Density-dependent effects of mortality on the optimal body size of a habitat shift: Why smaller is better despite increased mortality risk?

Many animal species across different taxa change their habitat during their development. An ontogenetic habitat shift enables the development of early vulnerable-to-predation stages in a safe ‘nursery’ habitat with reduced predation mortality, while less vulnerable stages can exploit a more risky, rich feeding habitat. Therefore, the timing of the habitat shift is crucial for individual fitness. We investigate the effect that size-selectivity in mortality in the rich feeding habitat has on the optimal timing of the habitat shift using a population model and the adaptive dynamics approach. We show that the size-selective nature of mortality in this habitat affects density-dependent body growth rate in the nursery habitat and thus the optimal timing of the habitat shift. This is caused by the effect exerted by size-dependent mortality on the size distribution of the population that results in strong competition in the nursery habitat. We furthermore find that, as a consequence of this effect, increased size-selectivity in mortality in the rich feeding habitat causes the optimal body size to shift habitat to decrease. Our results reveal the interdependence between population structure and life history traits, and highlight the need for integrating ecological interactions in the study of the evolution of life histories.