144 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



historic relations. Thus the species density upon an oceanic island will 

 in general be less than that upon an island of equal size near a conti- 

 nent, other conditions being equal. Under optimum conditions, the 

 species density is very great but decreases as the environment becomes 

 less favorable. Two biotopes with equal population density may be 

 very unequal in species density. 



"Each biocoenotic area has the greatest mass of life in every gen- 

 eration that it is capable of producing and supporting. All of the 

 usable food-materials present in it will be claimed by the life produced 

 there," says Mobius. 8 This applies, however, especially for animals, 

 only to the extent to which the members of the biocoenosis are able 

 to make use of these organizable substances. The full use of the food 

 materials prepared by the higher plants of a region may be prevented 

 by the absence of animals, due to historic factors. The utilization of 

 vegetable food requires certain adaptations and is not possible to all 

 animals. The principal herbivorous groups of the land are the land 

 snails, insects, birds, and mammals. In all the remaining groups of 

 animals, herbivores are only sparingly represented. The colonization 

 of several regions, especially islands, such as New Zealand, the Falk- 

 lands, and others, with hoofed animals, has shown how much more 

 animal life they were able to support than was actually present on 

 them; animals which were able to utilize the available food supplies 

 were absent. 



In equal areas with similar conditions of food supply the total mass 

 of life produced, i.e., the animal population density, will not be very 

 different. From this results the peculiar inverse proportion between 

 number of species and number of individuals (discussed above on 

 p. 30) . Thus the polar seas have a smaller number of species of most 

 groups of animals than the tropical waters, but not a smaller popula- 

 tion density. Plankton catches are mostly quantitatively larger in 

 summer in the arctic seas than in the tropics, but single species of 

 Radiolaria, Copepoda, and fishes appear there in enormous numbers 

 of individuals, while in the tropics, with a greater wealth of species, 

 the number of individuals of any one is smaller. This is a rather direct 

 corollary of Mobius' proposition. The oligochaetes of the seacoast, 

 which are necessarily euryhaline on account of the varying salt content 

 of the water, are present in very great numbers when food conditions 

 are favorable, though in small variety of species. 9 Only a very few 

 adaptable animals inhabit the ooze banks of the shallow North Sea, 

 but as these banks are rich in food matter, the few animals are present 

 in enormous numbers. 10 The slimeworms (Tubifex) , which feed on 

 the rich deposits of food materials, share this biotope with only a few 



