- It is agreed that, for senescence to occur, the intensity of natural selection must decline with age. Measures of the change in the intensity of natural selection with age include reproductive value and sensitivity of fitness to changes in survival and fecundity.
- To investigate the performance of these indices in predicting the pace and duration of life, which must be inversely related for senescence to occur, we quantified the temporal distribution of these measures employing a generalised logistic distribution tailored for this purpose. This distribution has three parameters two of which measure pace (units: per time) and one which measures duration (units: time). We hypothesised that, given their influence on the shape of the distribution, the time distribution parameters would also be correlated with specific life-history attributes. We tested these hypotheses employing demographic projections for a sample of 207 perennial plant species of varied life form and ecology.
- The results confirmed the expected relationships for the time distribution parameters of reproductive value, but not in general for other indices. In particular, a tight inverse relationship between one of the parameters of pace and the duration parameter of the time distribution of reproductive value ordered species along a fast–slow continuum where these two attributes compensate each other. That is, reproductive value was spread over a temporal scale that was in inverse proportion to its accruement.
- Synthesis. The tight negative power relationship between the pace and duration of life as measured on the time distribution of reproductive value provides the strongest support so far to the idea that the pace of life determines its duration and, as a corollary, the idea that reproductive value must be directly proportional to the intensity of natural selection. Senescence is the unavoidable consequence of the devaluation of the reproductive value currency.