Mutualism and Symbiotic Relationships

Comprehensive vocabulary flashcards covering the definitions, categories, evolutionary dynamics, and experimental case studies of mutualism and symbiotic relationships.

Facilitation

Interaction where growth, reproduction, or survival of one or multiple species is increased when interacting with other species, none are harmed

Commensalism

A symbiotic relationship in which one organism benefits ($+$) while the other organism is unaffected ($0$), epiphytes growing on host trees.

Amensalism

A symbiotic relationship in which one species is harmed or suffers a negative effect while the other species experiences no effect ($0 / -$).

Mutualism

A symbiotic relationship in which both interacting species experience net benefits ($+ / +$) from their association.

Facultative mutualism

A type of mutualism in which the interacting species derive benefit from one another but can survive and reproduce independently without the relationship.

Obligate mutualism

A type of mutualism in which at least one species relies entirely on the partnership for survival and reproduction, making the relationship mandatory.

Resource-resource mutualism

A category of mutualistic partnership where both species exchange nutritional or physical resources, such as fungi providing nutrients and water while algae provide photosynthates in lichens.

Service-resource mutualism

A mutualistic partnership in which one species provides an ecological service (such as pollination or protection against herbivores) in exchange for a nutritional resource (such as nectar or food bodies).

Service-service mutualism

A mutualistic partnership where both interacting organisms exchange services with one another (such as clownfish and sea anemones exchanging shelter and defense).

Lichen

A mutualistic symbiotic association between fungi and photosynthetic algae or cyanobacteria, where the algae supply glucose from photosynthesis and the fungal partner provides structural shelter and access to water and nutrients.

Parasite-mutualist continuum

The evolutionary concept that species interactions can transition dynamically between mutualism, commensalism, and parasitism over time, driven by evolutionary pressures and mechanisms such as horizontal gene transfer (HGT).

Cheaters (in mutualism)

Organisms within a mutualistic system that exploit the interaction by taking benefits without paying the expected reciprocal costs.

Selective flower abortion (Yucca)

A policing response where yucca plants drop flowers containing too many moth eggs to stop larvae from consuming too many seeds, retaining an average of $62\%$ of flowers with $1\text{--}6$ moth eggs but only $11\%$ of flowers with $9\text{--}12$ moth eggs.

Mycorrhizae

Mutualistic associations between plant roots and soil fungi, in which the fungi facilitate water and nutrient uptake in exchange for host plant photosynthates.

Ectomycorrhizae

Mycorrhizal fungi that envelop the root surface with a fungal mantle and form an intercellular network known as the Hartig net between plant root cortical cells without penetrating inside the cells.

Arbuscular mycorrhizal fungi (AMF)

Mycorrhizal fungi that penetrate the cortical cell walls of plant roots and form highly branched internal structures called arbuscules to mediate nutrient exchange.

Hartig net

A network of inward-growing fungal hyphae that extends between cortical cells in plant roots associated with ectomycorrhizal fungi, serving as the site of nutrient exchange.

Arbuscules

Branched, tree-like invaginations formed by arbuscular mycorrhizal fungal hyphae inside the cortical cells of plant roots, facilitating the transfer of nutrients between fungus and plant.

Functional equilibrium model

A model stating that the availability of belowground resources (nutrients, water) and aboveground resources (light) governs plant energy allocation: plants allocate more energy to roots and mycorrhizae in nutrient-poor soils and less in fertilized soils.

Root:shoot ratio

The proportion of plant biomass allocated belowground to roots versus aboveground to shoots; it is higher in unfertilized soils (to scavenge limited nutrients) and drops when nutrients or mycorrhizae are supplied.

Bullhorn acacia-ant mutualism

An obligate mutualism where bullhorn acacias provide food bodies and hollow thorn shelters to ants, and the ants vigorously defend the acacia against herbivorous insects and competing vegetation.

Chemical repellent (in acacia inflorescences)

A floral compound that deters protective mutualist ants from visiting flowers, creating spatial separation between aggressive defending ants and visiting bee pollinators.

Extrafloral nectaries

Nectar-secreting plant organs located outside of flowers (such as on leaves or stems) that attract predatory ants to defend the plant against herbivores.

Helianthella quinquenervis

The aspen sunflower, a temperate plant that maintains a facultative mutualism with ants via extrafloral nectaries to reduce seed predation; the relationship remains facultative because recurrent late spring frosts kill flower heads.