10. THE SUBSTRATUM 



The atmosphere, in its lower reaches, is a 

 much used medium for active locomotion 

 by flying animals or for passive transport, 

 especially for smaller plants and animals 

 and their disseminules. Unlike water, which 

 is denser and therefore more suitable for 

 buoyant support, the air is too tenuous a 

 substratum to furnish the physical base in 

 wliich an animal can pass its entire hfe his- 

 tory. The biosphere is a relatively thin shell 

 over the earth. It extends down to the 

 greatest depths of the sea, and, on land, 

 bacteria have been reported from coal at 

 depths of up to 1089 meters (ZoBell, 1946). 



The regions of contact between the at- 

 mosphere, hydrosphere, and lithosphere 

 make the great interphases in the physical 

 environment. The surface of water, the air- 

 water interphase, is a sharply defined film. 

 The surface of the harder rocks, whether 

 covered by air or by water, also is a pre- 

 cise, definite boundary. The meeting of 

 earth and air, or of hthosphere and hydro- 

 sphere, usually furnishes a transition zone 

 of some depth that provides a series of habi- 

 tats related to the degree to which subter- 

 ranean air penetrates the soil, or to which 

 water penetrates the underlying lithosphere 

 to form a "terraqueous" zone. 



The surface of water is an important sub- 

 strate for life, though not nearly so impor- 

 tant as the surface of land. Both water and 

 the upper layers of the lithosphere, whether 

 covered by water or air, furnish important 

 mechanical substrata for plants and animals 

 at the surface and for some distance below 

 it. The extent to which these surfaces are 

 occupied by organisms depends on other 

 conditions in addition to fitness for physical 

 support. The limiting action of other factors 

 can be illustrated by relations to light. Even 

 in the presence of a suitable substrate, 

 green plants grow only on the lighted areas 

 on the surface of the land and in a thicker 

 but still relatively shallow region in aquatic 

 environments. 



In this whole general section we are dis- 

 cussing the relation of animals to their 

 nonliving physical environment, and here 

 we come squarely upon an essential artifi- 

 ciality of our basis for classification of the 

 environment of animals. Living organisms 

 are important substrates for surface-dwell- 

 ing forms and for internal symbionts and 



parasites, and the bodies of dead plants ano 

 animals also support varied small biotic 

 communities. In many instances, animals 

 appear to react to certain other animals and 

 to many plants as they do to similarly solid, 

 nonHving objects in comparable positions. 

 The carapace of a mature horseshoe crab 

 (Limulus) on a tide flat may carry about 

 the snail, Crepidula, a common barnacle, 

 and some hydroids and other animals char- 

 acteristic of neighboring rocks, and on land 

 a puma may lie in wait for its prey in the 

 lower branches of a tree as it might atop an 

 advantageously placed rock. The prehensile 

 tail of American monkeys and other animals 

 of the tropical forest are understandable in 

 relation to the hard, round branches of trees 

 as a type of physical substrate. At present 

 we shall restrict ourselves to the non-living 

 environment. 



WATER SURFACE 



The air-water interphase is characterized 

 by a surface-tension film, both sides of 

 which are important for animals. A whole 

 ecological assemblage of birds, the so-called 

 water birds, if not in flight, typically float 

 or swim on the water. Many float relatively 

 high, as do swans, while others are nearly 

 submerged, hke the cormorant. In addition 

 to such flotation, many small objects are 

 supported directly by the surface film of 

 water, wliich is a tiansparent false bottom 

 of considerable strength. The classical and 

 easy demonstration is made by floating a 

 slightly oiled needle on the surface film. 

 The needle does not sink in quiet water 

 despite its high specific gravity. The upper- 

 most film of water has a tensile strength of 

 from 10,700 to 25,000 atmospheres result- 

 ing from intermolecular attraction (Ter- 

 zaghi, 1942). Such a film can support real 

 weight, whether it impinges from above or 

 is attached from below. 



The organisms associated intimately with 

 the surface film of water have been called 

 the neuston, as contrasted with the plank- 

 ton immersed in the water and mainly at 

 the mercy of water currents, and with the 

 more eflSciently swimming nekton. The or- 

 ganisms of the neuston may be microscopic 

 or macroscopic; its animals and plants, as- 

 sociated at the surface film of fresh water, 



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