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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

The fundamental niche of a plant is set by its physiological limits and the resources it needs. Without a nitrogen‑fixing partner, a legume cannot acquire sufficient nitrogen and is therefore restricted to habitats where nitrogen is abundant.

A mutualistic Rhizobium supplies fixed nitrogen to the host in root nodules, removing this limiting resource. This effectively removes a constraint on the plant’s fundamental niche.

Consequently the realised niche of the legume expands: it can now grow in nitrogen‑poor soils that would otherwise be unsuitable.

This niche expansion reduces competition with non‑symbiotic species, as the legume can exploit a resource dimension (low‑nitrogen habitat) that competitors cannot, giving it a competitive advantage or allowing coexistence.

Examiner tips

  • Use the term ‘fundamental niche’ first, then explain the limiting resource; link the symbiont to removal of that constraint; state the realised niche expands; finish with the competitive advantage point.
  • Keep each point concise and use exam‑specific terminology such as ‘fixed nitrogen’, ‘root nodules’, ‘realised niche’.
  • Remember the answer must be 4 marks – one sentence per point is ideal.

Common mistakes

  • Confusing the host’s fundamental niche with its realised niche; not mentioning the removal of the nitrogen constraint; omitting the competitive advantage or coexistence aspect.

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 in this question

ecological niche · mutualistic symbiont

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