COMMUNITY ORGANIZATION: STRATIFICATION 467 



the diurnal prairie dog, this rattler pur- tern measuring nearly three and one-half 



sues its prey into the burrows. The sit- million inches (±60 miles); where wild 



nation is rendered more complex by the oats are grown in competition with weeds 



presence of burrowing owls, which may {„ six-inch rows with eighteen to twenty 



feed upon the young rodents or may plants per foot, the average for single wild 



Uye in abandoned prairie dog tunnels. ^^t plants is 38,452 inches (less than one 



This is far from the supposedly friendly ^.^^y ^j^^^ ^^.^^ ^^^ .^^^ ^^^^^^^ ^^^^ 



association or prairie doe, rattlesnake, and .„i . .i .. ■ ^ ■ Z- n 



, . 1 rr^i • 1-1 eigrity-tnree to ninety-nine times smaller 



burrowing owl. There is obviously no ^ ^ . -^ . . ■, . , 



mutualism involved (Brehm, 1914; Seton, ^^^t systems in competition than as single 



1909). These examples suffice to demon- P^^^*'' /^ ^PP^^" *^^ t^« occupation of 



strate how animals of different grassland ^^^ ^oil by native grassland plants in free 



strata move interstratally and utilize dif- """o* competition, and their development of 



ferent periods of the twenty-four-hour ^ dense root mat, seldom penetrates more 



cycle. than 7 feet below the surface. 



The subterranean stratum of the grass- Herbivores seldom go beneath this po- 



land community is of considerable impor- tential food reservoir. Most grassland insects 



tance. This is partially a consequence of do not penetrate below 3 inches (Camer- 



the relative and seasonal aridity of grass- on, 1917), and pocket gophers seldom be- 



land, and the relatively slight protection low 2 feet, usually between 4 to 12 inches 



afforded by the herbaceous stratum to high beneath the soil surface. At least two 



summer day temperatures and high light in- groups of prairie animals go beneath the 



tensities as contrasted with the analogous grass root substratum. Colonies of prairie 



protection of the canopy of forests. The im- ants may penetrate the soil for 9 feet 



portance of the fossorial habit in this con- (Hungerford and Williams, 1912), and 



nection has been discussed by Shelford the gregarious prairie dogs may extend 



(1915). This author used data compiled by their burrows to a depth of 14 feet. Car- 



Seton (1909) to compare Manitoba forest nivores preying upon these residents may 



and steppe mammals on the basis of strati- descend to this lowest substratum in search 



fication, as summarized in the following of their food. The prairie rattler doubtless 



Table 34. Comparison of Number of Species of Forest and Steppe Mammals 

 in Vertical Stratum Occupied (Based on Shelford, 1915, after Seton, 1909) 



Stratum Forest Mammals Steppe Mammals 



1. Subterranean 6% 47% 



2. Floor 68% 53% 



3. Vegetational ( Arboreal, etc. ) 26% 0% 



table. From these percentages it will be descends to the bottom of prairie dog bur- 

 seen that approximately one-half of the rows. The bull snake (Pituophis satji) en- 

 steppe mammals inhabit the subterranean ters the more superficial tunnels of the 

 stratum for all or a part of their lives. pocket gopher in search of food. In fact, 

 The primary constituents of the vertical it has been shown that the bull snake may 

 gradient-namely, the grassland plants-ex- actually remove earth from a lateral tunnel 

 tend their root systems for a vertical dis- of a pocket gopher's burrow in order to 

 tance into the soil for from 60 inches in reach the animal (Hisaw and Gloyd, 

 wild oats to 30 inches for Marquis wheat, 1926). 



wild mustard, and Hannchen barley. These The floor, or second stratum, is both in- 



figures of vertical penetration by no means termediate and transitional between the 



allow us to appreciate the f. mount of root subterranean and herbaceous strata in am- 



surface made available for consumption by plitude of such operating physical in- 



herbivores. One wild oat crown-root will fluences as light intensity, temperature, and 



have a combined length of all its root precipitation. Its matrix is subject to ero- 



branches of 4.5 miles. Pavlychenko (1937) sion by wind and water and consists of a 



showed that where a single wild oat plant complex of soil particles mixed M'ith waste 



grows in an area 10 feet square, free from products and organic debris from decom- 



all root competition, it produces a root sys- posing portions of animals and plants, from 



