Harmful Effects 317 



Harmful Effects 



An increase in numbers means an increase in competition for the 

 necessities of life. Rivalry between members of the same species is 

 typically keener than that between members of different species. 

 In the words of Darwin (1859), "the struggle will almost invariably 

 be most severe between the individuals of the same species, for they 

 frequent the same districts, require the same food, and are exposed 

 to the same dangers." Overpopulation therefore results in a serious 

 interference of one individual with another— sometimes in a passive 

 or indirect way, at other times as direct aggression or even cannibal- 

 ism. The accumulation of metabolites often curtails the further 

 growth of the species within the area, and the simple matter of oc- 

 cupying available space imposes a mechanical limitation on some 

 populations. Since established land plants are fixed in position, the 

 sphere of influence of each individual, and the area from which it 

 must draw its necessities, are sometimes easier to observe than those 

 of active animals. When a population of plants has increased so that 

 the individuals are growing close together, their roots compete for 

 nutrients and for water and their tops compete for light. As com- 

 petition becomes more intense, growth rate is correspondingly re- 

 tarded. 



A ready-made record of the effects of competition was found in 

 the cross section of the trunk of a locust tree that had grown in Bel- 

 mont, Mass., and was blown down in the hurricane of 1944 (Fig. 

 9.3). By counting growth rings the date of the first year of the tree's 

 life was determined as 1929. At that time an open field was aban- 

 doned and a few trees seeded themselves in the area at widely spaced 

 intervals. For the first few years thereafter, the tree grew rapidly, 

 but, as size increased, competition with neighboring trees for nutrients, 

 for water, and particularly for light became progressively more se- 

 rious. The result may be obsei-ved in the diagram as progressively 

 smaller growth rings in 1934, 1935, and 1936. In the spring of 1937 

 the land was cleared of most of the trees for the construction of a 

 house. With competition removed the growth of this locust tree 

 was "released," to use the term of the forester. More wood was 

 added during the growing season of 1937 than had been added in 

 any of the previous 4 or 5 years, and growth continued at a high rate 

 for the remainder of the life of the tree. 



The form of development of plants as well as their growth is af- 



