Answer: Mutualism is an interspecific interaction in which both the interacting species benefit (+, +); often the association is obligatory.
Examples:
- Lichen: the fungus gives shelter, water and minerals, and the alga (or cyanobacterium) supplies food made by photosynthesis.
- Mycorrhiza: fungi help plant roots absorb water and minerals (especially phosphorus), and the plant gives the fungus carbohydrates.
- Pollination: insects get nectar/pollen and the plant gets pollinated, e.g. fig and fig wasp, orchid Ophrys and bees.
- Rhizobium in root nodules of legumes fixes nitrogen and gets food and shelter.