752 ANIMALS AND THEIR ENVIRONMENT 



enough oxygen when immersed. Knowledge of the limits of water toler- 

 ance is helpful in attacking insect and other pests. Wire worms have 

 rather narrow limits of tolerance to water and are most sensitive as 

 larvae and pupae. They can be killed by flooding the infested fields or 

 by planting alfalfa or wheat to dry out the soil below the limit of tol- 

 erance of the wire worm larvae. 



Other Factors. The supply of oxygen and carbon dioxide is usu- 

 ally not limiting for land organisms except for animals living deep in 

 the soil, on the tops of mountains, or within the bodies of other animals. 

 Animals living in aquatic environments may be limited by the amount 

 of dissolved oxygen present; the oxygen tension in stagnant ponds or in 

 streams fouled by industrial wastes may become so low as to be incom- 

 patible with many forms of life. Some parasites have adapted to the 

 low oxygen tension within the host's body by evolving special metabolic 

 pathways by which energy can be released from foodstuffs without the 

 utilization of free oxygen. 



The trace elements necessary for plant and animal life are limiting 

 factors in certain parts of the world. The soil in certain parts of Aus- 

 tralia, for example, is extremely deficient in copper and cobalt and is 

 unsuitable for raising cattle or sheep. Other trace elements which may 

 be a limiting factor are manganese, zinc, iron, sulfur and boron. 



The amount of carbon dioxide in the air is remarkably constant, 

 but the amount dissolved in water varies widely. An excess of carbon 

 dioxide may be a limiting factor for fish and insect larvae. The hydrogen 

 ion concentration, pH, of water is related physicochemically to the 

 carbon dioxide concentration and it, too, may be an important limiting 

 factor in aquatic environments. 



Water currents are limiting for a number of kinds of animals and 

 plants; the fauna and flora of a still pond and of a rapidly flowing stream 

 are quite diffierent. Winds may have a comparable limiting effect upon 

 land organisms. 



The type of soil, the amount of topsoil, its pH, porosity, slope, 

 water-retaining properties, and so on, are limiting factors for a variety 

 of plants, and hence indirectly for animals. The ability of many animals 

 to survive in a given region depends upon the presence of certain plants 

 to provide shelter and cover, as well as food. Grasses, shrubs and trees 

 on land each provide shelter for certain kinds of animals, and seaweeds 

 and fresh-water aquatic plants have a similar role for aquatic animals. 

 Some animals require special shelter for breeding places and the care of 

 the young. In many different kinds of birds, mammals, crustaceans and 

 other animals, each animal or pair establishes a territory, a region which 

 supplies food and shelter for it and its offspring, and which it defends 

 vigorously against invasion by other members of the same species. 



In summary, whether an animal can become established in a given 

 region is the result of a complex interplay of such physical factors as 

 temperature, light, water, winds and salts, and biotic factors such as 

 the plants and other animals in that region which serve as food, com- 

 pete for food or space, or act as predators or disease organisms. 



