The Ecological Complex 17 



dent may have at first gained an impression of all the faetors of the 

 environment impinging upon the organism in a one-sided aetion. 

 With further understanding of the natural situation, however, it eame 

 to be realized that not only do these factors affect the organism, but 

 also the organism affects its environment. This reciprocal action is 

 seen first of all in relation to the physical features of the habitat. For 

 example, light plays a part in controlling the growth of trees, and 

 at the same time the trees control the amount of light beneath 

 them. Similarly, the dissolved nutrients and oxygen in pond water 

 affect the growth of the aquatic organisms that live there, but the very 

 activity of these organisms in turn modify these factors. The growth 

 of plants depletes the supply of nutrient salts. The respiration of ani- 

 mals consumes oxygen and increases the amount of carbon dioxide in 

 the water. Thus the environment receives materials from the organ- 

 isms living in it and loses material to them as they grow. The fact 

 that regular changes in the environment are brought about by the life 

 activities of the inhabitants was especially emphasized in a series of 

 Lowell Institute lectures given by A. C. Redfield in 1941, which he 

 entitled The Physiology of the Environment. 



The animals and plants modify the biological features of their 

 environment just as they do the non-living factors. In thinking of the 

 activity of a carnivore, such as a fox preying on rabbits, we realize first 

 perhaps the influence of the predatory action in killing the rabbits and 

 depleting their numbers. But by turning the picture around it be- 

 comes clear that the abundance of rabbits also influences the fox popu- 

 lation. A rapid growth of the individual foxes and a high rate of 

 reproduction is made possible if the supply of rabbits for food is large, 

 but the number of foxes may be drastically curtailed if rabbits be- 

 come extremely scarce. In the same way when we observe a flock of 

 sheep in a pasture, we tend to think first of the activity of the sheep 

 in grazing down the grass. It is also true, however, that the sheep's 

 sharp teeth are clipping off tree seedlings which may be sprout- 

 ing in the turf and the animals are adding manure to the pasture. 

 If it were not for the presence of the sheep, the continued existence 

 of the turf would often no longer be possible. In many situations 

 trees would seed in, and as a forest grew up the turf would be killed 

 off. Clearly the sheep and the plants of the turf form an integrated 

 system. 



The concept that organisms and their environment form a recipro- 

 cating system represents the viewpoint of most modern ecologists. 

 In every natural situation the environment affects the organisms pres- 



