A multi-level theory of competition for resources applied to fruit production

We have developed a simple theory concerning the competition for resources within a metapopulation of fruit units defined at four different levels: 1) the fruit, where the units are the seeds and associated flesh; 2) the infructescence (e.g., grapevine cluster), where the unit is the fruit; 3) the plant, where the unit is the infructescence or the fruit for plants without infructescences, and 4),a collection of plants, where the unit is the fruits of a plant taken as a whole. The principles of the theory are that i) the fruit mass of each unit decreases when the number of units in the metapopulation increases, possibly after a plateau, (ii) the total fruit mass of the metapopulation increases as the number of units increases until total fruit mass reaches an optimum level, after which it decreases. Using previously published scientific studies and original data, we demonstrate the theory for wild and cultivated fruit plants and analyze the results in terms of density-dependence effects.