102 



THE RISE OF ANIMAL LIFE 



to life in a dry climate (Fig. 5-11). This 

 region is not truly desert; it is about half- 

 way between grassy plains and true desert. 

 The vegetation is composed chiefly of cacti, 

 yuccas, and other plants particularly well 

 adapted to prevent loss of water through 

 transpiration. They are protected from the 

 marauding attacks of hungry and thirsty 

 animals by their sharp stiff spines and tough 

 outer coverings. 



Several species of lizards and rattlesnakes 

 are permanent residents of this region. One 

 interesting bird, the so-called road runner 

 or chaparral cock, is commonly seen racing 

 down the road ahead of your car; it seems 

 to prefer using its powerful legs when pur- 

 sued rather than its wings. The most num- 

 erous mammals are the long-legged, long- 

 eared, swift-running antelope jack rabbits, 

 kangaroo rats, prairie dogs, and coyotes. 

 The last-named feed on the others, all of 

 which are vegetarians. Snakes and lizards, 

 like the coyotes, are also carnivorous and 

 feed on small mammals and some insects. 



Compared to the pond situation, the des- 

 ert food cycle is rather simple, as would be 

 expected where there is so little life. Like 

 the pond food cycle, the desert cycle begins 

 with the plants. These are preyed upon by 

 the various herbivorous animals, which in 

 turn are eaten by the carnivores, which 

 probably also eat one another. Eventually 

 death overtakes them and the elements of 

 which each is composed return to the soil 

 to be used again by the plants. 



There are many other communities of 

 animals in a wide range of habitats. Some 

 of these are the ocean, grassy plains, tun- 

 dra, forest, and mountains. In all of these 

 many niches have developed, inhabited by 

 certain species of animals which are similar 

 in respect to specific needs. Together they 

 constitute the life of each community and 

 the communities combined make up the 

 complex life on the earth. 



Population densities 



Much can be gained from studying the 



interrelationship of life in a community. 

 The study of populations alone has proved 

 valuable not only in the control of insect 

 pests and predators, in increased game and 

 fish, and other redistributions of animal life, 

 but it has also been very important in busi- 

 ness and government, as for example in the 

 formulation of insurance and retirement 

 plans. During every census more and more 

 information is gathered about people in 

 order to learn what is happening to the 

 status of our population. This makes it pos- 

 sible to predict future trends and also sheds 

 some light on what might be done to influ- 

 ence the ultimate outcome. This is a hope- 

 ful field of investigation and shows prom- 

 ise of being of great value as time goes on. 



Population, measured in terms of rela- 

 tive numbers and general distribution over 

 the surface of the earth, indicates the bio- 

 logical success or failure of any species of 

 animal. One of the important factors in 

 achieving success is the rate of reproduc- 

 tion of a species. Here we find vast differ- 

 ences among animals, from those like the 

 whale that produces one offspring every 

 few years, to the tapeworm that can pro- 

 duce the incredible number of 100,000 per 

 day. In general, the larger the animal the 

 fewer the young, while the smaller the 

 animal the greater the number of offspring. 

 Even in this there are wide variations — 

 rabbits, for example, can reproduce at such 

 a rate as to overrun the entire earth in a 

 matter of a few years. A single protozoan 

 could fill all the oceans of the world in 

 a few months if it were allowed to multi- 

 ply at full speed and none of its offspring 

 died. How successful these reproductive 

 powers have been is indicated by a glance 

 at some figures concerning numbers of ani- 

 mals. A quart of sea water may contain 

 over 1,000,000 microorganisms; an acre of 

 fertile soil, over 13,000,000 animals. Grass- 

 hopper eggs alone may exceed 200,000 per 

 acre in heavy infestations. 



Although each species of animal has the 

 potentiality for overrunning the earth, it 



