Deserts, prairies, mountain ranges, tundras, forests, oceans, rivers, and the 

 like each obstruct the migration of certain species, and at the same time fur- 

 ther the distribution of others. 



In any area the composition of the population changes through the arrival 

 of new species which happen to fit the conditions brought about by their 

 predecessors and which make, on the whole, better use of the existing situation. 



Through the interactions of plants and animals with each other and with 

 the soil, water and atmosphere, the composition of a population gradually 

 reaches a point which represents the optimum, or climax, for a region. 



The many different interdependent species in a region make up what Is 

 called a natural community. 



As man subordinates other forms of life to his purposes, he sometimes 

 destroys the very species upon which he depends for his further existence. 



As in the natural community, a growing human community makes it pos- 

 sible for new types to flourish; the pioneer becomes relatively inefficient in 

 every job he is capable of doing and so is replaced by interdependent special- 

 ists of many and diverse kinds. 



EXPLORATIONS AND PROJECTS 



1 To study the way in which earthworms mix and aerate the soil, place between 

 two vertical panes of glass an inch apart a layer of rich loam and above it a layer of 

 coarse sand. Introduce earthworms and observe for several days. (Keep soil rela- 

 tively moist, though not wet.) Describe what happens to soil and sand and explain 

 how this is related to the growth of plants. 



2 To investigate food chains, start with any species of animal that is convenient 

 and find out (a) upon what species it feeds and (b) what species feed upon it. Extend 

 the chain in both directions. That is, after each species in list a, enumerate in order 

 the species that supply it food, tracing as far back as possible; and similarly, after each 

 species in list ^, enumerate in order the species that use // as food, again tracing each 

 line as far as possible. The food chains and food cycles of such organisms as a lady- 

 bird beetle, an earthworm, a swallow, a crustacean, or a zebra would be interesting 

 to investigate. Represent your findings diagrammatically. 



3 To work out the food chains in a restricted habitat, find what organisms dwell 

 in it, and so far as possible determine what eats what. In such a habitat as a decayed 

 log one can study various relationships among fungi, sow bugs, millepedes, spiders, 

 caterpillars, ants, aphids, termites, protozoa, bacteria, seedlings, birds, centipedes, 

 snails, slugs, etc. Record and interpret your findings. 



4 To become familiar with various habitats, visit a varied stretch of countryside, 

 identifying different habitats, along with the dominant forms of life found in each. 

 Some interesting kmds of habitats to study are a moist woodland gorge, a pine wood, 

 a deciduous forest, a cypress swamp, a floating sphagnum-moss bog, a riverbank, a 

 seashore, a sand-dune, a second-growth brush, a meadow, a barnyard, a mountaintop. 



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