EVOLUTIONARY PERTURBATIONS OF OPTIMAL LIFE-HISTORIES

An optimal age-structured life history is perturbed by increasing the mortality factors specific to an age k. These can be density dependent (DD) or independent (DI), avoidable or unavoidable. The last two refe r to whether their effect on any individual depends or not on how much energy it devotes to defence. Age-specific trade-offs between the all ocation of energy to defence and fecundity exist: survival probabiliti es through each age x, P-x, are concave decreasing functions of the fe cundity per unit size at that age, b(x). These are constraints for the optimal life history. The changes induced by perturbation are evaluat ed by equations that predict whether some extra energy is diverted tow ards survivorship at the expense of fecundity or vice versa. The model predicts that for DI environments the degree of avoidability of the m ortality source perturbed, is a decisive factor for the strategy selec ted at age k, but not for any other age class. DD environments are mor e complex since all ages are simultaneously embedded in density effect s. The perturbations not only act directly - as in the DI situation - but also indirectly through their effect on equilibrium density, N. W hen any kind of mortality source becomes more intense at age k, N alw ays decreases and all ages react in consequence according to the effec t of density on each age-specific trade-off. Either coincidental or op posing reactions can be expected from direct and indirect effects. The resultant strategy for any age would be a matter of magnitude compari sons. Some possible general patterns are discussed.