Interactions among trade-offs
Although we have discussed trade-offs one at a time, our approach, as summarized in Figure 1, emphasizes that these behaviours and allocation decisions are all part of an integrated response to stressors. The adaptive behaviour or allocation for each trade-off depends on options and decisions for the other trade-offs. In the short term and small scale, how much an animal should behaviourally avoid stressors (trade-off 1) depends on the costs of being exposed to the stressors, which depends on the organism’s physiological coping capacity (tolerance) that, in turn, requires energy that often can only be acquired by taking risks (trade-off 2). In the longer term or larger scale, adaptive behaviour and physiological coping capacity depend on life history trade-offs (trade-off 3) and vice versa. Further, the decision to escape in space or time (trade-off 4) depends on the organism’s expected fitness in each possible situation which depends on how the organism balances trade-offs 1-3. Furthermore, given that success in escape in space or time is often state or condition-dependent, the adaptive balance of trade-offs 1-3 must include the need to maintain energy stores to preserve the option to escape in space or time. While this overall integrated response is complex, we believe that it is a reality that can usefully guide our analyses of each component and, consequently our overall understanding of organismal responses to stressors.
The integration of behavioural, physiological and life history decisions highlight the importance of differences between these types of plasticity in their relative speed and reversibility, relative to the rate of change in stressor levels. When should organisms escape versus stay and cope, using a mix of physiological and behavioral responses? If stressor levels increase slowly, organisms have time to both build higher physiological capacity and adjust behaviour, but if local stressor levels suddenly increase, this might exceed the speed of physiological plasticity. In that case, the notion of taking more risks to get more energy (trade-off 2) might not come into play, because even with more energy, organisms simply cannot mount the necessary physiological response quickly enough. Organisms can potentially still compensate behaviourally (avoid in space or time in the short term; trade-off 1), but if they cannot do that, they may be ‘forced’ to escape in space or time at a larger scale (trade-off 4).