WK8 - INTERACTIONS: Mutualism: Part 1: Types of mutualism

Species interact in various ways, with interactions being positive, negative, or neutral for each species involved.
Previously covered interactions include:
- Predation: Positive for one species, negative for the other.
- Competition: Negative for both species.
Other types of interactions:
- Commensalism: Benefits one species, no effect on the other (e.g., moss on a tree).
- Amensalism: Negative effect on one species, no effect on the other (e.g., a tree shading a small plant).
- Neutralism: No effect on either species (e.g., herbivores and insects).

Mutualism: Interactions between individuals of different species that benefit both partners.
Examples:
- Coral grouper and cleaner fish: Cleaner fish remove parasites from the grouper, gaining food; the grouper gets cleaned.
- Farming: Plants get water, nutrients, and protection from humans; humans get food.
- Buffalo and birds: Birds eat ticks and insects from the buffalo, gaining food; the buffalo gets pests removed and is alerted to danger.
- Lichen: A complex of algae and fungi where fungi provide structure and algae photosynthesize, allowing both to grow.

For mutualism to evolve and persist, the fitness benefit must outweigh the costs for both species.
If the benefits of interaction (red and blue species) outweigh the costs, both species coexist, and non-mutualistic individuals die out.
If the costs outweigh the benefits, natural selection will eliminate the interaction, and individuals will remain independent.
There is often a cost involved, such as a plant expending energy to provide food for a pollinator.

Nutritional Mutualism: Involves the exchange of nutrients.
- Mycorrhizae: Mutualistic relationship between plants and fungi, where there is a swapping of nutrition.
  $fungi \rightleftharpoons plants$
- Rhizobium bacteria: These bacteria also participate in nutritional mutualisms with plants.
Protective Mutualism: One species provides protection to the other.
- Plants and ants/mites: Plants offer food and shelter while ants/mites deter herbivores.
Transport Mutualism: Food is traded for transport.
- Gamete mutualism: Pollinators spread plant pollen in exchange for nectar.
- Propagule mutualism: Animals disperse seeds for plants in exchange for a reward like fruit.

Obligatory Mutualism:
- Interaction is required for survival.
- Example: The interaction between the fig and the fig wasp.
Facultative Mutualism:
- Provides a benefit, but it is not necessary for survival.
- Example: Seed dispersal by birds; many species of birds eat and disperse seeds from different species of fruit, and if one species goes extinct, it can be replaced.

Direct Mutualism: Two species interact directly, benefiting both.
Indirect Mutualism: One species benefits another indirectly through a third species.
- Example: A plant benefits from an "enemy's enemy" (e.g., parasitoid wasps eating caterpillars that consume the plant).

Mutualisms are crucial for the survival of many species.
Example: Darwin's orchids and sphinx moth pollinator in Madagascar (transport mutualism).
- The sphinx moth has a long proboscis to reach the nectar at the bottom of the orchid's long nectar spur.
- The orchid has only two pollinia (clusters of pollen), and pollination is very specific.
- The moth receives nectar, and the orchid gets pollinated. This reduces competition for the moth and ensures successful transport of pollen for the orchid.
Experimental evidence:
- Shorter floral tubes resulted in a lower percentage of successful pollinations.
- Full-length tubes of 40 mm resulted in higher pollination rates.

Nutritional mutualism between plants and mycorrhizal fungi/rhizobium bacteria.
Mycorrhizae:
- Form an association with plant roots.
- The fungal network is more extensive than plant roots, allowing them to gather more water and nutrients.
- Plants provide sugars to the fungi through photosynthesis.
- Up to 90% of plants have mycorrhizal interactions, which may have been a reason for plant's success on land.
Rhizobium Bacteria:
- Release chemical signals that induce plant roots to form nodules.
- Bacteria fix atmospheric nitrogen into a usable form for plants.
- In exchange, the bacteria get a home, and the plants create an anaerobic environment for them inside the nodules.
- This mutualism allows plants to thrive in nitrogen-poor soils.
- Rhizobia interactions are common in legume species.
Agropyroma study:
The study showed that Agropyroma, a weedy grass, experienced less border stress when associated with mycorrhizal fungi.
During a hot day, the grass associated with mycorrhizae maintained a higher leaf water potential (less water stress) compared to the group without mycorrhizae.
$water \space potential \downarrow stress \uparrow$