Biology Reference
In-Depth Information
BOX 1.1 THE OPTIMAL TRADE-OFF BETWEEN SURVIVAL
AND REPRODUCTIVE EFFORT (PIANKA AND PARKER,
1975; BELL, 1980)
The more effort an individual puts into reproduction, the lower its chances of
survival, so the lower its expectation of future reproductive success.
Reproductive costs include allocation of resources to reproduction which would
otherwise have been spent on own growth and survival and the increased risks
entailed in reproduction, such as exposure to predators. The optimal life history
depends on the shape of the curve relating profits in terms of present offspring
to costs in terms of future offspring.
The families of straight lines represent fitness isoclines, that is equal lifetime
production of offspring (Fig. B1.1.1). In a stable population, present and future
offspring will be of equal value and these lines will have slopes of −1. In an
expanding population, current offspring are worth more than future offspring
(current offspring gain a greater contribution to the gene pool) and the slopes
are steeper. In a declining population, future offspring are worth more and
slopes will be less than −1.
The point of intersection of the curves relating the trade-off between current
and future reproductive success, with the fitness isocline furthest from the
origin, gives the optimal reproductive tactic (indicated by a solid dot). When the
trade-off curve is convex (a), fitness is maximized by allocating part of the
resources to current reproduction and part to survival (i.e. iteroparity, or
repeated breeding). When the curve is concave (b), it is best to allocate all
resources to current reproduction, even at the expense of own survival
(semelparity, or 'big bang' suicidal reproduction). If maximal future reproductive
success is greater than maximal current reproductive success in case (b), then
the optimal tactic is to not breed and save all resources for the future.
(a)
(b)
Current reproductive effort
Fig. B1.1.1
