Figure 23-1. Survivorship curves (log percent) for R. hastatulus field (triangles) and undisturbed greenhouse (circles) populations.
was very low; only 9% of the undisturbed treatment plants were still alive post-flowering (day 525). Clearly, the developmental shift from vegetative to reproductive growth results in an increase in mortality. This may be an example of a pleiotropic trade-off between reproduction and survival, but the genetic correlations between these traits could not be measured with this experimental design. The mean life span in the undisturbed treatment in the greenhouse was 444 ± 259 days. There were no differences in mean life span for male and female plants that survived to flower.
The survival curves (Fig. 23-1) clearly show that, as predicted by Medawar, a larger proportion of the population in the greenhouse survived to later ages than in the field. Despite the fact that survivorship curves have traditionally been used in plant demography studies, it is in fact difficult to discern the pattern of mortality by looking at changes in the slope of a survival curve (Carey et al., 1992). Thus, in order to evaluate mortality patterns for the two greenhouse populations a plot of the age-specific mortality was calculated (Fig. 23-2). The mortality curve for the undisturbed treatment shows a mortality increase following reproduction and then a decline. There is a second, lower, increase in mortality following reproduction during the third growing season. For the 50 deflowered plants, 90% of the individuals were still alive at the post-flowering period during the second growing season. The mean life span for the non-flowering plants was 1064 ± 589 days. At the end of the third growing season, both populations showed a steady increase in mortality with increasing age of the plants, and there were no differences in mortality patterns for the undisturbed and non-flowering treatments.
The evidence from this study suggests that there may be multiple mechanisms that cause senescence in R. hastatulus. In a natural population, if random environmental factors do not cause a constant rate of mortality, then senescence may be manifest following the developmental shift to reproduction. If, on the other hand, flowering is inhibited, or plants
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