PHYSICOCHEMICAL AND BIOLOGICAL STRESSORS 



137 



u 

 z 

 < 



Q 

 Z 

 Z) 



m 

 < 



> 



O 



HIGH 



PHYSICOCHEMICAL 

 STRESS 



LOW 



Fig. 2 Effect of risk of mortality (solid bold line) on abundance 

 (thin line) of a species inhabiting a portion of a physicochemical 

 gradient. Sources of risk of mortality : abiotic stressors (white area 

 under bold line); biological stressors (shaded area under bold line). 



the ends of the gradient otherwise have Gaussian abundance curves, a 

 feature they share with very different organisms, e.g., some trees 

 (Whittaker, 1967) and birds (Terborgh, 1971) inhabiting other 

 gradients. The model shown in Fig. 2 could account for these 

 characteristic distribution patterns in groups that originated at one 

 end of a physicochemical gradient. As the risk of mortality from 

 usual or unusual weather decreases, risk from various biological 

 stressors increases. The abundance of the species in question mirrors 

 the combined risk of mortality from both abiotic and biological 

 stressors, with peak abundance corresponding to minimum combined 

 risk of mortality. The abundance curves of species result from actual 

 selective mortality if they lack life-history stages that can discrimi- 

 nate among habitats. In species having the potential for such 

 behavior, fitness is increased by avoidance of risky portions of the 

 gradient, and abundance curves result from both habitat selection 

 and mortality. The positions of species along the gradient, then, 

 reflect the relative degrees to which they have specialized toward 

 coping with physicochemical vs. biological stressors. 



ACKNOWLEDGMENTS 



I have profitted greatly from the essential balance of knowledge, 

 wisdom, and honesty provided by John S. Garth, Gerald J. Bakus, 

 William Stephenson, Alan D. Havens, Hugh Dingle, and Virginia 

 Fox-Norse. Many people helped in the field and laboratory, 



