the coiuimmity. Species exerting tliis iiii])ort;mt con- 

 trol are called dominatUs. Plants are more frequently 

 dominant in terrestrial communities than arc animals. 

 In aquatic communities, animals are relatively more 

 imiwrtant in this role, although dominance is often 

 not developed. 



Dominance is most commonly expressed in the 

 rfactions of an organism on its habitat (Clements 

 and Shelford 1939). Dominants shoulder the full im- 

 pact of the climate or the environment but modify 

 this effect for other organisms within the community 

 by tempering light, moisture, space, and other con- 

 ditions. Only those other organisms that find these 

 modified physical conditions tolerable can exist within 

 the community. Furthermore, dominants are ordi- 

 narily the most prominent species in the community, 

 make up its greatest mass of living material, and 

 serve as the major source of food, substrate, and 

 shelter for the animals that are present. In a forest 

 community, trees are dominant. They decrease light 

 intensity, increase the relative humidity, intercept 

 precipitation, monopolize most of the moisture and 

 nutrients in the soil, decrease wind velocity, and 

 furnish shelter and food for animals. Grasses play a 

 similar, though less conspicuous, role in prairie com- 

 munities ; sedges, rushes, and cattails in marsh com- 

 munities ; sagebrush in the arid habitat of the Great 

 Basin ; mussels and barnacles on a rocky seashore ; 

 and so forth. 



Sometimes dominance is demonstrated in coac- 

 tions. direct effects of organisms on each other. In some 

 fresh-water ponds, carp and suckers may consume 

 much, perhaps all, of the submerged vegetation. This 

 coaction thus prevents the plant constituents from 

 assuming their usual role in the community, and by 

 so much prevents the occurrence of animal species 

 that depend directly upon the plants. These fish also 

 react upon the habitat by stirring up the bottom, from 

 which they derive organic matter, thereby greatly in- 

 creasing the turbidity of the water. Penetration of 

 light into the water becomes poor, greatly handicap- 

 ping sunfish, bass, and other species which locate food 

 visually (Table 7-3). 



In primeval days, bison on our western great 

 plains fed on the luxuriant taller grasses more ex- 

 tensively than on the short grasses, with the conse- 

 quence that, over extensive areas, short grass species 

 replaced tall grasses almost entirely. Thus bison 

 were coactant with and dominant over the composi- 

 tion and character of the community (Larson 1940). 

 In a similar manner, when European meadow voles 

 are numerous they reduce the vigor and prevalence 

 of the grass dominants in consequence of their feed- 

 ing and tunnelling in the ground, and angiosperms 

 which are normally absent or scarce appear (Sum- 

 merhayes 1941). Overpopulations of the European 

 rabbit alter the character of the forest by frustrating 



re|jroduction of oak, beech, and hornbeam, upon the 

 seedlings of wiiich they feed to the exclusion of other 

 species. When introduced into Australia, the Euro- 

 l)ean rabbit converted grassy areas into desert-like 

 tracts (Mourliere 1956). 



Although animals are more common coactors than 

 plants. i)lants may occasionally exert dominance in 

 this way. As a notable example, chestnut blight (a 

 fungus) virtually eliminated the chestnut tree from 

 the deciduous forest of eastern North America during 

 the first few decades of the twentieth century. This 

 fungus infects the cambium, forms pustules under the 

 bark, and causes the bark to fall off and the leaves 

 to wilt. The blight has eliminated the chestnut from 

 the community, and the conse(|uent ojjening u]) of the 

 canopy has allowed the extensive invasion of new 

 species of shrubs, herbs, and animal inhabitants. 



Trees are the dominants in a forest community, 

 but species in the lower stratum of shrubs modify 

 the habitat still further and even the herbs exert some 

 control over the physical conditions on the surface 

 of the ground. A subdominant species must tolerate 

 the conditions established for it by the dominants; 

 hut it in turn is a modifier of the community composi- 

 tion in a secondary manner. 



Influence 



By influence is meant efTect upon the abun- 

 dance, health, and activities of other organisms in the 

 community but not to the extent of directly excluding 

 species. Influence is conspicuously expressed through 

 coactions, but it may be effected through reactions 

 as well. Insects may partially or wholly defoliate a 

 tree ; a pack of wolves may diminish a population of 

 deer over winter ; squirrels may bury acorns and nuts 

 and thereby aid germination of them ; parasitic or 

 poisonous plants may lower the vigor or destroy the 

 life of some other plants or animals ; animals may 

 burrow into the soil and thereby increase percolation 

 of water and air, a benefit to plants : and all organ- 

 isms, upon death, add organic matter to the habitat. 

 These and other actions influence the community, 

 but unless these influences become extreme they do 

 not absolutely determine whether or not other species 

 will occur in the community. Influence, then, is of 

 essentially the same nature as dominance but is less 

 vigorous in the modifying role that it plays. 



Evaluating and rlaxsifying 

 animals ecologically 



One of the most important, yet difficult, prob- 

 lems in ecology is evaluation of the roles the different 

 kinds of animals play in community dynamics. 



The biotic community 



