Ch 15: Mutualism

Interactions Between Individuals

Interactions between individuals can be categorized based on their effects:

  • Positive (+): Beneficial for both parties involved.(this would be considered mutualism)

  • Negative (-): Harmful to one party but not necessarily to the other.

    (Examples of this would be predation or parasitism, where one organism benefits at the expense of another. )

  • Neutral (0): No significant effect on either party.

Symbioses refer to close and often long-term interactions between different biological species.

* think of a relationship: The first half of the word has “symb” as in symbol, a relationship is a symbol of a long term connection between two or more entities working together towards mutual benefit.

Types of Biological Interactions

  • Mutualism: Interactions between individuals of different species that benefit both.( none are harmed)

    • Examples:

    • Honey bees collect nectar from flowers and pollinate them in return.

    • Nitrogen-fixing bacteria provide nitrogen to plants, receiving sugars in return.

    • Clownfish live among the stinging tentacles of anemones, which protect them, while clownfish provide nutrients to the anemones.

  • Predation/Parasitism: One organism benefits at the expense of another.

  • Competition: Organisms compete for the same resources, resulting in negative effects for both.

  • Commensalism: One organism benefits while the other is unaffected.

  • Amensalism: One organism is harmed while the other remains unaffected.

  • Neutralism: Both organisms are unaffected by the interaction.

Importance of Mutualism

Mutualism is crucial for numerous ecological functions:

  • Without mutualistic interactions, we would not have vital ecosystems such as coral reefs, bioluminescent fish, and animal-pollinated plants.

  • Approximately 90% of plants depend on mutualistic relationships like mycorrhizae (fungi-root associations).

Theory of Endosymbiosis

Lynn Margulis (1938-2001) proposed that all eukaryotic cells evolved through mutualistic ingestion of prokaryotes. Without mutualism, complex life forms, including plants, fungi, and animals, would not exist.

Types of Mutualism

  • Facultative Mutualism: Not essential for the survival of either species involved. ((think of the word “facilitated” as in the species helps one another but can live independently.)

  • Obligate Mutualism: Species are highly dependent and cannot survive without the relationship. (obligated” these species rely on one another for survical)

  • Service Mutualism: Provides services such as protection or transportation. (the word service basically means to provide something. In this case, and organism provided a service to another)

  • Resource Mutualism: Provides resources such as food or water.

Different Combinations of Mutualisms

  1. Transport-Resource Mutualisms: Example includes pollinators and plants.(this involves one organism transporting the reproductive cells of another which provides resources in return.)

    *Example: pollinators (bees or butterflies) transfer pollen from one plant to another, enabling plant reproduction, while they collect nectar pollen for food)

  2. Protection-Resource Mutualisms: Example includes ants protecting plants. (in this process, an organism receives protection from a predator or herbivorous threat while the other benefits by gaining resources. In this example, the ants protect plants from herbivorous pests in exchange for sugary substances produced by the plants, such as extrafloral nectar.)

  3. Resource-Resource Mutualisms: Example includes mycorrhizal fungi and plant roots. (in this relationship, both organisms provide each other with vital resourcesthat enhance survival and growth, showcasing the interdependence found in many ecological systems. An example is mycorrhizal fungi forming symbiotic relationships with plant roots, where fungi receive carbohydrates from the plants and, in return, assist the plants in nutrient absorption from the soil.

Mycorrhizal Relationships

  • Definition: Mutualistic association between fungi and plant roots. Fungi supply plants with inorganic nutrients and receive sugars in return.

  • Over 90% of plants form mycorrhizal associations, enhancing nutrient uptake from soil.

  • Types of Mycorrhizae:

    • Arbuscular Mycorrhizal Fungi: Develop arbuscules for nutrient exchange.

    • are endosymbiotic, arbuscules are the site of exchange, hyphae, are the fungal filaments extended into the surrounding fungi

    • Ectomycorrhizae: Forms a net-like structure around plant roots.

    • and they gather nutrients from the soil, which enhances the plant's ability to absorb water and essential minerals such as nitrogen and phosphorus.

Impact on Water Balance and Nutrient Uptake

Studies (Allen and Allen, 1986; Hardie, 1985) have shown that mycorrhizae increase plant ability to extract soil water:

  • Plants with mycorrhizae exhibit higher transpiration rates and better water availability.

  • Mycorrhizae promote root growth leading to better nutrient absorption.

Ants and Acacia Trees: A Case Study in Mutualism

Ants (Pseudomyrmex spp.) and Acacia trees engage in an obligate mutualism:

  • Ants receive shelter and food, while Acacia trees gain protection from herbivores.

  • Research by Janzen (1966, 1967) demonstrated that Acacias inhabited by ants showed increased growth rates and survival as compared to those without ants.

Coral Mutualisms

Coral reefs depend on mutualistic relationships with algae (zooxanthellae). These interactions are essential for:

  • Nutrient cycling and energy transfer, allowing corals to thrive in nutrient-poor waters.

  • Protecting corals from herbivory, which is vital for their health and integrity.

  • Main benefit for zooxanthellae is access to higher nutrient levels, namely nitrogen and phosphorus.(may also uptake ammonium)

  • corals receive organic compounds that are synthesized through photosynthesis by zooxanthellae, thereby enhancing their growth and resilience in harsh environmental conditions.

  • corals induce the release of organic compounds with signal compounds that alter the permeability of the zooxanthellae membrane cell membrane

  • corals control the rate of zooxanthellae population growth and density by influencing organic matter secretion(cause to secrete 90-99% of their fixed carbon for coral to use)

Coral Protection Mutualism

-glynn found 13 coral species are protected by crustacean mutualists( helps corals not get attacked by sea stars)

Additional Benefits:

-found that crab activity promotes coral health and integrity (without them, corals showed tissue death)

-pocillopora coral increases production of fat bodies in the presence of crabs(digestive tracts of crabs inhibiting corals contained by large quantities of lipids)

Evolution and Persistence of Mutualism

Mutualism persists when the benefits outweigh the costs for both parties:

  • Successful mutualists provide and receive benefits, while unsuccessful ones may only give or receive none.

  • The fitness of mutualists must be greater than non-mutualists for the interaction to survive evolutionarily.