ECOLOGY 775 



semble the "alarm response" induced in laboratory animals subjected 

 to physiologic stress. This similarity led J. J. Christian in 1950 to pro- 

 pose that their death, like the alarm response, is the result of an upset 

 in the adrenal-pituitary system. As the population density increases, 

 there is increasing physiologic stress on individual hares due to crowd- 

 ing and competition for food. Some individuals are forced into poorer 

 habitats, where the food is less abundant and predators more abundant. 

 The physiologic stresses stimulate the adrenal medulla to secrete 

 epinephrine which stimulates the pituitary to secrete more ACTH 

 (adrenocorticotropic hormone). This in turn stimulates the adrenal cor- 

 tex to produce corticoids, an excess or imbalance of which produces the 

 alarm response or physiologic shock. In the latter part of the winter of 

 a peak year, with the stress of cold weather, lack of food and the onset 

 of the new reproductive season putting additional demands on the 

 pituitary to secrete gonadotropins, the adrenal-pituitary system breaks 

 down, carbohydrate metabolism (normally under its control) is upset, 

 and low blood sugar, convulsions and death ensue. This is an attractive 

 theory but the appropriate experiments and observations in the wild 

 to test it have not yet been made. 



347. Population Dispersal 



Populations have a tendency to disperse, or spread out in all direc- 

 tions until some barrier is reached. Within the area, the members of 

 the population may occur at random (this is rarely found), they may be 

 distributed more or less uniformly throughout the area (this occurs 

 when there is competition or antagonism to keep them apart), or, most 

 commonly, they may occur in small groups or clumps. Aggregation in 

 clumps may increase the competition between the members of the group 

 for food or space, but this is more than counterbalanced by the greater 

 survival power of the group during unfavorable periods. Aggregation 

 may be caused by local differences in habitat, by weather changes, re- 

 productive urges or social attractions. Certain animals regularly are 

 found spaced apart; they establish and defend certain territories. Many 

 species of birds, some mammals, reptiles, fish, crabs and insects establish 

 such territories, either as regions for gathering food, or as nesting areas. 



348. Biotic Communities 



A biotic community is an assemblage of populations living in a de- 

 fined area or habitat; it can be either large or small. The concept that 

 animals and plants live together in an orderly maner, not strewn hap- 

 hazardly over the surface of the earth, is one of the important principles 

 of ecology. Sometimes adjacent communities are sharply defined and 

 separated from each other; more frequently they blend imperceptibly 

 together The unraveling of why certain plants and animals comprise 

 a given community, how they affect each other, and how man can con- 

 trol them to his advantage are some of the major problems of ecologic 

 research In trying to control some particular species, it has frequently 



