Lecture 14: Mutualism and Commensalism

Positive Interactions

• Positive interactions occur when one or both species benefit, and neither species is harmed

Mutualism

• Mutually beneficial interaction between individuals of two species

• BOTH BENEFIT

Commensalism

• Individuals of one species benefit, while individuals of the other species do not benefit and are not (significantly) harmed

• ONE BENEFITS, OTHER NOT AFFECTED

• Many species have commensalism relationships with organisms that provide habitat

• Ex. Lichens that grow on trees, bacteria on your skin, species that depend on trees for habitat but do not harm the trees

Mutualism Example: Leaf-Cutter Ants

• Ant farmers nourish, protect, and eat fungal species that they grow, forming a relationship that BENEFITS BOTH

• The ants cannot survive without their fungi, and many of the fungi cannot survive without the ants

• Leaf-cutter ants cut bits of leaves from plants and “feed” them to the fungi

• The ants chew the leaves to a pulp, fertilize them with their own droppings, and “weed” the fungal gardens to help control bacterial and fungal invaders

• The fungi produce specialized structures called gongylidia, on which the ants feed

Both ants and fungi benefit from the realtionship

• The ants scrape a waxy covering from the leaves that the fungi would otherwise have difficulty penetrating

• The fungus digests and detoxifies the chemicals that plants use to deter insect herbivores

Symbiosis

• Symbiosis is a relationship where the 2 species live in close physcial contact with each other, such as corals and algae

• Symbioses can include parasitism (+/-), commensalism (+/0), and mutualism (+/+)

Mutualism Example: Mycorrhizae

• Mycorrhizae is the symbiotic associations between the roots of plants and various fungi

• The fungi increase the surface area for the plant to take up water and soil nutrients (1cm to 3m of plant root)

• In turn, the plants supply the fungi with energy in the form of carbohydrates

Ectomycorrhizae

• The fungus grows between root cells and forms a mantle around the root

Arbuscular Mycorrhizae

• The fungus grows into the soil, extending away from the root

• Also penetrates into some of the plant root cells

Coral Mutualism With Photosynthetic Organisms

• The coral provides the alga with a home, nutrients (nitrogen and phosphorus) and access to sunlight

• The alga provides the coral with carbohydrates produced by photosynthesis

Obligate Positive Interactions

• Obligate means it is not optional for either species

• Ex. The leaf cutter ants and fungus cannot survive without each other, and both have evolved unique features that benefit the other species

Obligate Pollinator-Flower Relationship: Tropical Figs

• Usually, the plant that is being pollinated has a very specific pollinator, and only that pollinator can pollinate it

• Tropical figs are pollinated by fig wasps

• The species have coevolved, and neither species can reproduce without the other

• The wasps have complex reproductive behaviours in the fig receptacle that ensure pollination: Wasp helps move the pollen into the female part of the flower

• Wasp larvae develop by eating some of the fig seeds

Facultative Positive Interactions

• Facultative mutualisms and commensalisms means that it is not obligate or required

• These facultative relationships can occur with multiple partners and show few signs of coevolution

• Ex. Bigger plants give shade and provide habitat for other species (nurse plants)

  • This relationship is facultative because it doesn’t have to be a specific tree which gives the shade and habitat, it can be any

Facultative Mutualism Example: Seed Distribution by Herbivores

• Various large herbivores (deer, moose) consume seeds of herbaceous plants

• Many pf the seeds pass through unharmed and are deposited with feces

  • **Feces are a dispersal mechanism

  • The plants benefit by having their seeds dispersed, the herbivores benefit from the food

Positive vs. Negative vs. Neutral Interactions

Positive

• Benefits > Costs

Negative

• Costs > Benefits

Neutral

• Benefits = Costs

  • Costs and benefits can vary in space and time

  • Ex. Soil temperature determines whether wetland plants are commensals or competitors, and temp changes throughout the day

Relative Neighbour Effect (RNE)

• Target species’ growth with neighbours present minus its growth with neighbours removed

• Positive RNE means it is better to be with neighbours, Negative RNE means it is worse to be with neighbours

Biotic Shifts/Shifts in Interaction Types

• Interactions can shift from one to another, ex. can change from mutualism to commensalism to parasitism

• Ex. Some ants protect treehoppers from predators, and the treehoppers secrete honeydew (sugar solution) which the ants feed on

• Treehoppers always secrete honeydew and the ants always have this resource, but if there are less or no predators, the tree hopper will get no benefit

• Interaction shifts from mutualism (+/+) to commensalism (+/0)

Types of Mutualisms

Trophic Mutualisms

• Mutualist receives energy or nutrients from its partner

  • Ex. Leaf cutter ants and fungus, Mycorrhizae, Coral-alga symbiosis

Habitat Mutualisms

• One partner provides the other with shelter, living space, or favourable habitat

  • Ex. Pistol shrimp dig burrows that they share with goby fish, and the goby fish get habitat and in return act as a “seeing eye fish” for the nearly bind pistol shrimp

Service Mutualisms

• One partner performs an ecological service for the other (pollination, dispersal, defense against herbivores or predators or parasites)

What are Cheaters?

• Individuals that overexploit their mutualistic partner to increase their own reproductive fitness

• Interaction will probably not persist

  • One side is getting more benefit than the other, the relationship turns parasitic and the arms-race begins

Dealing With Cheaters: Yucca Plants

• Cheaters can get “penalties”

  • Ex. In an obligate mutualism between a yucca and its exclusive pollinator, the yucca moth, the female moth collects pollen in one yucca and lays eggs in another, depositing the pollen in this flower

  • Larvae complete development by eating the seeds in the flower

• The flower is benefitted because its pollen is being dispersed, the moth is benefitted because its larvae eats the flower seeds

• Cheating can occur if moths lay too many eggs and the larvae consume too many seeds

• Cheating is solved because yucca plants abort flowers with too many eggs, resulting in a loss of food for the larvae and death

Ecological Consequences of Positive Interactions

• Positive interactions can influence community composition

• Many coral reef fish have service mutualisms with smaller organisms (cleaners) that remove parasites from the fish (clients)

• The benefit the client receives is greater than the energy benefit it could gain by eating the cleaner

• Removing these cleaner fish would cause an increase of parasites on client fish, and the number of fish and the abundance would decrease