Facilitation and mutualisms

What are facilitative/positive interactions?

• Encounters between organisms that benefit at least one of the participants and cause harm to neither

• Interactions in which the presence of one species alters the environment in a way that enhances the growth, survival, or reproduction of a second neighbouring species

How can positive interactions occur?

When one organism makes the local environment more favourable for another

What are 2 types of positive interactions?

• Direct positive interactions

• Indirect positive interactions

What is an example of a direct positive interaction in the face of physical stress?

• Sea kraits forage in the ocean (27 ºC) and digest and nest on land (42 ºC)

• Their temperature tolerance is 33 ºC

• Sea kraits nest in shearwater burrows (32 ºC)

• Benefits:

  • Temperature regulation

  • Maintain water balance

  • Potential nest sites

What is an example of indirect positive interactions in the face of consumer pressure?

• Bald eagles eat seabirds

• Gulls eat seabird eggs

• Peregrine falcons deter predators 

What is an example of a positive interaction that affects the niche?

Intertidal seaweeds reduce thermal and dessication stress → other organisms extend their distribution up the shore

What is mutualism?

• A facilitative/positive interaction between organisms where both species benefit

• Involves an exchange of goods or services:

  • Food/nutrients

  • Defense against parasites/predators

  • Refuge from competition

  • Transport of gametes/propagules

  • Refuge from physical stress

• Services may have a cost

• Benefits > costs for both species

• Benefits and costs of a service can change as environmental conditions change, resulting in changes in the strength of the mutualistic interaction that can have additional consequences for the community

What is an example of mutualism with an exchange of food/nutrients?

Plants and mychorrhizal fungi

• Plants gain greater access to water and nutrients

• Mychorrhizal fungi gain sugars from plants

What are endomycorrhizal fungi?

Fungal hyphae grow into root cells between the cell wall and cell membrane

What are ectomycorrhizal fungi?

Fungal hyphae grow between root cells

What is an example of mutualism with defense against predators?

Aggressive red wood ants defend aphid colonies in return for sugar-rich secretions of honeydew

• Increased survival rate in the presence of ants

• Decreased body size in the presence of ants

What is an example of an obligate mutualism?

Yucca and yucca moths

• Female yucca moths collect pollen from yucca plants with their unique mouthparts

• Female moves to another plant, lays eggs in the ovary of a flower, and places pollen on the stigma, pollinating the plant

• Hatched larvae eat the seeds that develop in the ovary of the plant, but never eat all the seeds

Why don’t female yucca moths lay more eggs?

More eggs → lower proportion of flowers that mature → fewer seeds