lions, 863 bobcats, 7388 coyotes, and 30 wolves 

 trapped or shot, mostly by government hunters. 



The wolves were exterminated, and the other 

 predators were markedly reduced. The deer in- 

 creased from an estimated 4000 animals in 1906 to 

 an enormous herd of nearly 100,000 in 1924. The 

 deer's habits and the topography of the country pre- 

 vented a scattering of animals to adjacent ranges. 

 The over-populations of deer consumed all new 

 growth of young trees and browsed the foliage of ma- 

 ture trees as high as they could reach, yet the popu- 

 lation could not secure enough food to keep it in good 

 physical condition. The population far surpassed the 

 carrying capacity of the range. In September 1923, 

 it was estimated that 30,000 to 40,000 animals were 

 on the verge of starvation and during the winters of 

 1924-25 and 1925-26, an estimated 60 per cent of the 

 population died. Plainly the balance of nature was 

 upset in this instance, with dire results. Hunting was 

 again permitted in 1924, and the herd was reduced to 

 about 10,000 by 1939. This history of the Kaibab 

 deer herd is not unique ; similar cases have been re- 

 ported in about a hundred other areas (Leopold, 

 Sowls, and Spencer 1947). Over-populations in all 

 areas have followed reductions in the number of 

 predators, although other factors were involved. 



Constancy of balance 



When populations of species with differing food 

 habits are in equilibrium, the surplus of prey species 

 resulting from reproduction is destroyed and con- 

 sumed by the predators. If predation does not de- 

 stroy the total yearly surplus, the prey population 

 increases in size : if predation takes more than the 

 surplus the population of the prey decreases in size. 

 Actually, even in entirely natural communities un- 

 disturbed by man, a strict balance of nature is prob- 

 ably never maintained for any appreciable period of 

 time. It is characteristic for populations to vary in 

 size, but these fluctuations tend to vary rather closely 

 around a certain constant population mean. Depend- 

 ing on the length of its life cycle, a species population 

 in an area may fluctuate from day to day, from sea- 

 son to season, or from year to year. 



Many factors other than food coactions cause 

 fluctuations in the abundance of animals, and preda- 

 tion is not always the most important (see Chapters 

 16, 17). Close interdependency between populations 

 of prey and predator species occurs most commonly 

 when a prey species has only one or two important 

 species preying upon it, while the predator species is 

 largely restricted to that one species of prey (Pen- 

 nington 1941). It is obvious, however, that an equi- 

 librium of a sort exists between different species, and 

 this is what is referred to by the concept of balance 

 of nature. 



TROPHIC LEVELS 



In order to analyze the intricate coactions 

 involved in the food web and balance of nature, it is 

 desirable to simplify the relationships into nutritional 

 or trophic levels (A) (Thienemann 1926, Lindeman 

 1942, Allee et al. 1949, Odum 1953). The lowest 

 level (P) is composed of photosynthetic plants that 

 are able to use solar energy for the manufacture of 

 food, and certain types of bacteria that use either the 

 free energy of unstable inorganic compounds or are 

 activated by light to synthesize a limited amount of 

 new organic matter. These are the producers. At the 

 second level (Ci) come the herbivores, or primary 

 consumers; at the third level, the smaller carnivores, 

 or secondary consumers (C2) : and at the fourth level, 

 the larger carnivores, or tertiary consumers (C3). 

 Occasionally there may be quarternary consumers 

 (C4). The terms "producer" for plants and "con- 

 sumer" for animals were used, and the essential rela- 

 tionship understood by Dumas in 1841. These two 

 groups are also distinguished as autotrophic and 

 heterotrophic, respectively. 



The levels in subdivision of consumers are not 

 sharply defined, as the feeding behavior of some spe- 

 cies involves them simultaneously in several levels. 

 Actually, the more remote an organism is from the 

 initial source of energy (solar radiation), the more 

 likely it is that it will prey on two or more levels. 

 This need not confuse the essential relationships in- 

 volved (Lindeman 1942). Omnivores overlap be- 

 tween levels Ci and any of the higher levels. Large 

 saprovores, the heterotrophic bacteria, and fungi de- 

 rive their nourishment from the excreta and dead 

 bodies of organisms from all trophic levels. Since 

 they are reducers or decomposers, they may for sim- 

 plification be grouped with the autotrophic bacteria, 

 and be called transformers (T), since their total 

 effect is to convert dead organic matter into nutrients 

 that green plants can again absorb. 



Figure 13-7 illustrates how a complicated food 

 web may be simplified, somewhat arbitrarily, into 

 trophic levels. Detritus, derived from the disintegra- 

 tion of dead organisms and excreta from organisms, 

 is worked over by the bacterial transformers, and the 

 detritus and bacteria represent an independent base 

 of the food web separate from the green plants. 



A characteristic of trophic levels in most com- 

 munities is that the nearer a level is to the source of 

 energy, the greater the diversity of species involved. 

 Thus in Fig. 13-7, the primary consumers (Ci) in- 

 clude some twelve taxa ; the secondary consumers 

 (Co), four; the tertiary consumers (C3), three;. and 

 the quaternary consumers (C4), but one. Species in 

 the lower trophic levels have a higher rate of repro- 

 duction than those in the higher levels to compensate 

 greater predation. 



196 Ecological processes and dynamics 



