Mutalism Flashcards
Mutualism
Symbiotic relationship between 2 organisms in which both species benefit
Classification of mutualism
Degree of physical integration
Degree of dependency
Degree of specificity
Degree of shared evolutionary history
Nature of the benefits
Degree of physical integration
Symbiotic when the 2 partners are in intimate physical contact for most or all of their life cycles
Lichens
Fungus (mycobiont) + photobiont (alga or cianobacterium)
- photobiont provides carbohydrates
-Mycobiont provides structure and protection
Degree of dependency
From obligate to facultative for each of the 2 partners
Degree of specificity
From specialised to generalised for each of the 2 partners
Shared evolutionary history
Coevolved or not coevolved
Nature of benefits
Transportation= of partners or of partners’ gametes
Protection = defence against biotic/abiotic environment
Nutrition = provisioning of limiting nutrients
Transportation mutualisms
- Pollination- transport of pollen grains between flowers
- Fungi- bark beetle mutualism
- Seed dispersal through fruit
Pollination- benefits for plant
Pollen transport/ovule fertilisation
But costly in terms of rewards (nectar, flower damage)
Pollination- benefits for the animals
Pollen, nectar oil, oviposition sites, heat
But costly in terms of energy
Protection mutualisms
- Ant-plant protection mutualism- ants produce toxin, plant produces domatia
- Bioluminescence -bacteria used by the squid to camouflage itself from potential predators
- Cleaning
- Fungal endophytes- eg ryegrass seed, endophyte produces alkaloids, plant protects fungi
Nutritional mutualisms
- Legume- rhizobium
- Mycorrhiza’s - root + fungi
- Fungus gardens of ants and termites
Legume-rhizobium
Many plant species in the Fabaceae + N-fixing bacteria:
Plant supplies with carbohydrates and mineral nutrients, the bacteria supply the plant with ammonia in return which is fixed from atmospheric N2
Cheating is prevented by reducing oxygen supply by the plant
Mycorrhizas
root + fungi
Most plant species, 5 main types
Plant gives carbohydrates to the fungus in exchange for minerals from the soil
Fungus gardens
Fungal gardens are cultivated underground
Atta ants bring bits of leaves and chew them, mixing them with fungi that breaks the cellulose
Byproduct
the producer generates a byproduct (generally a secretion or excretion) that a receiver feeds upon or benefits from.
E.g. Bacteria excreting waste, predators leaving portion of kills, ungulates that generate excreta…
Origins of mutualism
Byproduct
Unexpected encounters among species eg expansion of a species’ geographic range
From existing antagonist interactions such as parasitism and predation.
E.g. seed predators becoming pollinators
Maintenance of mutualism
Byproducts: there is minimal or zero fitness costs so there is no selective pressure to benefit further.
Screening: not to associate with an exploiter in the first place. Again, costs are minimal.
Partner fidelity: mutualism is favoured via feedbacks from partners that reciprocate fitness benefits
Partner-choice sanctions: penalties are imposed to non-rewarding partners
Plants- arbuscular mycorrhizal fungi
Plants can discriminate and reward the best fungal partner with more carbohydrates. Fungus increases nutrient transfer only to roots providing more carbohydrates
N-fixing rhizobia
non-fixing strains are selectively killed by decreasing O2 to the nodules:
Cheaters
mutualists that have evolved to exploit their partners
Exploiters/parasites
species from outside the mutualism that takes advantages of the resources or services
Why are cheaters rare
Difficult to detect because fitness of the partners and phylogenetic data to resolve the origins of the mutualism are needed
Cheating strategies are evolutionary unstable. Once a cheater spreads in a population, mutualism can rapidly dissolve
Mutualism originate only under restrictive conditions that disfavour the emergence of cheating
How is mutualism visualised
Network theory
Third-party species
Third-party species shape the outcome of the mutualism by changing:
The density of mutualists
The rewards offered by one of the mutualists to another
The costs of reward production or receipt
How does mutualism affect communities
Can reduce species diversity be enhancing their performance and the competitive dominance of their partner species
Can increase species diversity by permitting rare species to persist at lower densities or in harsher environments
Mutualisitic outcomes are context-dependent
The outcome of interactions between species can vary over time and space along a continuum from strong mutualism to commensalism or even to parasitism
Mutualism usually changes from mutualism to parasitism depending on the abiotic and biotic context
E.g. P content in soil explains mycorrhizal benefit:
Life without mutualism
Less species-rich
Less ecologically and life-form diverse\richer in extreme specialists and incompetent generalists
Higher competition
