566 



INTERRELATIONSHIPS OF ORGANISMS 



practically unlimited, at least in terrestrial situations, and the space in a 

 field suitable for ant nests or gopher burrows is not badly crowded. Simi- 

 larly, we find that there is much unoccupied space for grasses and herbage 

 on the floor of a well-shaded forest, however crowded the individual 

 grass plants may be on a lawn. 



Here we see a principle in operation that is often termed Leibig's law 

 of the minimum. Every organism has a rather long list of necessities, 

 and it is that necessity that exists in minimal quantity that produces the 

 most stringent competition and limits the size of population. The ant 

 population in a field is probably limited by the food supply; and the 



Fig. 33.2. A parasitic fungus. Gymnosporangium, that has two hosts. Left, cluster-cup 

 (aecial) stage on apple. Spores from this stage infect juniper, producing the peculiar object 

 at the right — the teliospore stage. Spores from this stage again infect the apple. (Photo by 

 Prof. E. B. Mains.) 



severe physical conditions and low supply of raw materials for food 

 manufacture limit the grass and herb population of an oak scrub. On 

 the forest floor, the minimal necessity is probably light, although grasses 

 are poorly equipped in several ways to compete with the typical forest 

 plants. 



Competition occurs both between the individuals of the same species 

 and between individuals of different species. Because the requirements 

 of all members of the same species are alike in practically all respects, 

 intraspecific competition is particularly severe, as it is between different 

 species that have very similar total requirements. Interspecific competi- 

 tion in most instances, however, is only partial, and each species can 

 usually find at least restricted areas or habitats where conditions are 

 nearer to its own optima than to those of its rivals and in which it there- 

 fore has some advantage over the latter. 



