Coexistence of fish species with strongly similar life histories - population dynamical feedback forces species to pick sides

Fish generally grow several orders of magnitude between the larval and adult stage. Many ecological properties of organisms are related to body size, and hence small fish often have very different ecological roles than large conspecifics. This also implies that omnivory, the feeding on more than one trophic level by individuals of the same species, is a common phenomenon in fish. Intraguild predation is omnivory in its simplest form, where two species compete for the same resource, but one of the species can also eat its competitor. In models, persistence of both species in such a configuration is difficult to obtain. In marine fish communities however, it is observed routinely. One way in which persistence of omnivorous species can be established is by incorporating it as an ontogenetic diet shift, where small individuals of both species compete, and large individuals of one can feed on the small individuals of the others species. We show in this study that this mechanism can not only lead to persistence of a single omnivorous species, but also to persistence of multiple omnivorous species. This is possible given that the adults have sufficiently different diets. It is shown that, while adults of both species can feed both on small competitors or on the basic resource, due to the population dynamical feedback, one species will in practice act as a predator, while the other acts as prey. This way, a system with two omnivores in practice persists as a tritrophic system. Which of the species assumes which role depends on the specific community characteristics. We show here that by incorporating complex size-dependent feeding relationships in food webs, many more species may be able to coexist than is possible based on either species-level considerations or size spectrum models which do incorporate within-population size differences, but relate diet only to individual body size irrespective of species identity. The mechanism underlying our result may be part of the explanation why fish species with seemingly similar life histories do coexist in marine ecosystems.