ZOOGEOGRAPHY 377 



cover under optimal conditions. Applying the scale for varying density of 

 species population previously used, the total estimated population may be 

 derived proportionally: 



465,000 : 10 as a: : 14,000 



maximum maximum total 



population species estimated 



per sq. mi. per sq. mi. species 



This utilizes the precautionary steps taken in estimating number of 

 species, since it has been held that species populations are populations of 

 competing individuals. Therefore the estimated population of neotropical 

 Pselaphidae is 651,000,000. 



Naturally many species are uncommon, others very abundant, but these 

 figures enable us to set the average population at 46,500 individuals per 

 species. This brings up a picture of many species with relatively small popu- 

 lations, annually maintained, competing in a relatively densely populated, 

 relatively uniform biome. 



There remains the biomass (Pickles, 1937), or living weight per unit of 

 area. Weight data are at hand for the Chicago area only. These are the un- 

 published thesis figures of one of my students, Dr. Elizabeth Lunn, since 

 used by Williams (1941), in which the average weights were obtained by 

 weighing numbers of animals in a chemical balance. These were of beetles of 

 the forest floor beyond the size range of pselaphids, as well as leaf and log mold 

 oribatid mites and collembolans upon which pselaphids feed. Dr. Lunn's data 

 were of organisms from a rich forest in the Chicago area. The beetles averaged 

 0.03 gram, the mites and collembolans 0.000025 gram. They are frankly aver- 

 age weights and in using them here we have to assume that beetles of the 

 same general size have the same general weight. This Chicago figure is about 

 seven times too heavy for pselaphids. If the figure of .004 gram is used, then 

 the 651,000,000 neotropical pselaphids have a biomass of 2,604 kilograms. 



These ideas on the population may be summarized: 



Table XII 



THE NEOTROPICAL PSELAPHID POPULATION 



Number of described species 895 



Number of species estimated to exist 7000 to 14,000 



Number of individuals estimated 651,000,000 



Biomass estimated in kilograms 2,604 



Average species population 46,500 



