# Explain how mutualistic relationships between two species can affect the survival and abundance of each partner within a community.

> IB DP Biology Standard Level (2023 syllabus) — C4.1 Populations and communities · Explain · 4 marks

> Mutualism is a type of interspecific interaction in which both participating species benefit. Examples include mycorrhizal associations between fungi and plant roots, and the relationship between nitrogen-fixing bacteria and legume root nodules.

## Mark scheme (4 marks)

1. In mutualism, both species benefit from the interaction, increasing the fitness/survival of each partner.
2. One partner receives a resource or service it cannot produce/obtain efficiently alone, for example a plant receives mineral ions/fixed nitrogen or water from the fungal/bacterial partner.
3. The other partner receives organic compounds / carbohydrates / photosynthate produced by the plant, providing an energy source that supports its growth and reproduction.
4. Because both species are better able to survive and reproduce, mutualism can increase the population size/abundance of each partner compared with living without the association, particularly in nutrient-poor or otherwise limiting environments.

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