WK8 - INTERACTIONS: Mutualism: Part 2: Mutualism in ants

Ants are renowned for their diverse mutualistic relationships with plants, insects, and animals.
Ants' ecological success is attributed to:
- Phyletic longevity: Evolved ~140-168 million years ago, compared to humans (~2 million years).
- Species diversity: ~15,000 species.
- Wide distribution: Found in all terrestrial habitats except polar regions and high mountaintops.
- High population density.
- Complex sociality.
Research on mutualism heavily features ants, highlighting their importance.
Certain mutualistic categories are reliant on the presence of ants.

Example: Cecropia peltata (umbrella tree) and ants.
Umbrella trees are early successional, light-demanding species that require rapid growth.
Mechanism:
- The tree's hollow stems provide nesting chambers for ants.
- Plants produce food bodies, which provide carbohydrates and nitrogen to the ants.
- In return, ants protect the plant from herbivores and remove invading vines.
Experimental Evidence:
- Study comparing herbivore presence on trees with and without ants. White boxes are trees lacking plants, black boxes have ant.
- Ants significantly reduce chewing insect presence but have little impact on sucking insects.
- $x$ axis indicating that measurements of insects were taken a multiple points in time.
- Cecropia trees with ants exhibit faster growth rates and greater heights than those without ants.
- Graph showing tree height (y-axis) over time (days - x-axis). Trees with ants (dashed line) grow faster after ~300 days.

Facultative mutualism observed in various ant and Acacia (wattle) species.
Acacias produce elaiosomes, protein-rich, fatty structures attached to seeds.
Elaiosomes attract seed dispersers like ants and birds.
Elaiosome Differences based on Dispersal Agent:
- Bird-dispersed seeds: Larger, brightly colored elaiosomes to attract birds.
- Ant-dispersed seeds: Smaller, lightly colored elaiosomes, as ants aren't visually driven.
Seed size is generally consistent regardless of the dispersal agent.
Reward (elaiosome) is larger and more nutrient-rich for bird-dispersed seeds due to their higher energy requirements.
Seed dispersal patterns:
- Ant dispersal: Seeds typically fall to the ground.
- Bird dispersal: Seeds remain in the canopy.
Dispersal Syndromes: Correlated traits associated with a specific disperser.
Elaiosome Importance:
- Removal of the elaiosome significantly reduces ant-mediated seed dispersal.
Benefits of Ant-Mediated Seed Dispersal:
- Long-distance transport: Average of 2.25 meters, up to 50-400 meters in some species.
- Reduced intraspecific competition: Seedlings get away from parent plants and siblings.
- Escape from seed predators: Predators congregate near parent plants.
- Dispersal to nutrient-rich areas.
- Increased survival from stochastic events (e.g., fire) due to wider seed distribution.

Tri-trophic Interactions: Involving three organisms.
Example: Ants, Scale Insects (Homoptera), and Plants.
- Scale insects suck plant sap and produce honeydew.
- Ants feed on honeydew and protect scale insects from predators, sometimes farming them.
- Mutual benefit between ants and scale insects, but plants are harmed by scale insects and indirectly by the ants protecting them.

Leafcutter Ants (Atta): House fungi in their nests; ants cut leaves to feed the fungus, then harvest fungal hyphae for food.
- Mutualism: ants and fungi.
- Plant loses to fungus.
- Fungus gets leaves delivered by ants.
Microfungus pathogen (Escovopsis) attacks the cultivated fungus.
Ants harbor Pseudonocardia bacteria on their exoskeleton that secrete an antifungal compound to protect the cultivated fungus from the pathogen.
Five-way interaction: ants, cultivated fungus, plant, pathogenic fungus, and bacteria.