Practice question

Defensive, resource-based, and dispersive interactions are functional classes of mutualism because each identifies a reciprocal exchange that raises both partners’ fitness. In defensive mutualism, one partner receives protection while the defender obtains food or shelter, as in many ant–plant systems. Resource-based mutualism involves exchange of complementary resources, exemplified by plant carbon traded for fungal mineral nutrients in mycorrhizae. Dispersive mutualism includes pollination and seed dispersal: animals receive nectar, pollen, or fruit while moving plant gametes or propagules. Cannibalism instead occurs when an organism consumes a member of its own species. It is a positive–negative interaction for consumer and victim and is intraspecific rather than an exchange between mutualistic species. Cannibalism can provide nutrition, reduce competitors, or occur under crowding, but the consumed individual suffers complete loss of future fitness. Therefore, it cannot be grouped with interactions defined by reciprocal net benefit, even though indirect population-level consequences may sometimes benefit surviving conspecifics.