Mutualism & Commensalism

What is mutualism?

A +/+ interaction where both species benefit.

What is commensalism?

A +/0 interaction where one species benefits and the other is unaffected.

What is parasitism?

A +/- interaction where one species benefits and the other is harmed.

Q: What is competition?

A: A -/- interaction where both species are negatively affected.

Q: What is predation?

A: A +/- interaction where one organism kills and eats another.

Q: What is facultative mutualism?

A: Partners benefit but are not dependent on each other.

Q: What is an obligate mutualism?

A: Both species depend on each other for survival.

Q: What is symbiosis?

A: A close ecological relationship where a smaller symbiont lives in/on a larger host. Can be mutualistic or not.

Q: What is a symbiont?

A: The smaller partner living in a symbiotic relationship.

Q: Difference between symbiotic and non-symbiotic mutualisms?

• Symbiotic: species live in close physical association (e.g., coral–algae).
  

• Non-symbiotic: species benefit but do not live together (e.g., plant–pollinator).

Q: Example of a symbiotic mutualism in plants?

A: Rhizobium bacteria in legume roots fix nitrogen in exchange for carbon.

Q: Example of gut mutualism?

A: Wood-eating insects depend on protists that digest cellulose.

Q: What do pollinators gain in plant–pollinator mutualisms?

A: Nectar or pollen as food. The plant gains pollen transfer.

Q: What are mycorrhizae?

A: Mutualisms between plants and fungi that increase nutrient and water uptake.

Q: Benefits of mycorrhizal fungi to plants?

A: Improved phosphorus, nitrogen, and water uptake; disease protection.

Q: Example of a marine mutualism?

A: Coral and zooxanthellae: coral provides structure and nutrients; algae provide sugars and oxygen.

Q: Example of habitat mutualism?

A: Pistol shrimp digs burrows for itself and a goby; goby warns shrimp of predators.

Q: Ant–plant mutualism example?

A: Ant–Acacia: plant provides nectar, Beltian bodies, domatia; ants defend from herbivores.

Q: What are trophic mutualisms?

A: Mutualists exchange nutrients or energy (e.g., mycorrhizae, Rhizobium).

Q: What are habitat mutualisms?

A: One partner provides shelter or habitat (e.g., shrimp–goby).

Q: What are service mutualisms?

A: One partner provides ecological services like pollination, dispersal, or defense.

Q: What are “cheaters” in mutualism?

A: Individuals that take benefits without providing their share, threatening mutualism stability.

Q: Example of penalties for cheating?

A: Yucca plants abort flowers if moths lay too many eggs that would overconsume seeds.

Q: How do cleaner fish affect reef communities?

A: Removing cleaners increases parasite loads and decreases fish abundance and diversity.

Q: How do mutualisms influence populations?

A: They can increase population sizes, change species persistence, and alter community structure.

Q: What distinguishes the Lotka–Volterra equations for mutualism from competition?

A: Mutualism adds positive interspecific terms instead of subtracting them.

Q: What prediction does the mutualism model make?

A: Populations of both species can exceed their carrying capacities due to positive feedback.

Q: What defines a facultative mutualism in the model?

A: Populations increase but do not depend entirely on the partner to persist.

Q: Why is commensalism often hard to prove?

A: Because it is rare for one species to be completely unaffected. Environmental context often alters outcomes.

Q: Marine commensal example?

A: Sponge Callyspongia vaginalis hosts brittle stars and amphipods that benefit without harming the sponge.

Q: What are epiphytic plants?

A: Plants that grow on other plants for support but get nutrients from air; mainly benefit through access to light.

Q: How can commensal interactions shift?

A: They can become mutualistic or parasitic based on environment or population densities (e.g., gut bacteria).

Q: Be prepared to differentiate between which two types of mutualisms?

A: Symbiotic vs. non-symbiotic mutualisms.