Some disjunctive interrelations are competition, association of 

 species, pollination, dispersal of seeds and fruits, antibiosis, and 

 grazing. Conjunctive interrelations include parasitism, mutual- 

 ism, and commensalism. 



The interrelations of species as they affect the grouping of 

 organisms will be considered under six principles, four disjunc- 

 tive and two conjunctive. 



DiflFerences of Species in Competitive Capacity 



Competition is the state or relationship that exists between in- 

 dividuals of the same or different species when the resources of 

 the ecosystem in which they are living are insufficient to supply 

 the needs of all the plants in it. In a rigorous sense, "competition" 

 is not the most precise term. In a soil where the water content is 

 approaching the wilting percentage, the plants with deeper 

 roots or superior water economy may continue to live until 

 more water comes; the wilting and death of many plants are 

 caused by insufficient soil moisture, not by any direct action 

 of one plant upon another.'''* The term "competition," as usually 

 used, refers to the greater growth in vigor of some individuals as 

 compared to others when a certain requirement is in short supply 

 for one or more individuals. The capacity to compete depends 

 upon the requirements, ecological amplitude, and efficiency in 

 resource use of the various biotypes in an area. 



Many qualities, both genotypic and phenotypic, influence the 

 competitive capacity of plants. Some of these are rate of growth, 

 size, ability to produce tillers, tolerance of drought or shade, 

 annual or perennial habit of growth, and the number of leaves 

 and roots. The plants that are most efficient in utilizing resources 

 in short supply have the best chance of survival, but even under 

 severe competition some less efficient biotypes will survive 

 because of their location in more favorable microhabitats. Adap- 

 tations such as a shallow root system and early spring growth that 

 reduce competition with other deep-rooted, summer-growing 

 species enable some species to survive. 



Competition between individuals of closely similar competitive 

 capacity, as within an ecotype population, is very intense in thick 

 stands of seedlings of young plants (Figure 1-11). The struggle for 



Ecological CHaracteiristics of Species &. Populations • 27 



