66 LUGO 



give as examples of unpredictable environments boreal and tropical 

 sea bottoms with low dissolved-oxygen concentrations (<2 to 5% 

 saturation) and seasonal temperature fluctuation (~5.5°C), sewer 

 outfalls and outfalls of canneries, the edges of anoxic zones in 

 estuaries and lakes, and deserts. Hickman (1975) considered the 

 shallow soils of the upper slopes of mountain peaks to be 

 unpredictable environments for plants growing on them. Bleakney 

 (1972) suggested that acyclic extreme tides prevented the occurrence 

 of genetic selection and behavioral adaptation in littoral commu- 

 nities. Ehrlich et al. (1972) documented the extinction of butterflies 

 and the lowering of population densities in subalpine ecosystems as a 

 result of unusual weather (a wet June followed by heavy, late 

 snows). 



These environments are obviously harsh, v^th factors that exhibit 

 a relatively wide amplitude of variation, but are they really 

 unpredictable? Slobodkin and Sanders (1969) defined an unpredict- 

 able environment as one in which "the variance of environmental 

 properties around their mean values are relatively high and unpredict- 

 able both spatially and temporally." This definition is subjective and 

 presents a few problems when it is used to evaluate the degree of 

 unpredictability of natural environments. First, data are usually 

 meager and do not allow the necessary calculations. Second, seasonal 

 variations may appear to be unpredictable when they are, in fact, 

 very predictable over a longer period of observation. Third, how do 

 we know what is unpredictable to a natural system? The problem 

 with the use of the terms predictable and unpredictable was made 

 obvious in the discussion following the presentation of the paper by 

 Slobodkin and Sanders at the Brookhaven symposium in 1969. 

 Slobodkin could not agree with L. C. Cole on whether cave, 

 hot-springs, and salt-lake ecosystems were or were not predictable 

 environments. Yet, we would intuitively consider them very stable 

 environments. In fact. Brock (1970) showed this to be true for hot 

 springs, and Poulson and Culver (1969) described the environmental 

 constancy of the cave environment. 



Colwell (1974) conceptualized the term predictability as the sum 

 of two separate components, constancy and contingency. Contin- 

 gency represents the degree to which time determines a state (the 

 value is minimal when the probability of occurrence of each state is 

 independent of season), and constancy is a measure of sameness of 

 state from year to year. Predictabihty, according to Colwell, is 

 essentially a measure of the variation among successive periods in the 

 pattern of a periodic phenomenon. When the variation is low, 

 predictability is high. He argues that the same degree of predict- 



