The Relationship between Fecundity, Phenology, and Longevity: A Systems Approach

A simple systems model is constructed based on the assumption that semelparous organisms can afford to invest more in a single reproductive phase than iteroparous ones. The semelparous system is represented without, and the iteroparous is represented with, negative feedback from parental growth requirements on fecundity. At the theoretical level, systems control analysis indicates that the system without feedback reacted less accurately and more sluggishly to a test (parental desired-growth rate) input and was more sensitive to environmental perturbation. Indirect experimental and more circumstantial evidence suggest that this behavior is typical of real semelparous species and provides some confidence in the systems representation adopted. These conclusions lead to the hypothesis that reproduction in semelparity is an element of senescence and that lengthening life cycles requires an increase in intensity of feedback on reproduction. Types of feedback involved are listed. Finally the merits of a systems-theory approach are discussed.