THE BIOTIC ENVIRONMENT OF ORGANISMS 



565 



The resistance produced by the organisms that form a part of the 

 individual's environment is in part direct and more or less evident, and 

 in part indirect and difficult to discover or evaluate. The more direct 

 restraints placed upon an organism by the other forms of life about it 

 are those of the predators that prey upon it, the parasites that live at 

 its expense, and the competitors that vie with it for various limited neces- 

 sities for existence and reproduction. 



Predators and parasites. By a predator, we mean an animal that 

 catches and kills another animal: wolves, foxes, weasels, spiders, and 

 snakes are familiar examples. The food strainers and trappers referred 

 to on pages 551 and 552, and a few 

 plants such as the bladderworts, 

 sundews, and pitcher plants that 

 trap animals for food are hardly 

 predators in the restricted sense but 

 may also be classified here. By a 

 parasite, we mean an organism that 

 lives on or in the tissues of another 

 organism, the host, feeding on the 

 host's tissues, its digested food, or 

 the products of its metabolism. 

 Parasitic habits and adaptations 

 are more varied than predatory 

 habits; parasites include both ecto- 

 parasites, such as fleas and lice, and 

 endoparasites, such as tapeworms, 

 hookworms, the plasmodium of 

 malaria, and literally thousands of 

 other animal and plant organisms. 

 The plant and animal kingdoms furnish innumerable examples of para- 

 sites and of host organisms. Plants parasitizing plants, plants parasitizing 

 animals, animals parasitizing plants, and animals parasitizing animals 

 are all common and well-known phenomena. Both predators and parasites 

 take a tremendous toll of the organisms that they feed upon and tend to 

 increase their proportionate toll whenever circumstances permit the 

 abundance of their prey or their hosts to increase. 



Competitors. Many of the necessities that any organism requires 

 for continued existence are limited in quantity and distribution. Suitable 

 food or raw materials for food manufacture, nesting sites, areas in which 

 physical factors do not exceed the limits of toleration, and refuges in 

 which animals may escape their predators are all limited in amount or in 

 extent. In any situation some of the necessities for existence may be much 

 less restricted than others. The necessary oxygen for respiratory needs is 



Fig. 33.1. Cordyceps dipterigena, a parasitic 

 fungus that attacks only flies. (Photo by 

 Prof. E. B. Mains.) 



