Information Concept in Ecology 143 



COMMUNITY STABILITY 



It is almost axiomatic in ecology that structurally complex 

 communities are intrinsically more stable than simpler ones. The 

 standard illustration is to contrast the highly stable biota of 

 tropical rain forests with the comparatively unstable assemblages 

 of the Arctic tundra. Community stability and complexity can be 

 related in the following manner. 



At any specified stage in its development, a community con- 

 tains m species, s,, of plants and animals with frequencies A'',- such 

 that 



[21 Y.Ni = N. {i=\,2,...,m) 



1=1 



The uncertainty per individual of selecting the i ' species is 



where P is probability. The total uncertainty, N < D > , referred 

 to as community diversity, is 



f4] D = -XlAMogPCs,). 



(=1 



Although the diversity problem is not of specific concern here, it 

 should be mentioned that there has been considerable develop- 

 ment of this subject along informational lines (13, 14, 15), repre- 

 senting the most extensive application of information theory which 

 has so far been made to an ecological problem. 



MacArthur (16) has proposed that community stability be 

 equated to the complexity of the food web as given by an entr'opy 

 measure: 



[5J S= -Z^^(7.)logP(r/,), 



where S is stability and Piqj) the probability of energy traversing 

 a particular path qj. The rationale of this suggestion is that removal 

 of a species and consequent destruction of the pathways leading 

 to and from it would be less disruptive to a community with a 

 high value for S than to one with a lower value. Since the P{qj)'s 

 are obviously functions of the A^,'s, it follows that stability and 

 diversity are related, the more diverse systems being the more 

 stable. Since greater stability implies greater success in meeting" 



