Analysing hierarchically-structured fitness and modular dynamics in plants: integration of concepts from population dynamics

This paper analyses the reproductive strategies of a modular organism by means of demographic analysis of its reproductive units. The buds of the reproductive structure of Retama sphaerocarpa (L.) Boiss., a perennial shrub with a simple modular structure, were considered as ‘individuals’ of a population of modules within each individual plant. The development of the buds leads to the production of subpopulations of new units (inflorescences, flowers, fruits) of a lower hierarchical level. Fitness and hierarchical fitness (defined by the integration of developmental hierarchical levels in the plant) were analysed at the shoot module level, starting from the analogy provided by classical population dynamics and taking into account the different subpopulations of modules that take part in the reproductive process. Consideration of the dynamics of such subpopulations reveals the way in which demographic strategies partitioning the reproductive effort are exerted throughout the plant, and their consequences for fitness. The analysis of the reproductive process in R. sphaerocarpa shows a critical developmental transition from flowering to fruiting buds, which is a consequence of a low survival rate of the subpopulation of flowers. Despite the specificity of the empirical information used in this investigation, the fitness analyses proposed in this paper are fully applicable to any hierarchically-structured biological system, either modular or unitary.