Perturbation-Independent Systems 



Disturbance 



Perturbation-Dependent Systems 



Fire-Flood 



Hurricane, etc. 



Time — 



Figure 4. Disturbances in general ecosystems create vegetational setbacks and 

 complete recovery is slow, whereas disturbances in perturbation-dependent 

 ecosystems usually stimulate pulses of growth which rapidly decline unless 

 disturbed again. 



ment of natural systems, but they also provide the genetic and species resources 

 necessary to restore damaged ecosystems. Refuges distant from damaged ecosystems 

 but with suitable reinvading species could still be extremely helpful if human 

 assistance were given to the invasion process. 



Little attention has been given to the preservation of natural systems that would be 

 suitable for this purpose. Since ecosystems continue to be damaged at an alarming 

 rate, the development of refugia to aid the rehabilitation process should have a high 

 priority. 



ECOSYSTEM HYPERSENSITIVITY 



At a symposium held in 1976. Cairns and Dickson'*" speculated that ecosystems 

 might lose their resilency (ability to "snap back" after successive displacements) 

 following repeated e.xposure. This is termed hypersensiiiviiv by Rapport et al. '' It is 



229 



