8 Viewpoint of Modern Ecology 



salesman, their life expectancy is low. Man is a notable exception. 

 Civilization has brought about an increasingly higher life expectancy 

 in man— and with economic and sociological consequences to which 

 we must adjust. 



The great majority of plants and animals in nature die young, not 

 because of flaws in their internal mechanism, but because of their 

 failure to cope successfully with the external world. When fish eggs 

 are raised under the best conditions in a hatchery, most of them sur- 

 vive; veiy few die because of a mechanical or a physiological break- 

 down in their embryological development. When seeds are sown 

 in a well-prepared greenhouse bed, germination of most of the popu- 

 lation is expected. On a bag of grass seed, you will usually find a 

 statement that tests of the seed have shown a germination of 80 to 95 

 per cent. In a laboratory culture the larvae of a marine copepod were 

 raised to the adult stage with an 80 per cent survival, although 

 a survival of less than 0.5 per cent would have been sufficient to main- 

 tain the population on the basis of the number of eggs laid (Johnson 

 and Olson, 1948 ) . In the laboratory the copepod was protected from 

 predators, diseases, and the exigencies of the physical and chemical 

 factors of its usual environment, which would normally have killed 

 off more than 995 out of every 1000 young animals. 



Of course, some individuals are bora with congenital flaws in their 

 anatomy or physiology. Little information exists on the frequency 

 with which lethal genes occur in wild populations, but their influence 

 is probably very slight. Studies of wild fruit flies indicate that only 

 about 20 per cent of the eggs laid fail to develop into adult insects 

 because of genetic causes. Most of these 'lethals" succumb because 

 the genetic changes cause failures in their relations to the environment, 

 rather than failures in their internal adjustments. 



The few animals or plants that have escaped death long enough 

 to reach maturity are leading a very precarious existence. A sword 

 of Damocles hangs over their heads! The threat is ever present that 

 the action of the environment may become a little more severe and 

 wipe the population out. This sometimes happens. A striking ex- 

 ample of wholesale destruction of a bird population occurred in Min- 

 nesota when hundreds of Lapland longspurs were killed during a late 

 winter storm (Fig. 1.4). Another illustration of mass mortality is the 

 suffocation of fish under the ice of a lake (Fig. 1.5). In these in- 

 stances certain physical features of the environment became too 

 severe and the entire local population was killed. 



Sometimes a biological influence goes beyond tolerable limits. 

 Older residents of northeastern United States tell of Sunday excur- 



