XVI. EXERCISE ON ECOLOGY 



Ecology is that part of biology which studies living organisms in their natural 

 environments. Its problems are: (i) to locate every species of plant and animal 

 in the place which it naturally inhabits; (2) to find out and measure all of the 

 factors which together make up its surroundings, as physical factors (light, 

 temperature, moisture), chemical (oxygen content, carbon dioxide content, salts 

 present), physiographic and metereological (climate, seasonal changes, composi- 

 tion of soil, etc.), and biological (other organisms present); (3) to discover 

 what structures the animal possesses which enable it to maintain itself success- 

 fully in its environment (e.g., if an animal living in a swift stream had not some 

 means of hanging to objects, it would be swept away) ; (4) to determine how its 

 behavior enables it to continue to live in the environment in which it is found and 

 how it responds to the various stimuli present in that environment, a field of 

 work called animal behavior; (5) to study how environments change through 

 physiographic or other processes, and how this effects the organisms inhabitating 

 those environments. All of these matters must be determined not only for the 

 adult but also for all stages of the life-cycle. The problems of ecology are there- 

 fore exceedingly complex and difficult ones. 



The class should be conducted if possible on an excursion into the field, into 

 any characteristic habitat, as a pond, swift stream, woods, etc. Each student 

 should carry a number of Mason jars into the field and bring back to the labora- 

 tory living animals upon which experiments can be performed in the laboratory. 

 The animals are to be placed as soon as possible into conditions imitating the 

 natural environment. As this is easiest in the case of pond animals, the pond 

 environment will prove one of the most suitable for an exercise of this kind. 

 While in the field the student will note carefully the various possibilities for 

 animal habitats in each general environment, as in a pond, bottom of the pond, 

 on vegetation, free in the water, under floating objects, etc., and note which 

 animals live in these variqus places. 



As a sample exercise in ecology the following experiments on the pond snail 

 are presented. The common pond snails are Limnaea, having a long spiral 

 shell; Planorbis, flat spiral shell; and Physa, short spiral shell, with the last 

 chamber much larger than any of the others. The instructor should furnish 

 references for reading in connection with this exercise. 



i. Natural environment of the pond snail. Where were the animals found 

 in the field? What were they doing? What are they doing now in the laboratory 

 aquaria where they have been kept since collected? What are the factors of 



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