply is in excess, and 

 hence competition for water is absent. This is most evident in the case of 

 submerged plants. 



Woodhead (1906) distinguishes conmnmities as competitive when the domi- 

 nants occupy the same soil layer, and complementary when the roots are in 

 different layers. It is one of the most important tasks of ecology to determine 

 the root and shoot relations of commxmal plants, but it seems much better to 

 apply Woodhead's terms to the species concerned and not to the whole com- 

 munity. It is the species which are competitive ,or complementary, and not 

 the commimity. Moreover, species which are complementary as to roots may 

 be competitive as to shoots, and vice versa. In addition, the individuals of 

 each species are competing more or less actively, and this is the case with the 

 secondary species also, both as to themselves and the dominants. Finally, 

 the complementary relation in many cases, if not in all, is merely the outcome 

 of the more or less complete success of certain species by which competition 

 is changed into dominance. Our knowledge of both competition and domi- 

 nance at present is quite too rudimentary to warrant drawing distinctions, 

 except as suggestive working hypotheses. 



E61e of competition in succession. — ^As already indicated, competition affects 

 the amount of water and of light, even to the point of complete control when 

 success in competition becomes dominance; hence its effect upon ecesis is 

 direct and often critical. It is seen in the behavior of the seedlings of species 

 already in possession, as well as in that of new invaders. Competition ia 

 most decisive during the development of the seedling and at the time of repro- 

 duction, particularly in the case of perennials and woody plants. Accordingly 

 it plays a large part in determining the relative number of occupants and in- 

 vaders in each stage of a sere, and thus helps to control the course of develop- 

 ment. In analyzing the r61e of competition in the latter, it is desirable to 



