Bees get nectar. Flowers get pollinated. Both benefit. That is mutualism, a relationship where both interacting species gain from the association. The term covers a wide range of partnerships. Some are obligate, meaning the species cannot survive without each other. Figs and fig wasps are an example. The wasp pollinates the fig and lays its eggs inside. Neither can reproduce without the other. Other mutualisms are facultative, meaning the partners can survive alone but do better together.

Mutualisms take many forms. Cleaner fish eat parasites off larger fish, getting a meal while the larger fish gets rid of pests. Mycorrhizal fungi connect to plant roots and help them absorb water and minerals; in return, the plants give the fungi sugars. Ants protect acacia trees from herbivores and receive shelter and food in return. These relationships are not always equal. One partner may cheat, taking benefits without providing services. Over time, cheating can destabilize the mutualism.

Common mutualisms

• Pollination: bees, butterflies, and flowers.
• Cleaner fish and their hosts.
• Mycorrhizal fungi and plant roots.
• Ants and acacia trees.

Mutualism is not altruism. Each partner is pursuing its own interest. The relationship persists because both come out ahead.