ECOLOGY AND ZOOGEOGRAPHY 



629 



environment may be called a population, 

 and it is populations, and not a particular 

 individual, with which modern ecology is 

 chiefly concerned. 



The modern ecologist is also well aware 

 of the importance of studying the physical- 

 chemical environment and population, in 

 terms of each other, at all times. This ap- 

 proach takes account of the constant inter- 

 action between the two and is called the 

 ecosystem approach. Odum defines an 

 ecosystem as "any entity or natural unit that 

 includes living and non-living parts inter- 

 acting to produce a stable system in which 

 the exchange of materials between the living 

 and non-living parts follows circular paths." 



THE ENVIRONMENTAL 

 OR HABITAT APPROACH 



Before one can understand the interac- 

 tions between environments and popula- 

 tions one must first learn of what an 

 environment consists. The separate charac- 

 teristics are called factors and may be classi- 

 fied as physical, chemical, and biotic. If the 

 presence or absence of such a factor in a 

 certain minimal quantity is necessary for 

 the success of a population, it is called a 

 limiting factor. Physical factors in the en- 

 vironment that are often of great importance 

 as limiting factors are light, temperature, 

 wind, water currents, fire, soil texture, pH 

 (hydrogen ion concentration— acidity or 

 alkalinity) of water and soil, presence or ab- 

 sence of certain inorganic salts, and the con- 

 centration of oxygen, carbon dioxide, and 

 other gases. Biotic factors may exist in rela- 

 tion to the food supply, predation, or para- 

 sitism. A few of the more important en- 

 vironmental factors will be considered. 



Physical factors 



Light 



Light is an important physical factor. We 

 have seen what an influence light has on 

 various types of animals such as the amoeba, 



euglena, paramecium, hydra, earthworm, 

 crayfish, insect, and frog. Without light, 

 life on earth would be impossible. Only in 

 the presence of light does photosynthesis 

 take place for the manufacture of food for 

 all organisms. By means of light, animals 

 are able to move about freely and to carry on 

 the necessary activities of life. Sense or- 

 ganelles, which detect light, are present in 

 the protozoans, such as the euglena, and in 

 most of the higher invertebrates and verte- 

 brates. Thus we have encountered lensless 

 eyes in planarians, eyes with lens and cornea 

 in the sandworms, compound eyes in cray- 

 fishes and insects, eyes similar to those of 

 vertebrates in the squid, and various types 

 of vertebrate eyes in fish, frogs, turtles, birds, 

 cats, and man. 



The character of the lighting of an area 

 has a profound effect upon the animals that 

 live there. Everyone knows that certain 

 plants will thrive in the shade, whereas 

 others will not. This is likewise true of ani- 

 mals, not only of individuals, but also of 

 aggregations. For example, the length of the 

 period of daylight appears to be one of the 

 factors that stimulate the migration of birds. 

 In oceans and lakes many of the plankton 

 organisms, that come to the surface at night, 

 migrate downward by day. Many animals are 

 nocturnal; some possess very large eyes with 

 pupils that wfll admit a great number of rays 

 in dim light. Certain fish that live in the 

 ocean at depths where the light rays are few 

 also have large eyes; many deep-sea fish 

 manufacture their own light by means of 

 luminescent organs. Among terrestrial ani- 

 mals that are nocturnal and possess large 

 eyes are the tarsiers, night monkeys, and 

 owls. Nocturnal animals, sometimes without 

 large eyes, are provided with sensitive bris- 

 tles on the face as in the shrews, or with 

 some other means of finding their way 

 around. Among the well-known animals that 

 manufacture their own light by means of 

 luminescent organs are the protozoan 

 Noctiluca, ctenophores, certain marine 

 worms, fireflies, glowworms, and certain 

 deep-sea fish. 



