Chapter 45.3: Mutualistic Interactions

Mutualism

An interaction between species that benefits both participants

• Each side receives benefits but also bears the respective costs 

  • Occurs when benefits > costs: Specifically when there's an increased reproductive success 

    • This can shift over time:

1. Mutualism → Antagonism: When one partner becomes hostile

2. Commensalism → Mutualism: When both partners begin to benefit 

• Persistence through natural selection as it increases fitnesses of both species

Generalized Mutualism

• Symbiotic relationships where two species benefit from an interaction that is not limited to a single partner

  • Making them more resilient and potentially able to expand their geographic range

Obligate Mutualism:

• One or both species cannot survive without the other 

Facultative Mutualism

•  One or both species cannot survive without the other  

Evolutionary transitions

Facultative interactions can become obligate over time as selection strengthens the relationship

Symbiosis

A close and long-term interaction between species (mutualistic or antagonistic)

• Parallel Case: Mitochondria and Chloroplasts which coevolved through mutualisms within eukaryotic cells 

• Digestive Symbioses:  Microbiomes → most animals have gut microbes that help digest food

  • Eg, Herbivores: Depend on microbial symbionts to digest cellulose and lignin from plants

Commesalism

One species benefits, the other is left unaffected

Amensalism

One species harmed, the other is left unaffected

Facilitation

When one species indirectly benefits another by altering the environment 

• Includes both mutualisms and commensalisms because one species benefits while the other is unharmed or also benefits.