ECOSYSTEM RESPONSES TO STRESS 105 



response. The stress of a specific ecosystem cannot be discussed 

 without reference to the nature of the input changes, the system's 

 past and present states, and the observer's characterization of the 

 system, abihty to detect changes in the system, and his time scale. 

 These considerations not only require that the concept of ecological 

 stress be discussed in precise terms but also provide a practical 

 approach to assessing stress in a specific ecosystem. 



An ecosystem's stress response can be described in several ways. 

 A system's ability to resist the effects of exogenous input changes 

 can be measured by the magnitude of its deflection from the normal 

 operating range (Webster, Waide, and Patten, 1975) and by the time 

 required for an initial displacement to occur (Hurd and Wolf, 1974). 

 The first characteristic can be referred to as resistance and the second 

 response time. The trajectory of the stressed system must be 

 statistically defined so that significant deviations from the normal 

 operating range can be assessed. After a system has been displaced 

 from its normal operating range, it may or may not return to its 

 original, statistically defined condition. If it fully recovers to its 

 original normal operating range, the time required is a measure of its 

 resihence to the stress (Webster, Waide, and Patten, 1975). (These 

 concepts are depicted in Fig. 1.) 



Since resistance and resilience are not mutually exclusive 

 properties, another measure is necessary to describe the divergence of 

 the perturbed trajectory from that of the unstressed system. The 

 integral difference between the normal operating range and the 

 perturbed trajectory can be defined as total relative stability 

 (Webster, 1975), which is illustrated by the shaded area in Fig. 1. 

 This reflects the total impact of a stress on an ecosystem and is a 

 function of both the specific system and the altered input. 

 Ecosystems can be compared by dividing resistance and total relative 

 stability measures by the mean of the observations used to define the 

 normal operating range. The resilience measure can be made 

 independent of resistance by dividing the recovery time by the 

 relative resistance. This model provides a means of identifying 

 stresses and of describing an ecosystem's stress response. 



Many hypotheses have been proposed to describe system 

 characteristics that influence an ecosystem's stress response or 

 stability. The best known of these has been the hypothesis that 

 increased diversity leads to increased stability [see the review by 

 Goodman (1975)]. Large abiotic nutrient reserves have also been 

 proposed as a major influence on an ecosystem's response to stress. 

 Pomeroy (1975) suggested that increased nutrient availability leads 

 to increased resistance and resilience, but Webster, Waide, and Patten 



