Mutualism & Commensalism (4)

What is mutualism?

A (+/+) interaction where both species benefit.

What is commensalism?

A (+/0) interaction where one species benefits and the other is unaffected.

What is facilitation?

Any positive interaction that increases survival, growth, or reproduction of another species.

Are positive interactions common?

Yes, they are ubiquitous across ecosystems.

Example of a positive interaction: plants and oxygen

Plants produce oxygen, benefiting other organisms.

Example of a positive interaction: endosymbionts

Organisms living inside others and providing benefits (e.g., bacteria).

Example: corals and zooxanthellae

Zooxanthellae photosynthesize and provide energy to coral hosts.

Example: gut bacteria

Gut microbiota aid digestion and immune regulation.

Example: mitochondria and chloroplasts

Originally endosymbionts that became permanent cell organelles.

Example: mycorrhizae

Fungal partners that assist plant nutrient uptake.

Example: plants and pollinators

Pollinators disperse pollen while receiving nectar rewards.

What is an obligate mutualism?

A mutualism where both partners require the interaction for survival.

Example of obligate mutualism

Figs and fig wasps.

What is a facultative mutualism?

A mutualism that is beneficial but not required for survival.

Example of facultative mutualism

Birds dispersing seeds that can also germinate without bird assistance.

Why have positive interactions been historically understudied?

Because ecology traditionally focused on negative interactions. :contentReference[oaicite:2]{index=2}

What negative interactions dominated past ecology research?

Predation, competition, and disturbance.

How does competitive exclusion relate to this bias?

Coexistence problems focused attention on competition rather than mutualism.

What distinction in niche theory connects to this?

Fundamental vs realized niche, emphasizing negative pressures.

What is a foundation species?

A species that creates habitat for many others.

Characteristics of foundation species

Numerically abundant and ecologically important.

Examples of ecosystems driven by foundation species

Forests, mangroves, salt marshes, coral reefs, oyster reefs.

Example of foundation species effect

Trees create canopy structure, vertical habitat, and litter for decomposers.

What is a keystone species?

A species with an ecological impact disproportionate to its abundance.

What is a dominant species?

A species that is numerically abundant but ecologically replaceable.

What is an ecosystem engineer?

A species that physically modifies the environment to its benefit.

Examples of ecosystem engineers

Beavers, oysters, corals.

Are ecosystem engineers always abundant?

No; they may be less abundant than foundation species.

Are ecosystem engineers often keystone species?

Yes, many have large ecosystem effects despite low abundance.

Are ecosystem engineers typically involved in coevolution?

Less likely; interactions are often commensal rather than coevolved.

What is the stress gradient hypothesis?

Positive interactions become more common in high-stress environments.

Why do positive interactions increase with stress?

When abiotic stress dominates, competition for resources weakens and facilitation becomes advantageous.

How can consumer pressure mimic stress effects?

High predation or herbivory can create conditions where species benefit from grouping or protection.

What is diversity-invasibility?

The relationship between community diversity and vulnerability to invasion.

What is biotic resistance?

High-diversity communities resist invasion by filling niche space.

Why do high-diversity communities resist invaders?

Most niche space is occupied, leaving few opportunities for newcomers.

Why do low-diversity communities allow invasion?

More empty niche space is available.

Outcome for invader in high-diversity systems

Invader usually unsuccessful.

Outcome for invader in low-diversity systems

Invader more likely to establish.

How is biotic resistance a positive interaction?

Native species collectively reduce invader success, indirectly benefiting one another.

Why might ecology have historically ignored positive interactions?

Because negative interactions are more visible and easier to observe directly. :contentReference[oaicite:3]{index=3}

What cultural factors may have biased ecology toward negative interactions?

Speculation includes gender imbalance in science or cultural frameworks like capitalism.

Why are positive interactions harder to detect?

They often involve subtle or long-term benefits instead of visible harm or mortality.