Mutualism and Community Dynamics in Ecosystems

Mutualism

A relationship where both species benefit.

Obligate mutualism

Mutualism where both species require the relationship to survive.

Facultative mutualism

Mutualism that is beneficial but not essential.

Symbiosis

Symbiosis includes all close associations (mutualism, parasitism, commensalism), not just beneficial ones.

Ectomycorrhizae

Fungi that form sheaths around plant roots and extend into soil.

Arbuscular mycorrhizae

Fungi that penetrate root cells and form arbuscules for nutrient exchange.

Ecological community

A group of interacting species in a shared environment.

Components of community structure

Species richness, relative abundance, species composition, and diversity indices.

Lognormal distribution

Most species have intermediate abundance, few are rare or dominant.

Shannon-Wiener diversity index

A metric combining species richness and evenness.

Rank abundance plot

To compare species evenness in communities.

Factors affecting species diversity

Disturbance, habitat complexity, productivity.

Intermediate Disturbance Hypothesis

Moderate disturbance supports highest species diversity.

Human impact on communities

By altering habitats, reducing diversity, introducing invasives, and pollution.

Food web

A network of feeding interactions in a community.

Direct interaction

One species directly affects another (e.g., predation).

Indirect interaction

One species affects another via a third species.

Tri-trophic interaction

An interaction across three trophic levels (e.g., plant-herbivore-predator).

Apparent competition

Two species negatively affect each other via a shared predator.

Indirect commensalism

One species benefits indirectly from another with no effect on the intermediary.

Keystone species

A species with a large effect on community structure despite low abundance.

Example of a keystone species

Sea otters that control sea urchin populations, maintaining kelp forests.

Main energy source for ecosystems

Sunlight (via photosynthesis).

Chemosynthesis

Using chemical energy to produce food, often in deep-sea or cave ecosystems.

Photosynthetic pathways

C3, C4, and CAM.

CAM photosynthesis

Found in arid/dry environments like deserts.

Gross Primary Production (GPP)

Total photosynthetic energy captured by producers.

Net Primary Production (NPP)

GPP minus energy used in respiration by producers.

Importance of NPP

It indicates how much energy is available to consumers.

Limits to terrestrial primary productivity

Light, temperature, water, and nutrients (especially nitrogen).

Limits to aquatic primary productivity

Light and nutrients (nitrogen and phosphorus).

Marine NPP highest

Near coasts and upwelling zones.

Secondary production

Energy transferred to consumers for growth and reproduction.

Calculation of secondary production

Ingested = egested + respired + production.

Essential nutrients

Nitrogen, phosphorus, iron.

Limits to herbivores in energy/nutrients

Low nutrient content in plants.

Limits to carnivores in energy/nutrients

Difficulty finding prey.

Structural plant defenses

Physical features like thorns that deter herbivores.

Chemical plant defenses

Toxins produced by plants to deter herbivores.

Morphological prey defenses

Camouflage adaptations that help prey avoid detection.

Behavioral prey defenses

Hiding or altering behavior to avoid predation.

Chemical prey defenses

Poisons produced by prey to deter predators.

Keystone species removal

Can lead to drastic changes in community structure, such as loss of biodiversity or collapse of specific trophic levels.

Mutualism to parasitism shift

Occurs when the cost of the interaction outweighs the benefit for one partner, such as drought reducing plant benefits to mycorrhizae.

Intermediate Disturbance Hypothesis experiment

Introduce varying frequencies of disturbance to similar plots and measure species richness over time.

Human-induced disturbance effects

Often occur at larger scales, are more persistent, and can introduce novel stressors, reducing recovery and resilience.

Trophic level change effects

Can alter populations or behavior of connected species, leading to trophic cascades or shifts in competitive relationships.

High productivity and biodiversity

Excess productivity can lead to dominance by fast-growing species, excluding others and reducing diversity.

Plant structural defenses effectiveness

Deter generalists and physical grazers.

Plant chemical defenses effectiveness

May deter specialists and reduce herbivore fitness; effectiveness depends on herbivore adaptations.

CAM plants dominance reason

CAM is energy-intensive and less efficient under normal conditions, giving C3 and C4 plants a competitive edge.

Nitrogen inputs effect on aquatic ecosystems

Eutrophication could occur, leading to algal blooms, oxygen depletion, and die-offs of aquatic organisms.

Low abundance species role

Such species may control key processes or interactions, like predation, nutrient cycling, or habitat formation.

Primary and secondary production relationship

Secondary production depends on the energy fixed by primary producers; low NPP limits herbivore and predator biomass.

Long-term ecological research benefits

Reveals trends, resilience patterns, and delayed responses that short-term studies might miss.

Shannon-Wiener Index usefulness

Quantifies both richness and evenness, making it useful for comparing complexity, but doesn't capture species identity or functional roles.

Grassland ecosystem management strategy

Apply moderate, periodic disturbances like grazing or fire to prevent dominance and maintain niche variety.

Indirect commensalism benefits

Can increase coexistence and resilience in fluctuating environments by buffering species from direct competition or predation.