Chapter 15 - Lecture

Chapter 15: Mutualism

Overview

  • Mutualism: Interactions between individuals of different species that benefit both partners.

  • Types of Mutualism:

    • Facultative Mutualism: Can survive without the partner.

    • Obligate Mutualism: Dependent on the mutualistic relationship.

  • Margulis and Fester suggest that eukaryotes originated from mutualistic associations.

Concept 15.1: Plant Mutualisms

  • Plants engage in mutualistic partnerships with bacteria, fungi, and animals.

  • Key Roles:

    • Nitrogen fixation.

    • Nutrient absorption.

    • Pollination.

    • Seed dispersal.

Concept 15.2: Coral Mutualisms

  • Coral reefs rely on mutualistic relationships with algae and animals.

  • Vital for ecological integrity, with exceptional productivity and diversity despite surrounding nutrient-poor oceans.

  • Mutualistic relationship with zooxanthellae (a type of algae).

Concept 15.3: Predictions in Mutualism Evolution

  • Mutualism evolves when benefits exceed costs.

15.1 Plant Mutualisms

Benefits of Mycorrhizal Fungi

  • Mycorrhizal fungi enhance plant access to inorganic nutrients (K, Cu, Zn, N).

  • Common Types of Mycorrhizae:

    • Arbuscular Mycorrhizal Fungi: Forms arbuscules (exchange sites), hyphae, and vesicles.

    • Ectomycorrhizae: Forms a mantle around roots and netlike structures around root cells.

Impact on Water Balance

  • Mycorrhizae help maintain higher leaf water potentials in plants like Agropyron smithii.

  • Indirect benefits may come from increased access to phosphorus and improved root growth, leading to better water extraction.

Nutrient Availability and Mycorrhizal Relationships

  • Fertilization can influence mutualistic dynamics.

  • In nutrient-poor soils, plants release more carbohydrates to support fungal partners.

  • Mycorrhizae become more aggressive in nutrient-rich soils, seeking carbohydrates.

15.2 Coral Mutualisms

Relationship with Zooxanthellae

  • Zooxanthallae live within coral tissues and provide nutrients through photosynthesis in exchange for organic compounds from corals.

  • Corals can manipulate the zooxanthellae’s permeability to enhance nutrient exchange.

Protection Mutualism

  • Crustacean mutualists (e.g., certain crabs) enhance coral health by preventing attacks from predators like sea stars.

  • Active protection and maintenance of coral integrity occur due to crustacean activity.

15.3 Evolution of Mutualism

Theoretical Predictions

  • Mutualism is likely to evolve where benefits outnumber costs.

  • Categories of Mutualists:

    • Successful Mutualists: Both give and receive benefits.

    • Unsuccessful Mutualists: Give without receiving.

    • Nonmutualists: Offer no benefits in either direction.

Conditions Favoring Mutualism

  • For mutualism to persist, mutualists must have fitness levels that outperform unsuccessful or nonmutualists.

  • In facultative ant-plant mutualism, benefits derived from ant protection must outweigh costs of nectar production.

  • Factors that enhance mutualism:

    • High probability of attracting beneficial ants.

    • High potential for herbivory.

    • Ineffectiveness of alternative defenses.

  • Successful mutualism dynamics depend on these conditions.