Mutualism

In mutualism both interacting species benefit.

Lichens are an intimate partnership between a fungus and photosynthesising algae or cyanobacteria.

In mycorrhizae, fungi live in partnership with the roots of higher plants: the fungus helps absorb nutrients from the soil, and the plant supplies energy-rich carbohydrates.

Plant-animal mutualisms are the most striking. Plants need animals to pollinate flowers and disperse seeds, and pay them with pollen and nectar or with juicy, nutritious fruit.

The system must also guard against 'cheaters', such as animals that steal nectar without pollinating. So flower and pollinator often co-evolve.

Many fig species have a strict one-to-one partnership with a wasp species: only the partner wasp can pollinate that fig.

The female wasp pollinates the fig inflorescence while looking for egg-laying sites; it lays eggs in the fruit, and the fig gives some developing seeds as food for the wasp larvae.

Many orchids have flower patterns that attract a particular pollinator, such as bees and bumblebees, but not all orchids offer a reward.

The Mediterranean orchid Ophrys uses 'sexual deceit': one petal resembles a female bee in size, colour and markings. The male bee 'pseudocopulates' with it, picks up pollen, and carries it to the next flower.

If the female bee's colour pattern changed during evolution, the orchid would have to co-evolve its petal to keep pollination success.