STRESS AND ECOSYSTEMS 



73 



to a system must be coupled with increased availability of other 

 essential resources. Otherwise the input energies cannot be usefully 

 converted. Table 2 summarizes literature examples of push— pull 

 effects of different stressors. 



These examples suggest that, as the inputs of energy to a system 

 change (including energies that push and/or pull), the biotic 



TABLE 2 



EXAMPLES OF POSITIVE (PUSH) AND NEGATIVE (PULL) 



EFFECTS OF STRESSORS ON ECOSYSTEMS 



AND POPULATIONS* 



Stressor 



Pushing effect 



Pulling effect 



Ionizing 

 radiation 



Tidal 

 extremes 



Water flow 



Flooding 



Volcanic 

 eruptions 



Denudation, 

 clipping, 

 herb ivory, 

 or defolia- 

 tion 



Salinity 



Observed faster decomposition, 

 leaf fall, gi-owth, and recovery 

 rates for a variety of dosages 

 (Woodwell and Marples, 1968) 



Redistribute nutrients, sedi- 

 ments, organic matter, and 

 organisms (Odum, Copeland, 

 and McMahan, 1974) 



Brings nutrients and redistributes 

 larvae (Anderson and 

 Lehmkuhl, 1968); brings food 

 and O2 and removes toxic 

 substances (H. T. Odum, 1955) 



Removes competition; signals 

 the beginning of phenological 

 events (John, 1964) 



Allow for better nutrient, 

 moisture, and competitive 

 environments (Eggler, 1967) 



Stimulates plant productivity 

 (Churchill et al., 1964); 

 diversifies community at 

 low rates (Penfound, 1964) 



Allows higher gross productivity 

 in mangroves up to about 

 seawater strength (Hicks 

 and Burns, 1975) 



Can be lethal; disrupts 

 structure and increases 

 respiration rates; changes 

 species composition 

 (Ferens and Byers, 1972) 



Exposes organisms to 

 lethal conditions 

 (Glynn, 1968) 



Removes structure; causes 

 high energy maintenance 

 costs in plants and 

 animals (Anderson and 

 Lehmkuhl, 1968; 

 Moore, 1964) 



Increases energy mainte- 

 nance costs; temporarily 

 decreases number of 

 taxa and individuals 

 (Hoopes, 1974) 



Suffocate and kill plants 

 and animals (Eggler, 

 1967) 



Removes structure and 

 reduces diversity at high 

 rates; causes mortality 

 (Churchill et al., 1964) 



At values higher than 35 /qq, 

 increases respiration rates 

 and decreases transpira- 

 tion and net productivity 

 rates (Lugo and 

 Snedaker, 1974) 

 (Table continues on following page.) 



