Tropical plants and Markov chains: life and death in a random world

2010. május 31 hétfő, 16:15

Senescence is not inevitable. Mortality plateaus at advanced ages have been found in many species, even though biological causes remain unclear. We study cohort dynamics for species in which size and environment are predictive of short term survival. As individuals traverse pathways of size and environment, age patterns of mortality for a cohort unfold. Cohort dynamics behave as a killed Markov process. Diverse patterns may emerge. Age-specific mortality always reaches a plateau at advanced ages, but the plateau may be reached rapidly or slowly, and the trajectory may follow positive or negative senescence along the way. In variable environments, a "doubly" Markov process applies. We find that birth state influences mortality at early but not late ages, although its effect on the level of survivorship persists. Using as examples an understory shrub in a subtropical hurricane-driven forest, and a set of nine canopy and emergent trees in a tropical treefall-gap driven forest, we illustrate general findings and learn some lessons of potential application to ageing in humans.