568 INTERRELATIONSHIPS OF ORGANISMS 



tensifies intraspecific competion, and favors an increase in the populations 

 of predators and parasites that feed upon the organism. 



This tendency of the biotic resistance to hold an organism's popula- 

 tion size in check when the physical resistance decreases leads to some 

 rather paradoxical relationships that appear to be well substantiated by 

 field observations. There is evidence, for instance, that an extreme reduc- 

 tion in the numbers of an organism's normal predators may be actually 

 harmful for that organism. In an environment in which food and physical 

 factors show a wide seasonal (or other periodic) fluctuation, an organism, 

 if freed from nearly all biotic resistance, will tend to build up so large a 

 population size in good times that it will completely exhaust all food 

 supplies before the next adverse season has ended. 



Something very like this was shown some years ago by the elk herd 

 at Jackson Hole, Wyo. Here the abnormally large summer increase in 

 the protected elk population, freed from the normal predation of moun- 

 tain lions and bears, resulted in such a large winter herd that they quickly 

 exhausted the limited winter forage and were faced with wholesale 

 starvation. Fortunately, in this case, the local forest rangers met the 

 emergency by the importation of enough hay to tide the elk over until 

 the reappearance of the next spring's abundant natural forage. 



COOPERATIVE RELATIONS AMONG ORGANISMS 



By no means all of an organism's relationships to the other forms of 

 life about it are adverse or inimical. Many types of interdependency 

 involve cooperation and mutual aid. Some of these cooperative rela- 

 tionships are among individuals of widely different species, or of different 

 orders, classes, phyla, or even kingdoms. Others are shown by the many 

 kinds and degrees of social behavior that exist among individuals of the 

 same species — the herds, coveys, schools, flocks', packs, and unnamed 

 family groups that confer various sorts of mutual advantage to some or 

 all of their members. 



Cooperation among widely different forms of life shows almost every 

 gradation from close, often obligate, partnerships in which the benefit 

 received by each of the participants is clear-cut and unmistakable, to 

 cases that are difficult to distinguish from true parasitism. An example 

 of the truly mutual relationship termed symbiosis (Greek, "a living 

 together") is shown by the lichens, a widespread group of lower plants in 

 which each "individual" plant comprises an alga intimately associated 

 with a fungus. The chlorophyll of the algal cells carries on photosyn- 

 thesis, and the tissues of the fungus protect the delicate algal cells and 

 collect and store water and minerals for their needs. Both the alga and 

 the fungus are maintained by the anabolic profits of the partnership. A 

 somewhat similar example is shown in the relationship between many 



