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Mutualism (biology)

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**Types of Mutualistic Relationships**:
– Resource-resource relationships:
– Mycorrhizal associations between plant roots and fungi involve biological barter.
– Plants provide carbohydrates to fungi in exchange for phosphate and nitrogenous compounds.
Rhizobia bacteria fix nitrogen for leguminous plants in return for carbohydrates.
– Metabolite exchange occurs between mutualistic species of bacteria.
– Service-resource relationships:
– Examples include pollination, cleaning symbiosis, and zoochory.
– Plants trade food resources for services like pollen dispersal.
– Phagophiles feed on ectoparasites, offering anti-pest services.
– Zoochory involves animals dispersing plant seeds for food resources.
– Service-service relationships:
– Rare interactions where mutual benefits are exchanged, like anemone fish and sea anemones.
– Ants nesting in acacia trees for protection from herbivores in exchange for food.

**Evolutionary Significance of Mutualism**:
– Crucial in driving ecological interactions and evolution.
– About 80% of land plant species rely on mycorrhizal relationships for nutrients.
– Tropical rainforest plants have seed dispersal mutualisms with animals.
– Led to the evolution of diverse biological forms like flower structures.
– Key events like symbiogenesis and plant colonization of land are linked to mutualistic relationships.

**Mathematical Modeling of Mutualism**:
– Mutualism models lag behind predator-prey interactions.
– Type I functional response is linear, while type II is saturating.
– Lotka-Volterra equations quantify changes in population densities.
– Mutualism is represented by a modified logistic growth equation.
– Including saturation in models is crucial to avoid unrealistic growth.

**Human and Intestinal Mutualistic Relationships**:
– Humans have mutualistic relationships with gut flora for digestion.
– Head lice infestations might trigger beneficial immune responses.
– Traditional agriculture uses companion plants for mutual benefits.
– Human intestinal microbiota coevolved with humans, beneficial for both parties.
– Commensal bacteria in the intestine play a role in host health.

**Network Structures and Breakdown of Mutualism**:
– Mutualistic networks have similar structures in different ecosystems.
– Network organization minimizes competition between pollinators.
– Mutualism breakdown can occur by evolution, like one mutualist shifting to parasitism.
– Measuring fitness benefit and defining closeness in mutualistic relationships is challenging.
– Localization impacts stability of ecological networks, critical for functionality.

Mutualism describes the ecological interaction between two or more species where each species has a net benefit. Mutualism is a common type of ecological interaction, one that can come from a parasitic interaction. Prominent examples include most vascular plants engaged in mutualistic interactions with mycorrhizal fungi, flowering plants being pollinated by animals, vascular plants being dispersed by animals, and corals with zooxanthellae, among many others. Mutualism can be contrasted with interspecific competition, in which each species experiences reduced fitness, and exploitation, or parasitism, in which one species benefits at the expense of the other.

Hummingbird hawkmoth drinking from Dianthus, with pollination being a classic example of mutualism

The term mutualism was introduced by Pierre-Joseph van Beneden in his 1876 book Animal Parasites and Messmates to mean "mutual aid among species".

Mutualism is often conflated with two other types of ecological phenomena: cooperation and symbiosis. Cooperation most commonly refers to increases in fitness through within-species (intraspecific) interactions, although it has been used (especially in the past) to refer to mutualistic interactions, and it is sometimes used to refer to mutualistic interactions that are not obligate. Symbiosis involves two species living in close physical contact over a long period of their existence and may be mutualistic, parasitic, or commensal, so symbiotic relationships are not always mutualistic, and mutualistic interactions are not always symbiotic. Despite a different definition between mutualistic interactions and symbiosis, mutualistic and symbiosis have been largely used interchangeably in the past, and confusion on their use has persisted.

Mutualism plays a key part in ecology and evolution. For example, mutualistic interactions are vital for terrestrial ecosystem function as about 80% of land plants species rely on mycorrhizal relationships with fungi to provide them with inorganic compounds and trace elements. As another example, the estimate of tropical rainforest plants with seed dispersal mutualisms with animals ranges at least from 70 to 93.5%. In addition, mutualism is thought to have driven the evolution of much of the biological diversity we see, such as flower forms (important for pollination mutualisms) and co-evolution between groups of species. Mutualism has also been linked to major evolutionary events, such as the evolution of the eukaryotic cell (symbiogenesis) and the colonization of land by plants in association with mycorrhizal fungi.

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