Mutualism

Ecosystems (Interaction and interdependence) · Populations and communities · note 6 of 9

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MutualismSpec C4.1.12

In short

Mutualism is an interspecific relationship that benefits both species. In root nodules of legumes (Fabaceae), Rhizobium bacteria fix nitrogen for the plant and receive sugars. Orchid mycorrhizal fungi supply sugars and minerals to the orchid and receive carbon compounds once it photosynthesises. Zooxanthellae in hard corals supply photosynthetic products; the coral provides carbon dioxide, nutrients and a sheltered, sunlit position.

In mutualism both species gain. Each partner provides something the other cannot easily obtain alone.

Three examples of mutualism and the benefit to each partner
Example	Benefit to the first partner	Benefit to the second partner
Root nodules in Fabaceae (legumes such as peas, beans, clover)	Plant: Rhizobium bacteria fix nitrogen gas (N₂) into ammonium, which the plant uses to make amino acids, so it grows in nitrogen-poor soil	Rhizobium: receives sugars and other carbon compounds from the plant's photosynthesis, and a protected, low-oxygen site inside the nodule
Mycorrhizae in Orchidaceae (orchids)	Orchid: fungal hyphae inside root cells supply sugars and mineral ions; orchid seeds are tiny with almost no food store, so seedlings depend on the fungus to germinate and grow	Fungus: receives carbon compounds such as sugars from the orchid once it has green leaves and photosynthesises (shown by isotope tracing in creeping lady's-tresses, Goodyera repens); a seedling gives nothing back, so the relationship becomes mutualistic only in the adult
Zooxanthellae in hard corals	Coral: the photosynthetic algae inside its cells pass sugars, other organic compounds and oxygen to the coral, supplying most of its energy	Zooxanthellae: receive carbon dioxide and nitrogen compounds (ammonia) from the coral's metabolism, and a protected, well-lit position near the surface

Zooxanthellae are dinoflagellates. If seawater gets too warm, corals expel them and bleach; without their partner they starve unless the algae return.

Three examples of mutualism. Legume roots with pink root nodules and an inset nodule cell packed with Rhizobium bacteria: Rhizobium passes fixed nitrogen as ammonium to the plant, and the plant passes sugars to the bacteria. Orchid root cells containing coiled fungal hyphae, with hyphae in the soil: the fungus passes sugars and mineral ions to the orchid seedling, and the adult orchid passes carbon compounds to the fungus. A hard coral polyp with tentacles, epidermis and gastrodermis, and an inset gastrodermis cell containing zooxanthellae: the algae pass sugars and oxygen to the coral, and the coral provides carbon dioxide, ammonia and a well-lit position. — Three mutualisms. Each arrow is a benefit, so **both partners gain**.

Exam tip:

For every example, give a benefit to each partner. Answers that give a benefit to only one partner lose marks. Common or scientific names are both accepted.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What is carrying capacity in biology?

Carrying capacity is the maximum population size of a species that an environment can support. It is set by limited resources such as food, water, light, space or nesting sites. Near carrying capacity, competition, predation and disease increase, so density-dependent factors push the population back towards it by negative feedback.

How do you estimate population size using the Lincoln index?

Catch and mark a sample (M), release it and let it mix, then catch a second sample (N) and count the marked individuals in it (R). Population size = (M × N) ÷ R. The method assumes marks are not lost, marking does no harm, and there is no migration, birth or death between samples.

Why does a population grow exponentially at first?

A population grows exponentially at first because resources are plentiful, so there is little competition, and predators and pathogens are scarce. The birth rate is far higher than the death rate, so numbers multiply at a constant rate. Later, density-dependent factors slow growth and the population levels off at carrying capacity.

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