STRESS AND ECOSYSTEMS 69 



Truly unpredictable events are, perhaps, earthquakes and many 

 human impacts. Human interferences with natural ecosystems are 

 unpredictable because they do not necessarily follow a recognizable 

 pattern and they do not operate long enough to allow for 

 adaptations to develop. Many activities of man, while nonadaptable 

 for many natural systems, however, do select for interface ecosys- 

 tems that quickly adjust to human interference and develop 

 short-term steady states depending on continuous human interven- 

 tion. These systems have many of the characteristics of the 

 physically controlled communities described by Sanders (1969, 

 Table 1 ) and are dominated by exotic or successional species. 



TABLE 1 



PROPERTIES OF PHYSICALLY CONTROLLED, BIOLOGICALLY 

 ACCOMMODATED, AND STRESSED ECOSYSTEMS* 



Physically Controlled 



Small numbers of species per numbers of individuals 



Widely fluctuating physical conditions that are not rigidly predictable 



Organisms exposed to severe physiological stress 



Environment of recent past history 



Biologically Accommodated 



Large numbers of species per numbers of individuals 



Physical conditions constant and uniform for long periods of time 



Environment of high predictability 



Ecosystems Under Stress 



Succession arrested or set back periodically 



Few successional stages between pioneers and climax; autosuccession and cyclic 



succession common 

 Plants may show stress by aberrations or leaf deformations 

 Changes in the intensity of stressors cause shifts in species composition; 



generally species diversity decreases with successional time 

 P/R ratios of 1, > 1, or < 1 at steady state 

 Speed of succession is a function of physical environment and the point in the 



system where disordering energies act 

 Species show zonations that reflect gradients of stress; zones should not be 



confused with successional stages 



*Data on "physically controlled" and "biologically accommodated" eco- 

 systems are from Sanders (1969). 



