We happy few: Using IPMs to determine the decisive events in the lives of exceptional individuals
In any population, some individuals make it big: they are among the few that produce many offspring, grow to large size, etc. What distinguishes the lives of these happy few? We show how to use integral projection models (IPMs) to determine how the lucky got to be where they are. As an example, we work with a published IPM fitted to eight tree species, calculating quantities such as the probability of becoming large (>= 20 cm dbh), expected growth and survival as a function of age and size for those destined to be large versus those that will never become large, and the sensitivity of the probability of becoming large to quantities such as seedling survival and the standard deviation of growth.
For the trees in this study, becoming large is mostly a matter of surviving the seedling stages, which in turn is a matter of surviving long enough to reach the sapling stages. Seedlings and small saplings that eventually become large have substantially higher size-specific growth rates than those that do not, but this by itself has only a small impact on the chance of becoming large. Growth of these species is slow and nearly deterministic, however, and having higher-than-average sapling growth may be more important when growth variance is higher.