XVI 



THE RELATION OF MAMMALS TO SOIL TURNOVER, 

 EROSION, DAMS, DIKES, RESERVOIRS, CANALS 



The activity of earthworms in moving soil, continually bringing 

 fresh soil to the surface, has long been known and much discussed. 

 That burrowing mammals move considerable soil is also very apparent 

 to the most casual observer of outdoor phenomena, but it has not been 

 so much discussed, and the enormity of mammalian activity in this 

 direction and its results are but little realized. It affects soil fertility, 

 water conservation, water run-off, growth of vegetation and erosion. 

 Grinnell 1 estimates that a single group of rodents, the gophers, in Yo- 

 semite Park alone raise 8000 tons of earth an average height of eight 

 inches per annum. Extend this to include the whole world and add to 

 it the quantity moved by all other burrowing animals the world over, 

 such as moles, rats, mice, squirrels, rabbits, prairie-dogs, badgers and 

 a multitude of others, then multiply the result by the thousands of years 

 during which such burrowing has been in progress, and the total would 

 run far up into millions of tons. Moles observed in captivity made 1252 

 pits in 65 days, a daily average of ig. 2 White-footed mouse (Peromys- 

 cus polionotus) mounds are very large for so small a species, some of 

 the burrows being a foot deep and some of the mounds containing sev- 

 eral cupfuls of soil. 3 



The burrowing helps to aerate the soil, and the soil brought to the 

 surface is subjected to more rapid weathering, thus releasing minerals 

 needed by the plants. The freshly exposed soil buries portions of the 

 surface vegetation, which decomposes and helps to enrich and mellow 

 the soil. The loosening of the soil on pasture ground that is not stirred 

 by the plow to some extent serves to counteract the packing of soil due 

 to the trampling of livestock, admitting to the soil the air essential to 

 plant growth. The loosened soil more readily absorbs moisture from 



1 Grinnell, The burrowing rodents of California as agents in soil formation, 

 Journ. Mammalogy, iv, 137-149, 1923; reprinted in Ann. Kept. Smithsonian Inst. for 

 1923, PP- 339-350- Henderson, Geology in its relation to landscape, pp. 135-137, 1925. 



2 Hisaw, Observations on the burrowing habits of moles (Scalopus aquaticus 

 machrinoides), Journ. Mammalogy, iv, 79-88, 1923. 



3 Sumner and Karol, Notes on the burrowing habits of Peromyscus polionotus 

 Journ. Mammalogy, x, 213-215, 1929. 



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