ECOLOGY 779 



Each of these strata has a distinctive animal population. Even such 

 highly motile animals as birds have been found to be restricted to cer- 

 tam layers. Some birds are found only in shrubs, others only in the tops 

 of tall trees. There are daily and seasonal changes m the populations 

 found in each stratum and many animals are found first in one layer 

 and then in another as they pass through their life history. These strata 

 are strongly interdependent and most ecologists consider them to be 

 subdivisions of one large community rather than separate communities. 

 Vertical stratification, by increasing the number of ecologic niches in a 

 given surface area, reduces competition between species and enables 

 more species to exist in a given area. 



350. The Dynamic Balance of Nature 



The concept of the dynamic state of the body constituents was dis- 

 cussed in Chapter 4, and we learned that the protein, fat, carbohydrate, 

 and other constituents of both animal and plant bodies are constantly 

 being broken down and resynthesized. Biotic communities are con- 

 stantly undergoing an analogous reshuffling and the concept of the 

 dynamic state of communities is an important ecologic principle. Not 

 only are plant and animal populations constantly subject to changes in 

 their physical and biotic environment to which they must adapt or die, 

 but communities undergo a number of rhythmic changes— daily, lunar, 

 seasonal, tidal, etc.— in the activities or movements of their component 

 organisms which result in periodic changes in the composition of the 

 community as a whole. A population may vary in size, but if it outruns 

 its food supply, like the Kaibab deer or the lemmings, equilibrium is 

 quickly restored. Communities of organisms are comparable in many 

 ways to a many-celled organism, and exhibit growth, specialization and 

 interdependence of parts, characteristic form, and even development 

 from immaturity to maturity, old age and death. 



Questions 



1. Define an ecosystem. Discuss an aquarium of tropical fish as an example of an eco- 

 system. 



2. Differentiate clearly between a habitat and an ecologic niche. 



3. Discuss the various pathways of the nitrogen cycle. What can man do to increase the 

 supply of nitrates? 



4. Define: range of tolerance, hibernation, photoperiod, biologic potential, environ- 

 mental resistance. 



5. Define and give examples of commensalism, mutualism and parasitism. 



6. What is meant by a food chain? Why is the number of steps in a food chain limited? 

 Describe a food chain ending in a bird hawk. 



7. AVhat is meant by a survi^al curve? Discuss the importance of such curves to a life 

 insurance company. 



8. Discuss the factors that tend to keep relatively constant the size of a population of 

 animals in the wild. 



9. \Vhat factors tend to cause cyclic variations in the size of a population of anmials in 



the wild? 



