78 LUGO 



signature), attempts to account for the factors that control ecosys- 

 tem productivity, and has been tested widely (Holdridge et al., 

 1971). The life zone is a bioclimatic zone of the earth with a unique 

 set of conditions and capacities to support biological complexity. 

 For each climatic life zone of the world (Holdridge identifies about 

 120), a maximum level of ecosystem performance is possible. 

 Performance can be measured in terms of gross energy flow or total 

 ecosystem complexity. Holdridge (1967) has already shown that the 

 complexity of forested terrestrial systems varies according to life 

 zone. This complexity is equal to the product of maximum tree 

 height (m), number of species per 0.1 ha, basal area (m' /O.l ha), tree 

 density per 0.1 ha, and a constant, 10~^ . Life zones can be compared 

 with one another in terms of their relative potential to support 

 ecosystems of different complexity. The impact of additional 

 stressors on a given type of ecosystem should vary with life zone. 

 Ecosystems in humid life zones obviously respond differently from 

 those in arid life zones. For example, Ewel (1977) reported slower 

 responses to stressors (cutting and herbicides) in dry tropical climates 

 than in humid tropical climates and described two different 

 successional strategies for ecosystems in these two contrasting 

 environments. 



His results and those of others suggest that care should be taken 

 in comparing ecosystems across latitudinal or climatic gradients since 

 they should not be expected to exhibit the same speed of response to 

 stressors. The optimism of Slobodkin and Sanders (1969), who 

 suggest that latitude has no fundamental importance on species 

 diversity considerations, is premature. They believe that environ- 

 mental constancy is more important; however, environmental con- 

 stancy per se, without adequate energy inputs, cannot sustain 

 complexity. 



The life-zone concept has not been applied to lakes and streams, 

 but these systems are so dependent on external inputs that there is 

 no reason not to expect a life-zone pattern within them. Since 

 climate controls water availability and runoff, nutrient runoff from 

 the land, and evapotranspiration, it should also influence the patterns 

 of response of lakes and rivers. 



Model of the Action of Stressors on Ecosystems 



The analysis of individual ecosystems requires detailed examina- 

 tion of the forces acting on them. To analyze the possible ways in 

 which stressors affect individual ecosystems, I first develop a 

 conceptual model of an idealized unstressed ecosystem (Fig. 4). In 

 this simple model, we can see the relationship between four state 



