14 – Mutualism

Mutualism definition

An interaction between individuals of two different species that results in a benefit to both partners.

Facultative mutualism

A relationship where the species benefit from each other but can survive independently.

Obligate mutualism

A relationship where the species are dependent on each other and cannot survive in the absence of the other.

Mycorrhizae

A mutualistic association between fungi and plant roots.

Mycorrhizal exchange

Fungal hyphae extract environmental nutrients for the roots, while the plant provides sugars (photosynthate) to the fungi.

Mutualism-Parasitism Continuum

The concept that a relationship can shift between beneficial and harmful depending on environmental conditions.

Johnson (1993) experiment result

Plants grown with fungi generally had higher shoot mass and more flowers, but the benefit varied by soil nutrient levels.

Effect of high nutrients on mycorrhizae

In nutrient-rich soil, plants rely less on fungi; fungi may take sugars without providing nutrients, becoming more parasitic.

Fungi from unfertilized vs. fertilized soil

Fungi from unfertilized soil are more beneficial (mutualistic)

fungi from fertilized soil are less beneficial (more parasitic).

Acacia-Ant mutualism (Acacia provides)

The plant provides "extrafloral" nectar for food and hollow thorns for living space.

Acacia-Ant mutualism (Ants provide)

The ants protect the plant by attacking herbivores and clearing away competing vegetation.

Evidence for Acacia-Ant mutualism

Experimental plots showed that acacias with ants had higher growth, higher survival, and fewer herbivorous insects.

How acacias prevent ants from killing pollinators

The plant separates flowers from nectaries and produces chemical repellents that keep ants away from the flowers.

Coral-Zooxanthellae mutualism

Zooxanthellae (algae) live in coral tissues, performing photosynthesis and transferring up to 90% of their products to the coral.

Products transferred from algae to coral

Glucose, amino acids, and lipids.

Benefits of zooxanthellae to coral

Provides energy for metabolism, growth, reproduction, and the building of the calcium carbonate skeleton.

How corals regulate zooxanthellae

Corals control the release of algal compounds and provide/retain ammonia to stimulate algal growth.

Quantifying the evolution of mutualism

Mutualism evolves only when the fitness benefits of the interaction outweigh the costs.

Successful mutualist

An individual that gives benefits to a partner and receives benefits in return.

Unsuccessful mutualist

An individual that gives benefits but receives none in return.

Nonmutualist

An individual that does not engage in the exchange of benefits at all.

Conditions for mutualism persistence

Mutualism persists when the fitness of successful mutualists is higher than that of unsuccessful mutualists and nonmutualists.