754 



ANIMALS AND THEIR ENVIRONMENT 



(Cornivoret) 



Stcondary consumars 



Zooplonkfon 



(HerbivofM) 



Primory contuintr* 



Bocleria ond fungi 

 (Reducers) 



Gottom forms (HerblvorcO 

 Primary consumers 



Figure 37.1. A small fresh-water pond as an example of an ecosystem. The pro- 

 ducer, consumer and decomposer (reducer) organisms plus the nonliving parts are 

 indicated. (Villee: Biology.) 



lake include the water, dissolved oxygen, carbon dioxide, inorganic 

 salts such as phosphates and chlorides ol sodium, potassium and calcium, 

 and a host of organic compounds. The living organisms may be sub- 

 divided into producers, consuiners and decomposers, according to their 

 role in keeping the ecosystem operating as a stable, interacting whole. 

 The producer organisms are the green plants that manufacture organic 

 compounds from simple, inorganic substances. There are two kinds of 

 producer organisms in a typical small lake: the larger plants growing 

 along the shore or floating in shallow water, and the microscopic float- 

 ing plants, mostly algae, distributed throughout the water, as far down 

 as light will penetrate. Such small plants are collectively known as 

 phytoplankton; they are usually invisible unless present in great abun- 

 dance, when they give the water a greenish tinge. The phytoplankton 

 are usually much more important as food producers for the lake than 

 are the larger plants. 



The consumer organisms include insects and insect larvae, Crus- 

 tacea, fish, and perhaps some fresh-water clams. The plant eaters are 

 called primary consumers, the carnivores that eat the primary consumers 

 are called secondary consumers, and so on. The ecosystem is completed 



