466 Dynamics of the Ecosystem 



self-sufficient ecosystem rarely occurs in nature, but situations ap- 

 proaching this condition may be found. A balanced aquarium 

 represents an artificially established, self-contained system in which 

 the number and type of plants and animals are adjusted so that a glass 

 cover can be placed over the top and the organisms within will main- 

 tain themselves indefinitely. A pond with no inlet or outlet, or with 

 very little water exchange, may approach self-sufficiency. This idea 

 was first set forth by Forbes (1887) in his classical essay, "The Lake 

 as a Microcosm." In the following quotation it is clear that Forbes 

 was thinking primarily of the animals in the lake and was not taking 

 the complete ecosystem into consideration, but he states the essence 

 of the concept of self-sufficiency, and his essay has become a landmark 

 in the development of ecological thought: 



The animals of such a body of water are, as a whole, remarkably isolated 

 —closely related among themselves in all their interests, but so far independ- 

 ent of the land about them that if every terrestrial animal were suddenly 

 annihilated, it would doubtless be long before the general multitude of the 

 inhabitants of the lake would feel the effects of this event in any important 

 way. It is an islet of older, lower life in the midst of the higher, more re- 

 cent life of the surrounding region. It forms a little world within itself— 

 a microcosm within which all the elemental forces are at work and the play of 

 life goes on in full on so small a scale as to bring it easily within the 

 mental grasp. 



If an isolated and self-sufficient microcosm cannot be found, a 

 habitat may be used for study in which exchanges with other areas 

 are slight and regular, or can be measured. Thus, the living and non- 

 living materials entering a pond by way of the inlet and the substances 

 being carried away by the outlet can sometimes be measured, and 

 suitable allowance can be made. Another possibility is the study of 

 a unit area, or unit volume, within a uniform region. The operation 

 of ecological forces may be investigated in the ecosystem represented 

 by 1 hectare in the middle of a large tract of forest, or in 1 sq km of 

 grassland in range country, or in a certain area of the ocean. In these 

 instances we know very well that the unit we select is not isolated from 

 neighboring units, but we may assume that transfer out of our unit is 

 compensated for by transfer into it from surrounding areas so that 

 ecological activity within our area may be treated as if the study unit 

 were self-contained. 



Principal Steps and Components 



The fundamental steps in the operation of the ecosystem are: (1) 

 reception of energy; (2) production of organic material by producers; 



