Mutualism: relationships where both species benefit
What mutualism is
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Mutualism is a relationship between two species in which both benefit. It is a +/+ relationship.
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It is not co-operation in any intentional sense. Each species behaves in the way that maximises its own survival and reproduction; the benefit to the partner is a by-product that happens to be reciprocated.
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Mutualisms are classified by how dependent the partners are:
- Obligate — one or both species cannot survive without the other.
- Facultative — both benefit, but each can survive alone.
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The essential test for mutualism is that each partner must gain more than it loses. Providing a benefit is costly — nectar is sugar the plant could have used for growth — so a mutualism only persists while the return exceeds that cost.
Types of mutualism
- Nutritional mutualism — one partner supplies nutrients the other cannot obtain.
- Transport mutualism — one partner moves the other's gametes or seeds.
- Protective mutualism — one partner defends the other in exchange for food or shelter.
Nutritional mutualisms
- Mycorrhizae — associations between fungi and plant roots.
- The fungus gains glucose from the plant's photosynthesis.
- The plant gains water and mineral ions, especially phosphate and nitrogen.
- The fungal hyphae are far finer than root hairs and extend much further through soil, so they enormously increase the surface area available for absorption.
- This is critical in New Zealand's often phosphate-poor soils, and virtually all native forest trees form these associations.
- Nitrogen-fixing bacteria in root nodules.
- Bacteria convert atmospheric N2 into ammonium, which the plant uses to make amino acids and nucleotides.
- The plant supplies glucose and an oxygen-controlled environment, because the nitrogen-fixing enzyme is destroyed by oxygen.
- In New Zealand, tutu (Coriaria) forms nodules with Frankia bacteria, allowing it to colonise raw, nitrogen-free substrates such as landslides and riverbeds.
- Lichens — a fungus and an alga or cyanobacterium.
- The photosynthetic partner supplies glucose; the fungus supplies structure, anchorage, water retention and protection from UV and desiccation.
- Together they colonise bare rock, which neither could do alone.
- Gut microorganisms — bacteria digest cellulose, which no animal can digest itself, releasing fatty acids the host absorbs. The host supplies a warm, anaerobic, food-rich habitat.
Transport mutualisms
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Pollination. The plant gains gamete transfer between individuals; the animal gains nectar or pollen as food.
- Tūī, korimako and hihi pollinate kōwhai, harakeke (flax) and rātā, feeding on nectar and carrying pollen on their heads.
- The pekapeka (short-tailed bat) pollinates Dactylanthus, a root parasite whose flowers sit at ground level and produce a strong scent and copious nectar — an association found almost nowhere else.
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Seed dispersal. The plant gains movement of seed away from the parent; the animal gains fruit.
- The kererū is now the only surviving New Zealand bird with a gape wide enough to swallow the large fruits of taraire, karaka, tawa and miro whole. Without it, those species' seeds fall beneath the parent tree.
- Seeds pass through the gut undamaged, and are deposited away from the parent, with a supply of faeces as fertiliser.
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Dispersal away from the parent is a large advantage because it reduces competition with the parent for light and nutrients, reduces density-dependent attack by seed predators and pathogens concentrated near the parent, and allows colonisation of new sites.
Protective mutualisms
- One partner defends the other in return for food or shelter — for example ants that attack herbivores on a plant that supplies them with nectar from special glands.
- Cleaner relationships, in which one species removes parasites from another, benefit the cleaner with food and the host with reduced parasite load.
Selective advantage
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The advantage must be stated for both partners, and both must end in survival or reproduction.
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Mycorrhizae.
- The plant absorbs far more phosphate and water, so it photosynthesises and grows faster and produces more seed. In phosphate-poor soil a plant without mycorrhizae is often severely stunted.
- The fungus obtains glucose it cannot make itself, since it has no chlorophyll, so it can grow and produce more spores.
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Pollination.
- The plant achieves cross-pollination, which produces genetically varied offspring, better able to survive changing conditions and disease than self-pollinated offspring. Animal transfer is also far more targeted than wind, so less pollen is wasted.
- The bird obtains nectar — a concentrated, reliable, easily digested energy source — supporting its own survival and breeding.
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Seed dispersal.
- The plant's seed escapes competition with the parent and the concentration of seed predators beneath it, so a higher proportion of seedlings survive.
- The bird obtains sugars and water from fruit.
Worked Example
Worked Example
A seedling of a native forest tree is grown in phosphate-poor soil.
- Seedlings grown in sterilised soil reach a mean height of 6 cm after one year, with pale leaves.
- Seedlings grown in unsterilised forest soil reach a mean height of 31 cm, with dark green leaves.
- Fungal hyphae are found growing within and around the roots of the second group.
- Radioactive carbon supplied to the leaves of the second group is later detected in the fungus.
- Radioactive phosphate supplied to the soil is later detected in the leaves.
Identify the relationship, explain how the evidence establishes it, and explain the selective advantage to each partner.
Answer:
Identifying the relationship. This is a mutualism — specifically a mycorrhizal association between a fungus and the tree's roots. It is a +/+ relationship, and the radioactive tracer results establish that directly.
How the evidence establishes it. Growth data alone would show only that the seedlings do better in unsterilised soil, which could have several explanations. The tracers are what identify a two-way exchange:
- Carbon supplied to the leaves appears in the fungus. The leaves are where photosynthesis occurs, so this carbon must be glucose made by the plant and transferred to the fungus. The fungus has no chlorophyll and cannot photosynthesise, so this is a genuine gain for it.
- Phosphate supplied to the soil appears in the leaves. This shows the fungus is absorbing phosphate from the soil and transferring it to the plant.
Because the transfer runs in both directions and each partner receives something it could not obtain alone, the relationship is mutualistic rather than parasitic or commensal. The sterilised-soil control is essential: it shows the difference is caused by a living soil organism, not by soil chemistry.
How the fungus achieves this. Fungal hyphae are far finer than root hairs and extend much further into the soil, so they greatly increase the surface area available for absorption and reach a much larger volume of soil. Phosphate ions are relatively immobile in soil, so the volume a root can reach is the main limit on uptake — which is exactly the limit the hyphae remove.
Selective advantage to the plant.
Phosphate is required for ATP, DNA, RNA and phospholipid membranes, so a shortage limits every aspect of growth — which is why the sterilised-soil seedlings were both short and pale. With mycorrhizae the plant absorbs far more phosphate and water, so it:
- builds more chlorophyll and photosynthesises at a higher rate
- grows taller, competing better for light in the forest understorey
- has more resources available for flower and seed production.
A seedling five times taller after one year is far more likely to reach the canopy before being shaded out, so it is much more likely to survive to reproduce.
Selective advantage to the fungus.
The fungus cannot photosynthesise, so it depends entirely on organic carbon obtained from elsewhere. The glucose supplied by the plant is a reliable, continuous supply, unlike the patchy dead organic matter a free-living soil fungus must compete for. This allows the fungus to grow more hyphae and produce more spores, so it leaves more offspring.
Why the relationship is stable. Each partner supplies something cheap for itself but scarce for the other — the plant has surplus fixed carbon but cannot reach distant phosphate; the fungus reaches phosphate easily but cannot fix carbon at all. Because the cost to each is far smaller than the benefit received, individuals of both species that form the association leave more offspring than those that do not, and the mutualism is maintained by selection acting independently on both.