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).