# Explain how the feeding relationships of a mutualistic symbiont can expand the ecological niche of its host species.

> IB DP Biology Higher Level (2023 syllabus) — B4.2 Ecological niches · Explain · 4 marks

> Leguminous plants such as clover (Trifolium spp.) form mutualistic associations with nitrogen-fixing bacteria of the genus Rhizobium in root nodules. These plants are often found thriving in nitrogen-poor soils where closely related non-symbiotic plant species cannot persist.

## Mark scheme (4 marks)

1. The fundamental niche of a species is determined by its physiological tolerances and resource requirements; without the symbiont the host is limited by a key resource (e.g. available nitrogen).
2. The mutualistic symbiont provides the host with access to a resource (fixed/usable nitrogen) that was previously unavailable or limiting, effectively removing a constraint on the host's niche.
3. As a result, the host's realised niche is broader than it would be without the symbiont — the host can now occupy habitats (nitrogen-poor soils) that were previously outside its realised niche.
4. This niche expansion reduces interspecific competition with non-symbiotic species because the host can exploit a resource dimension (low-nitrogen habitat) that competitors cannot, allowing coexistence or competitive advantage.

## Key terms

- [ecological niche](https://www.gradenine.co.uk/glossary/ecological-niche)
- [mutualistic symbiont](https://www.gradenine.co.uk/glossary/mutualistic-symbiont)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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