The shift from concern about point source effluents to lakes, rivers, and streams 

 (Public Law 92-500) to nonpoint source run-off (Section 408) in a sense also 

 illustrates a wider perspective. Effects of nonpoint source pollution came to light 

 primarily through efforts to clean up point source problems, which consisted largely 

 of concentrated organic wastes and acutely toxic discharges. Nonpoint source 

 pollution contributed less obvious nitrogen and phosphorus loading, sedimentation, 

 and sublethal or chronic effects of toxins on aquatic organisms. Agricultural land use 

 represents the major nonpoint source influencing most watersheds. ''"'*' 



Single species, presence/ absence, and toxicity tests (described by Mount and 

 Gillett) have often been replaced by approaches that attempt to integrate system 

 properties, e.g., some of the more recent diversity index formulations. Such indices 

 have major shortcomings, yet the approach they represent reflects an increased 

 awareness of the system as a whole. Recent interest in groups of tests*^ and trophic 

 chains, *'' as opposed to single species, as ecotoxicological models for study of 

 ecosystem contamination also stresses this view. Renewed interest in habitat 

 management*'''*^'''^ and ecosystem protection rather than management of a given 

 population or for a critical species may provide a sounder strategy for management*^ 

 and also illustrates the trend towards the broader ecosystem approach. Many 

 schemes of assessment currently in use concentrate on habitat evaluation rather than 

 resident species censuses.*** 



Failure to consider direct and indirect ramifications of actions and the interrelated 

 components of ecosystems has been in part responsible for the present ecological 

 dilemma. A broader consciousness of relationships inherent in the systems being 

 disturbed is one of the more important emerging features of environmental research 

 and management in the 1970s. 



Appreciation of Large Scale Events 



Another trend is a developing appreciation for differences between man-made 

 disasters and natural episodic events (such as fire, flood, and volcanic eruption). The 

 magnitude and timing of natural events are integrally related and/ or essential to 

 many ecosystem processes. For example, annual Hooding serves as a reset 

 mechanism which maintains the long-term community structure of running water 

 ecosystems**^ and the use of prescribed fire represents the return of a natural 

 ecological factor to the environment. '",71 



Concern for maximization or optimization of use of a particular resource has been 

 tempered with more concern for long-term stability of that resource. Systems are 

 dynamic, and man-induced changes frequently set in motion a response with 

 undetermined and unforeseen consequences.''- Long-term ecological records are 

 essential for distinguishing natural oscillations from aberrant ecosystem behavior. 

 This can be especially important in the management of fish and wildlife resources. '* 

 Among methods recently developed to analyze effects of man-induced or natural 

 changes in the environment is that of intervention analysis, '^'''' which gives the 

 probability that changes in mean level can be distinguished from natural data 

 variability. The method is particularly sensitive to the way in which data are 

 collected, and suggests (counterintuitively) that the post-intervention data record be 

 substantially longer than the pre-intervention period. Long-term studies are rare and 

 yet are often required for the recognition of thresholds beyond which habitat/ 

 ecosystem reclamation may become exceedingly difficult, if not impossible. 



Recognition of Limits 



The concept of threshold, or limit, denotes an absolute quantity as well as a level 

 beyond which, for example, a given population or system property cannot be 

 sustained. A prescribed area may have the potential (e.g., territory, food resource 

 base, nesting or spawning habitat) to adequately support a limited number of 



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