Lecture 17

Mutualism in Ecology

Outline of Topics Covered

  • Positive interactions between species

  • Plant-fungi mutualism

  • Plant-animal mutualism

  • Animal mutualism

  • Types of mutualism

  • Impact of mutualism on communities

Ecological Interactions

  • Definition: Ecological interactions refer to the relationships between species and organisms that affect each other's survival, growth, and reproduction.

  • Classification of Interactions:
      - Beneficial (+): one or both species gain
      - Harmful (-): one species is harmed
      - Neutral (O): one species is unaffected

  • Negative Interactions: These occur when energy is expended or when injury occurs.

Non-harmful Interactions

  • Commensalism:
      - One species benefits while the other is unaffected.

  • Mutualism:
      - Both species benefit from the interaction.
      - Can involve:
        - Facultative Mutualism: Partners can survive independently.
        - Obligate Mutualism: Partners cannot survive without each other.

  • Acknowledgment of Mutualism's Importance:
      - Once overlooked, mutualism is now recognized as vital for ecological interactions and relationships.
      - Definition of Mutualism: Interaction between two organisms in which both benefit, can involve symbiosis (+/+).

Positive Interactions Between Species

Examples of Mutualism

  1. Pollination Example:
       - Hummingbird feeds on nectar and inadvertently transfers pollen between flowers.
       - Benefits:
         - Hummingbird gains food.
         - Plant achieves pollination.
       - Outcome: Both species benefit.

  2. Digestive Mutualism in Herbivores:
       - A deer consumes plants, while microorganisms (protozoans and bacteria) in its stomach facilitate cellulose breakdown.
       - Benefits:
         - Deer obtains energy from digested plants.
         - Microbes receive food and shelter within the deer.
       - Outcome: Mutual benefit for both organisms.

Importance of Plant Mutualisms

  • Key Roles of Plant Mutualisms:
      - Support vital processes like nitrogen fixation, nutrient absorption, pollination, and seed dispersal.
      - Essential for the function and sustainability of terrestrial ecosystems.

Mycorrhizal Fungi and Plant Relations

  • Evolution: Mycorrhizae evolved around 400 million years ago.

  • Function:
      - Plants provide fungi with carbohydrates from roots.
      - Mycorrhizae enhance plant nutrient uptake, especially of phosphorus, nitrogen, copper, and zinc.

  • Types of Mycorrhizae:
      - Arbuscular Mycorrhizae: Fungi penetrate root cells forming arbuscules and vesicles.
      - Ectomycorrhizae: Fungi form a mantle around roots and create a network surrounding root cells.

Geographic Distribution of Mycorrhizal Associations

  • Mycorrhizal fungi associations cover the Earth's land surface, with specific geographic distributions indicated by color.
      - Regions colored dark green show the dominance of ectomycorrhizae with diverse fungal members from Ascomycota and Basidiomycota.

Mycorrhizae and Plant Water Relations

  • Mycorrhizal fungi enhance plant water uptake, crucial in dry or nutrient-poor soils.

  • Experimental Findings:
      - Grasses with mycorrhizal fungi exhibit higher leaf water potential, suggesting better water retention.
      - Red clover with mycorrhizae shows increased transpiration rates, indicating greater water loss through leaves.

Nutrient Availability and Mutualism

  • Not all fungi benefit plants equally; nutrient-rich environments affect mutualistic fungi dynamics.

  • Experiment by Nancy Johnson on Andropogon gerardii:
      - Seedlings grown in sterilized sand with different soil inoculums:
        - Fertilized soil inoculum (from fertilized plots)
        - Unfertilized soil inoculum (from unfertilized plots)
        - Sterilized control (no mycorrhizae)
      - Treatment combinations: +P, +N, +N +P.

Experimental Results Summary

  • Plants with mycorrhizae generally exhibited:
      - Increased height
      - Lower root:shoot ratios (showing better nutrient accessing ability)
      - Greater inflorescence production

  • In unfertilized soils, mycorrhizal fungi provided more significant benefits than those from fertilized soils.

Protective Mutualisms in Plants

  • Example: Ant–acacia relationships studied by Daniel Janzen where ants defend acacias from herbivores and competitive plants.

  • Mutualistic Partners: Bullhorn acacias and ants of the genus Pseudomyrmex (subfamily Pseudomyrmecinae).

Ants and Bullhorn Acacia Relationships

Ants' Characteristics:

  • Fast, agile runners with good vision

  • Foraging independently and maintaining large colony sizes

  • Aggressive behaviors toward any vegetation and animals encroaching the acacia

  • Active 24 hours a day outside nests, providing constant protection.

Acacia's Features:

  • Enlarged thorns with soft pith for nesting

  • Continuous leaf production

  • Foliar nectaries providing sugars

  • Beltian bodies supplying proteins and oils, making the plants attractive to ants and providing sustenance.

Mutual Benefits:

  • Ants Receive:
      - Shelter in acacia's thorns
      - Food (nectar and protein bodies)
      - Nesting opportunities (laying eggs in thorns).

  • Acacia Receives:
      - Protection from herbivores and competing plants, conserving resources such as light and nutrients.

Experimental Findings on Acacia Growth

  • Janzen's Experiments:
      - Removal of ants resulted in measured impacts on acacia growth, leaf production, and overall survival.
      - Acacias with ants exhibited:
        - Faster growth
        - Greater biomass
        - More foliage and thorns
        - Improved survival rates.

  • Conclusion:
      - Without the protective mutualism with ants, acacias had increased herbivore presence.

Plant and Animal Mutualisms

  • 45% of identified mutualisms involve pollination and/or seed dispersal.

  • Coevolution Examples:
      - Hummingbird bills and flower shapes (e.g., Heliconia in Caribbean islands).
      - Figures show polinators such as hummingbirds and butterflies coevolving with respective plants.

Example of Hummingbird and Flower Shape Coevolution

  • Caribbean Research:
      - The beak shape of the purple-throated carib hummingbird evolved in tandem with specific Heliconia flowers on islands such as St. Lucia and Dominica.
      - A differential evolution of flower and hummingbird shapes is observed between these two islands.

Example of Figs and Wasp Pollinators

  • There are 900 species of figs (genus Ficus), with each species having its distinct pollinating fig wasp.

  • Wasps gain advantages by laying eggs in the fig ovaries while pollinating the flowers.

Seed Dispersal Mutualisms

  • Mechanism: Plants benefit from having their seeds dispersed by animals, which often also fertilize them through defecation.

  • Animal Benefits: Animals gain a food source, such as the flesh of fruits (e.g., apples).

  • Attraction Mechanisms: Fruits are designed to be brightly colored or emit odors to attract dispersers, which can include birds, mammals, insects, and bats.

Mutualistic Symbiosis

  • Definition of Symbiosis: A close interaction between species that have co-evolved and maintain direct intimate contact over long periods.

  • Example:
      - Aphids and bacteria (e.g., Buchnera) which provide essential amino acids to aphids.
      - The mutualism between these entities has persisted for nearly 200 million years.

Microbial Mutualisms

  • Function: Microbial mutualism is critical in helping digest cellulose in ruminants (e.g., cows).

  • Mutualistic bacteria in the rumen digest plant matter for ruminants, making cellulose—the most abundant biomolecule—accessible.

Diversity of Mutualisms

  • Example:
      - Cape buffaloes gain energy from plant tissues with help from mutualistic microorganisms in their guts.
      - Meanwhile, the cattle egret benefits by consuming ticks and flies from the buffalo.

Types of Mutualisms Based on Obligation

  • Obligate Mutualism:
      - Co-evolved relationships where at least one partner cannot survive without the other (e.g., mycorrhizal fungi and plant roots).
      - In this scenario, the fungus receives carbohydrates while the plant gains nutrient access.

  • Examples of Obligatory Mutualism:
      1. Ruminants and gut bacteria.
         - Ruminants depend entirely on these bacteria to break down plant materials.
      2. Deep-sea fish and bioluminescent bacteria.
         - Fish provide habitat for bacteria while utilizing their light for communication and predation.

Types of Facultative Mutualisms

  • Definition: Looser associations that do not require co-evolution but lead to positive outcomes for fitness (e.g., flowering plants and their pollinators).

  • Examples:
      - Nurse Plants:
        - Adult desert plants create cooler, moist microclimates conducive to seed germination for other plants.
      - Deer as Seed Dispersers:
        - Deer consume seeds of various herbaceous plants, allowing seeds to pass through their digestive tract unharmed and deposited with feces as fertilization.

Categorization of Mutualisms According to Benefits

  • Types of Mutualism Benefits:
      - Trophic: Mutualist receives energy/nutrients.
      - Habitat: Mutualist receives shelter/favorable habitat.
      - Service: Mutualist receives ecological services (e.g., pollination, dispersal, predator defense).

  • Example Case:
      - In the plant-pollinator relationship: plants receive pollination (service mutualism), while pollinators receive nectar (trophic mutualism).

Importance of Mutualism

  • Essential for Ecological Integrity:
      - Elimination of mutualisms would lead to drastic ecological consequences.

  • Potential Consequences if Mutualism Disappeared:
      - Oceans:
        - Loss of reef-building corals, affecting coral atolls and communities.
        - Elimination of bioluminescent deep-sea organisms.
        - Hydrothermal vent ecosystems would face drastic reductions.
      - Land Ecosystems:
        - Absence of animal-pollinated plants (e.g., orchids, sunflowers, apples).
        - Fewer pollinators like bees and butterflies would lead to collapse of tropical rain forests.
        - 90% of plants dependent on mycorrhizae would be severely threatened.
        - Extinction of herbivores reliant on microbial mutualists for digestion, leading to a cascading loss of carnivores as well.
        - Overall, the biosphere would face biological impoverishment due to the collapse of mutualism.