CONDITIONS OF EXISTENCE 19 



ion concentrations, but in general we may safely conclude that the 

 importance of this factor (pH.) acting alone has been overempha- 

 sized. 27 (See also bog waters, Chapter XIX, and soil acidity, Chap- 

 ter XX.) 



Pressure. — With land animals variation in atmospheric pressure 

 is not an important environmental factor except in mountainous re- 

 gions and there the effects of high altitudes are associated rather with 

 oxygen deficiency than with decreased total pressure. With aquatic 

 animals the pressure increases one atmosphere for every 10 meters 

 depth. Even so, surface fishes with the air bladder empty may be sub- 

 jected to a pressure of 100 atmospheres without harm. This means 

 that under these conditions they can range through the upper 1000 

 meters without injury. If the air bladder is full, gases enter the blood 

 under the higher pressures. Then, if pressure is suddenly removed, 

 gas accumulates in the blood, causing gas-bubble disease and death. 

 Deep-sea fishes may live normally under a pressure of more than 800 

 and perhaps at 1000 atmospheres. Marine and lake fishes are known 

 to have limitations in their vertical ranges, and these are determined, 

 at least in part, by the prevailing pressures. 28 



Food. — Finally, a sufficient amount of organic food is an indis- 

 pensable condition for the habitability of an area for animals. 



The amount of available food is usually the deciding factor for 

 density of life, i.e., for the number of individuals in a given area. In 

 years when there is a richer development of zooplankton in the Eng- 

 lish Channel there is a larger mackerel catch. 29 The planting of kale, 

 cauliflower, turnips, and other vegetables in New Zealand was fol- 

 lowed by a disconcerting increase in the native insects. 30 In years of 

 abundance of mice, buzzards and owls gather in the mouse-infested 

 areas (see Fig. 11). Larger predatory animals are unable to live on 

 small islands, as they cannot find prey in sufficient quantity. 



Many animals are not particular about their food and have a large 

 menu; they are euryphagous or omnivorous. The migratory grass- 

 hoppers, some caterpillars, and, among vertebrates, the crows, may be 

 named as examples. Others are specialists and eat few or even only 

 one kind of food; they are stenophagous or even monophagous. In this 

 category belong the caterpillars of many Lepidoptera, such as the 

 Apollo butterfly {Parnassius apollo) , which feeds only on plants of 

 the genus Sedum; the larva of the zebra swallowtail, which feeds only 

 upon pawpaw; the oleander sphinx {Sphinx nerii) ; and the ant and 

 termite eaters like Myrmecophaga and Manis. Stenophagy tends to 

 limit distribution, euryphagy to extend it. The bird called the nut- 

 cracker is limited in Siberia to the occurrence of the nut pine, while 



