238 



ANALYSIS OF THE ENVIRONMEN T 



chain, if the final members (like the guano 

 birds of the Humboldt Current) have few 

 enemies and a vast food supply, the num- 

 bers of individuals of the successful species 

 simply breed up to the available food, and 

 the nonnal death rate returns an appre- 

 ciable amount of food for plants to the ini- 

 tial elements of the chain. In either food 

 chain or food pyramid, there is still a pop- 

 ulation level below which the survival of 

 the species is in hazard from external acci- 

 dent or from the longer cycles of the en- 

 vironment, so that in the great excess of 

 normal years there is a food surplus. The 

 correlation of the snowshoe-rabbit and 

 Canada lynx cycles of boreal America (with 

 the lag of a year or more in the lynx cycle) 

 aflFords an illustration of the influence of 

 surplus animal food. The now familiar fate 

 of the deer in the Kaibab Forest of the 

 north rim of the Grand Canyon in Arizona 

 illustrates the role of predator control in 

 relation to surplus (see p. 706). This whole 

 matter is discussed further in relation to 

 community metabolism and evolution (pp. 

 370 and 509). 



There are notable illustrations of the ex- 

 ploitation of food surplus at the nonevolu- 

 tionary level. The density of human popu- 

 lations may be traced to food supplies at 

 various levels of society. The riverbank vil- 

 lagers of the Sepik River in New Guinea, 

 dependent on the sago palm (of the low- 

 land swamp area) for a basic starch, sup- 

 plemented by fish from the river, exhibit a 

 narrow ribbon of dense population follow- 

 ing the river and its branches. The density 

 of animal populations of single species is 

 correlated with the surplus of a basic food 

 supply. For herbivores this will be diatoms, 

 grass, browse, or tree-top foliage. For car- 

 nivores (in a broad sense) it may be 

 plankton (e.g., as the food environment of 

 baleen whales), rodents, or artiodactyls. It 

 is evident that the distribution of a given 

 species of fish may be analyzed in terms of 

 its centers of population density as well as 

 in its total range, and that such centers are 

 as vitally important to the success of the 

 species as to the fisheries that develop in 

 them (see page 602 for the genetic- 

 evolutionary aspect of this phenomenon). 



The modem pattern of bird migration is 

 thought to have originated largely as a re- 

 sponse to the opening up of the north tem- 

 perate zone with the retreat of the Pleisto- 

 cene glaciers. With the establishment of ice- 



free summers, with seasonal abundance of 

 food, vernal expansion of the pre-existent 

 types of birds into the northern areas was 

 possible, but could develop only in correla- 

 tion with autumnal retreat to the south. The 

 various physiological mechanisms by which 

 bird migration is controlled are to be 

 thought of as regulatory rather than as 

 causal. More ancient Tertiary patterns of 

 bird migration may be discerned, for ex- 

 ample, in the relation of the North Ameri- 

 can bird fauna to that of South America 

 (Mayr, 1946). It is evident that the dis- 

 persal of birds has been to some degree 

 correlated with the capacity for migration 



In connection with the impact of the food 

 surplus as the food environment of the indi- 

 vidual animal, we may point to some of the 

 large-scale evolutionary implications as they 

 have aflFected the biotic environment. The 

 variety of structural types in the sea seems 

 to be correlated with maximum utilization 

 of food supplies, and in an obscure way the 

 variety of marine phyla may be compared 

 with the variety of species in the tropical 

 forest, which also appears to correspond to 

 the seizure by specifically adapted forms of 

 every possible food supply. In the sea adap- 

 tations to the principal types of habitat- 

 sand beach, rock beach, open water, deep 

 sea— evidently correlate with the presence 

 of food otherwise unutilized. Evolution to 

 exploit developing surpluses leads to 

 secondary food specializations for taking 

 special types of food, such as those of the 

 plankton-feeding whales, herrings, and ap- 

 pendiculates. 



The evolution of land animals into the 

 major habitats accomplishes the utilization 

 of the otherwise untapped food supplies of 

 the riparian, terrestrial, subterranean, arbo- 

 real, and aerial environments. Invasion of 

 more special or peculiar habitats, like the 

 alpine zone of mountains, the desert, the 

 polar regions, or caves, may likewise be 

 thought of in tenns of exploitation of a pre- 

 existing surplus, or at least of a surplus 

 developing step by step with its exploita- 

 tion. The invasion of fresh-water and ma- 

 rine habitats by land animals reflects their 

 attraction to food supplies, as is sufficiently 

 evident in such partially adapted riparian 

 forms as seals and sea lions. 



The more extreme specializations of ani- 

 mals to specific foods could scarcely become 

 possible without the marginal excess of 

 living matter. Uniform kinds of food, like 



