imiMBERS : STATISTICS 57 



at Rothamsted to be about 7J lb (3-4 kilograms) 

 per acre. This method is valuable in enabling com- 

 parisons to be made between species of different 

 sizes. It expresses to some extent the effective 

 biological success, or at any rate the ecological impor- 

 tance of different species, and enables the relative 

 importance of species occupying the same biological 

 niche to be studied. There is at present Httle informa- 

 tion along these lines and it is to be hoped that future 

 work will supply facts about the total weights per 

 acre of animals at different stages in a food-chain, 

 or in different years. 



We now see how ecological surveys and censuses 

 could be combined to build up a picture of the total 

 amount of animal tissue maintained on any given 

 area, so that we should be able to calculate the 

 actual biological turnover on different types of habitat 

 and at different times of year. It might then be 

 possible to express these ecological facts in terms of 

 some physiological or biochemical unit such as the 

 rate of oxidation or the rate of consumption of certain 

 organic substances. Chapman (1931, 329) summar- 

 ized some of the rather limited data which are avail- 

 able for estimating the total amounts of substance 

 present in a whole animal community, counting all 

 the species together. Three large American lakes 

 (Mendota, Monona, and Waubesa) have plankton 

 yields of 214, 238, and 215 lb. per acre of surface, or 

 1,974, 3,163, 4,398 milligrams of dry organic matter 

 per cubic metre. This is the amount at one time. 

 The annual total turnover of plankton in Lake 

 Mendota came to about 10,700 lb. per acre (dry 

 weight) or 107,000 lb. (about 48 tons) per acre of 

 living plankton. The dry weights of benthic ( bottom - 

 living) animals at middle and deep zones in Lake 

 Mendota were 43 lb. and 68 lb. per acre (p. 341). 

 Another important generalization concerns the Umits 

 to summer growth of zooplankton, depending on 

 limits to the amount of phytoplankton. The latter 



