THE ANIMAL AND ITS ENVIRONMENT 



99 



tats which are strangely similar no matter 

 where they are found on the earth. Like- 

 wise, groups of animals that require the 

 same set of conditions are usually aggre- 

 gated in one locality. The types of animals 

 present in neighboring habitats change 

 only slightly due to minor variations in 

 certain physical conditions. For example, 

 the oxygen content of the water in one lake 

 is slightly different from that of neighbor- 

 ing lakes. This tends to bring about the 

 accumulation of animals that require just 

 the amount of oxygen present in the lake, 

 and those animals that require more or less 

 are apt to seek water that satisfies their 

 optimal needs. 



Sometimes the introduction of a species 

 changes the community of animals, such as 

 the inadvertent planting of carp minnows 

 in the northern lakes of the Midwest. Fish- 

 ermen driving into the lake country fre- 

 quently obtain their minnow bait at some 

 southern point where carp minnows are 

 abundant. After the fishing is over the re- 

 maining minnows are usually dumped into 

 the lake where they propagate and flourish. 

 Because of their feeding habits, fecundity, 

 and general hardiness, they soon replace 

 the more desirable game fish. Even under 

 these conditions a balance will eventually 

 be reestablished and once it is the animal 

 community remains much the same for a 

 long period of time, although not indefi- 

 nitely, for there are always gradual environ- 

 mental changes that necessarily affect the 

 life in it. Another similar illustration is the 

 case of the introduction of the English spar- 

 row into the United States. 



In order to obtain some understanding of 

 animal communities let us consider two 

 diverse situations, one a fresh-water pond 

 and the other a desert recjion. An examina- 

 tion of these two communities may provide 

 some appreciation of the complexity of 

 ecological studies. 



A fresh-water pond. A pond is defined 

 as a small body of fresh water, usually not 

 more than two or three meters in depth, 



its temperature being approximately the 

 same throughout (Fig. 5-10). Many ani- 

 mals and plants live in such a limited en- 

 vironment and even within its confines 

 there are definite regions which support 

 specific animals. The open water is largely 

 devoid of both fish and plants, but the 

 shores support a variety of animal and 

 plant life, depending on the relative 

 amounts of mud, sand, or rocks. 



If the bottom is muddy, many plants, 

 such as water lilies, grow in profusion. In 

 protected places around the edge there may 

 be several varieties of fish, principally bass 

 and pickerel. Crayfish and small fish may 

 be seen darting here and there in search 

 of food. Tadpoles can be found near the 

 bottom. The water teems with tiny Crusta- 

 cea and larvae of insects like midges ( small 

 gnat-like flies in the adult stage), which 

 form the basic food for young fish. By 

 scooping up some of the mud in a fine 

 mesh net, many other animals can be noted, 

 including snails of various sizes and shapes 

 and perhaps a few leeches. 



Many different kinds of flying insects 

 make their home around the edge of the 

 pond. The dragon fly (Fig. 11-31) and May 

 fly nymphs can be found. An occasional 

 diving beetle (Fig. 11-36) may be picked 

 up. This is an interesting insect because it 

 is so well adapted to aquatic life even 

 though it must breathe air. It carries a film 

 of air under its wings which acts as a reser- 

 voir for underwater maneuvering. The hind 

 legs are large and beautifully designed for 

 swimming under water. 



The water boatman also carries a similar 

 air film. Its long hind legs covered with 

 hair, when in operation, remind one of a 

 man rowing, and hence its name. Other 

 insects such as the water strider (Fig. 2-2) 

 and the whirligig beetle skim over the sur- 

 face of the water, depressing but never 

 breaking the surface film (see p. 24). 

 Their food consists of air-borne insects that 

 are blown out over the water and accident- 

 ally fall on the surface. The whirligig beetle 



