72 ECESIC CAUSES. 



ary soils, on the contrary, afford more or less optimum conditions for germina- 

 tion and growth, and are invaded and stabilized with corresponding rapidity 

 (plate 17, A, b). 



COMPETITION. 



Nature. — Competition occm's whenever two or more plants make demands 

 in excess of the supply (Clements, 1904: 166; 1905:285; 1907:251). It is a 

 universal characteristic of all plant communities, and is absent only in the 

 initial stages of succession, when the pioneers are still isolated. It increases 

 with the increase of population in successive stages until the climax or sub- 

 climax is reached, after which it decreases again with the population. It is 

 necessarily greatest between individuals or species which make similar or 

 identical demands upon the same supply at the same time, and least or quite 

 lacking in associated plants with demands largely or quite unlike. 



In its essential nature, competition is a decrease in the amotmt of water and 

 light available for each individual, or for each species as represented by the 

 total number of individuals. It affects directly these two factors, and through 

 them the response of each plant. In a few cases, such as occur when radish 

 seeds are planted closely, it is possible to speak of mechanical competition or 

 competition for room. The crowding of the swelling roots is, however, only an 

 incident in the competition for water, and seems to have no counterpart in 

 nature. There is no experimental proof of mechanical competition between 

 root-stocks in the soil, and no evidence that their relation is due to anything 

 other than competition for the usual soil factors — ^water, air, and nutrients. 



Competition and dominance. — ^Properly speaking, competition exists only 

 when plants are more or less equal. The relation between host and parasite 

 is not competition, nor is that between a dominant tree and a secondary herb 

 of the forest floor. The latter has adapted itself to the conditions made by 

 the trees, and is in no sense a competitor of the latter. Indeed, as in many 

 shade plants, it may be a benefici9,ry. The case is different, however, when the 

 seedlings of the tree find themselves alongside the herbs and drawing upon the 

 same supply of water and light. They meet upon more or less equal terms, 

 and the process is essentially similar to the competition between seedlings 

 alone on the one hand, or herbs on the other. The immediate outcome will 

 be determined by the nature of their roots and shoots, and not by the domi- 

 nance of the species. Naturally, it is not at all rare that the seedling tree 

 succumbs. When it persists, it gains an increasing advantage each succeeding 

 year, and the time comes when competition between tree and herb is replaced 

 by dominance and subordination. This is the course in every bare area and 

 in each stage of the sere which develops upon it. The distinction between com- 

 petition and dominance is best seen in the development of a layered forest in a 

 secondary area, such as a bum. All the individuals compete with each other 

 at first in so far as they form intimate groups. With the growth of shrubs, the 

 latter become dominant over the herbs and are in turn dominated by the trees. 

 Herbs still compete with herbs, and shrubs with shrubs, as well as with younger 

 individuals of the next higher layer. Within the dominant tree-layer, indi- 

 viduals compete with individuals and species with species. Each layer exem- 

 plifies the rule that plants similar in demands compete when in the same area, 

 while those with dissimilar demands show the relation of dominance and sub- 

 ordination (plate 18, A, b). 



