574 - Heredity and Evolution 



possible to catch a glimpse of the chain of 

 circumstances that interlinks the species of a 

 region, even though biotic interrelationships 

 are extremely complex and difficult to study. 

 In 1931, for example, a fungus disease, caused 

 by a parasitic slime mold (Labyrinthula), 

 suddenly destroyed almost all of the eelgrass 

 (Zostera marina) that for centuries had flour- 

 ished in the shallow bays and inlets along 

 the Atlantic coast. Such a wholesale extermi- 

 nation of the eelgrass produced enormous 

 changes in the fauna and flora of the coastal 

 region. Most people noticed these changes, 

 because scallops and mussels became ex- 

 tremely scarce and because certain migratory 

 ducks (which feed upon the eelgrass) were 

 no longer available. But more important still 

 was the extermination of many species of 

 marine worms and other tidal organisms, 

 which died oil in prodigious numbers. With- 

 out eelgrass, the mud flats were no longer 

 anchored against erosion by the shifting tides, 

 and large areas of this richly populated habi- 

 tat were destroyed all along the coast. 



Another example of the interdependence 

 of species in a given locality may be cited 

 from the woi k of Darwin. For years Darwin 

 studied the periodic fluctuations in the 

 abundance ol red clover in an English coun- 

 tryside. Generally speaking, it was found that 

 red clover became exceptionally abundant 

 whenever the population of cats increased. 

 These changes were correlated with changes 

 in the population of field mice and bumble- 

 bees in the locality. The red clover produced 

 more seeds whenever there were larger num- 

 bers of bumblebees to pollinate the flowers; 

 and since field mice gain a living by preying 

 upon the nests of the bumblebees, the bees be- 

 came abundant when the current generation 

 of cats was adequate to hold down the num- 

 ber of field mice. Consequently, both bumble- 

 bees and red clover began to prosper when- 

 ever there were many cats to check the 

 depredations of the mice. 



Animal Populations and the Food Supply. Al- 

 though the green plants of a region provide 

 an ultimate nutritive base for the animal 



species, many large animals feed directly 

 upon smaller animals, and smaller animals, 

 in turn, gain a living by devouring a variety 

 of still smaller creatures. Moreover, large 

 carnivorous animals cannot survive unless 

 they find an abundance of smaller creatures 

 to feed on — as is witnessed by the fact that 

 an adult lion may kill as many as fifty zebras 

 every year (Fig. 30-1). As a result of these 

 factors, the carnivorous species of a region 

 tend to display a pyramidal distribution. 

 Generally one finds, at the base of the pyra- 

 mid, large numbers of small plants and ani- 

 mals, which provide food for the larger 

 animals. And at the apex of the pyramid 

 there are only a few very large animals which 

 feed upon the lesser species. 



Fig. 30-1. African lions at the kill (a zebra). (Courtesy 

 of the American Museum of Natural History, New 

 York.) 



This pyramidal distribution of animals, 

 according to si/e and the magnitude of the 

 food requirements, may be observed in a 

 variety of localized environments. In many 

 ponds, for example, there are billions of bac- 

 teria, but only millions of paramecia and 

 other large Protozoa can be supported by the 

 bacterial population. Then there will be a 

 lesser number — perhaps hundreds of thou- 

 sands — of semimicroscopic animals, such as 

 Daphnia and Cyclops, and only thousands of 

 easily visible creatures, such as carnivorous 

 beetle larvae and small fish. Finally, at the 

 apex of the pyramid, the number of truly 

 large predaceous fish may be so restricted 



