BIOTOPES AND BIOCOEXOSES 143 



gray fox and the bobcat are index mammals, or were before man's 

 influence became too important. 



Population density. — The amount of animal life present in an 

 area is different in each biotope. It depends primarily upon the amount 

 of food available in the biotope in question. If the association is a 

 closed one (autarchic), the amount of animal life will depend on the 

 amount of the plant element in the association, which in turn depends 

 directly on the biotope. Such a relation does not apply when the 

 greater part or all of the food comes from without, as in the associa- 

 tions on the sea bottom or in bird rookeries. 



The amount of animal life in a given biotope may be considered 

 from different viewpoints. The population density must be distinguished 

 from the species density. The former is measured directly by the mass 

 of animal substance present. If it were to be estimated from the 

 numbers of individuals, no comparable values would result, since small 

 forms such as hydroid polyps, midges, or even mice are generally pres- 

 ent in much larger numbers than larger forms, and associations with 

 larger animals, as well as smaller, should be considered richer than 

 those with only small forms, the numbers of individuals being equal. 

 It must be admitted that an exact estimate of the amount of animal 

 life in the terrestrial biocoenoses is excessively difficult. This factor, 

 however, is well established for marine and fresh-water plankton. 

 Species density, on the other hand, is well known in many regions. 

 This is the number of species which are encountered in an area of 

 considerable size, whether they are abundant or scarce. The knowl- 

 edge of this factor, from a species list, does not give a direct insight 

 into the composition of a biocoenosis. A picture of the butterfly life of 

 the palaearctic fauna gained from a species catalogue would be quite 

 wrong. "One would find Pyrameis with 4 species, Vanessa with 12, 

 Pieiis, Epinephele, and Pararge with 20, and Coenonympha with 25; 

 the genera Satyrus with 56 species, Sesia with 78, and Erebia with 80, 

 would be much better represented. He would accordingly expect on 

 an excursion, to catch principally Sesia, Erebia, and Satyrus species; 

 with good fortune he might catch a Pieris, a Coenonympha or an 

 Epinephele, but it would be too much to expect that he would meet 

 with a Vanessa or Pyrameis. The reality stands in complete contrast. 

 Pieris, Coenonympha, and Epinephele are present in great numbers; 

 he might encounter the species semele of the genus Satyrus, but of 

 the 78 species of Sesia he could not expect to find one!" 7 



Whereas the population density represents an absolute value, the 

 species density is a relative one. Species density depends on one hand 

 upon the selective action of the environment but on the other upon 



