COMMUNITY ORGANIZATION: STRATIFICATION 



461 



the photic zone in large part and, therefore, 

 are largely from plankton. 



The diatomaceous ooze is characteristic 

 of antarctic seas, and of the extreme north- 

 em portion of the Pacific Ocean at depths 

 between 1200 and 4000 meters. It is com- 

 posed almost exclusively of silicious diatom 

 shells. 



The pteropod ooze is essentially cal- 

 careous, comprising the shells of pelagic 

 mollusks (pteropods and heteropods) prin- 

 cipally, with some shells of Globigerina. 

 This ooze is typically deposited on tropical 

 sea floors at depths less than 2000 meters, 

 is chiefly formed in the deep sea zone, and 

 occurs in significant amounts only in the 

 Atlantic Ocean. 



The globigerina ooze is much more ex- 

 tensive. This deposit is formed in large part 

 by the shells of the foraminiferan, Globig- 

 erina biilloides, and in addition by cocco- 

 liths discussed in the next chapter. These 

 constituents make the ooze 60 to 70 per 

 cent calcareous. The globigerina ooze is de- 

 posited chiefly between 2000 and 5000 

 meters over about one-third of the lower 

 abyssal zone. 



The radiolarian ooze consists of a matrix 

 of red clay in which are silicious shells of 

 radiolarians. It is much less extensive, being 

 deposited between 5000 and 10,000 meters 

 in parts of the Indian and tropical Pacific 

 oceans. 



Lastly, the red clay covers about one- 

 third of the sea floor and is presumably not 

 organic in origin; organismal residues, at 

 any rate, form only a minor portion of this 

 sediment. It is composed largely of silicates 

 of such elements as iron, manganese, and 

 aluminum, in addition to volcanic and 

 meteoric "dust," and is especially typical of 

 the Pacific Ocean, where it covers about 

 half of the sea floor. 



Thus four of the five pelagic deposits are 

 of organic origjin. These pelagic materials 

 comprise roughly two-thirds of the floor of 

 the two inner horizontal strata of the sea. 

 In other words, the lower strata of the 

 marine vertical gradient are formed in a 

 manner ecologically equivalent to the for- 

 mation of the lower strata of the vertical 

 gradient in terrestrial communities, i.e. by 

 increment of organic materials from above- 

 notably leaves in grassland and forest 

 communities and the settling of decom- 

 posing plant and animal remains in 



aquatic communities. It should be remem- 

 bered that in both terrestrial and aquatic 

 gradients bottom strata are formed in part 

 by deposit from above and by evolution of 

 basic ingredients. This virtually homologous 

 parallel, embracing photosynthetic and non- 

 photosynthetic elements from the upper- 

 most stratum, is doubly notable. It re- 

 emphasizes the fundamental identity of 

 pattern in the organization of marine, fresh- 

 water, and terrestrial communities. It dem- 

 onstrates the sequence of events in this 

 pattern: namely, primary adjustment to the 

 physical gradients and secondary response 

 to the biological gradients, whether periph- 

 eral (photic zone, epilimnion, canopy), or 

 floor. 



STRATIFICATION IN TERRESTRIAL 

 COMMUNITIES 



These considerations prepare the back- 

 ground for an equally brief survey of strati- 

 fication in the terrestrial communities. 



In the first place, it should be noted that 

 terrestrial communities are geographically 

 distributed in broad climatic belts. From 

 either pole to the equator the mean air 

 temperature increases about 1 degree Fahr- 

 enheit for each degree of latitude. This 

 Humboldt Rule (Humboldt, 1850; Cut- 

 right, 1940) is paralleled by a similar in- 

 crease in mean air temperature with loss 

 of altitude (Chapman, 1933), which works 

 out at roughly 1 degree Fahrenheit for 

 about 300 feet elevation; that is, some 67 

 miles of latitude are equivalent to 300 feet 

 in altitude. This regular stratification in 

 temperature, and associated influences such 

 as hours of sunlight, impose a correspond- 

 ing disposition of vegetation zones horizon- 

 tally, through latitudinal change, and verti- 

 cally, through altitudinal change. Each 

 vegetational belt imposes restrictions upon 

 its associated animals, that are less apparent 

 for such mnltizonal components as migra- 

 tory birds and the more wide-ranging 

 mammals. From such a biogeographica] 

 vie\vpoint, the terrestrial organisms are in 

 concentric strata or zones from snow and 

 ice desert or tundra at high elevations or 

 subpolar latitudes, through coniferous for- 

 ests and high latitude steppe, deciduous 

 forest and temperate grassland, to tropica] 

 forest and grassland. 



This similarity of response of orgam'sms 

 to the environmental gradients, whether in 



