A coastal wetland ecosystem supports many different species of plants, invertebrates, fish and birds. A sewage works nearby begins discharging excess nutrients into the wetland. Over several years, fast-growing algae cover the water surface and most other plant species disappear. Explain why the loss of plant species in the wetland is likely to reduce the biodiversity of the whole ecosystem.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A coastal wetland ecosystem supports many different species of plants, invertebrates, fish and birds. A sewage works discharges excess nutrients into the wetland. Over several years, fast-growing algae cover the water surface and most other plant species disappear.
Model answer (5 marks)
Plants are primary producers, providing the main source of food and energy for other organisms in the wetland.
When the fast‑growing algae outcompete other plant species, the number of plant species falls.
Fewer plant species mean fewer distinct food sources and habitats for invertebrates, fish and birds.
Animals that rely on particular plant species for food or shelter will decline or become locally extinct.
This loss of species triggers a cascade effect through the food web, causing further reductions in other species.
Consequently, the overall biodiversity of the ecosystem decreases because there are fewer different species and less variety in the community.
When the fast‑growing algae outcompete other plant species, the number of plant species falls.
Fewer plant species mean fewer distinct food sources and habitats for invertebrates, fish and birds.
Animals that rely on particular plant species for food or shelter will decline or become locally extinct.
This loss of species triggers a cascade effect through the food web, causing further reductions in other species.
Consequently, the overall biodiversity of the ecosystem decreases because there are fewer different species and less variety in the community.
Examiner tips
- Use the term "primary producers" and link plants to food/energy supply. Show the chain from loss of plants to loss of dependent animals. Mention the cascade effect to justify the broader impact. Finish with a clear statement that biodiversity falls because of fewer species.
Common mistakes
- Confusing algae with other plant species; remember algae are not the same as vascular plants. Failing to mention the cascade effect or the role of plants as food sources. Using vague terms like "more animals" instead of specifying dependent species or food webs.
Mark scheme (5 marks)
- Plants are producers / the source of food / energy for other organisms in the ecosystem
- Fewer plant species means fewer food sources / habitats available for animals / invertebrates
- Animals / species that depend on specific plant species for food or habitat will decline / become locally extinct
- This reduction in species causes a further knock-on / cascade effect through food webs, reducing other species further
- Overall biodiversity is reduced because there are fewer different species / the variety of species in the ecosystem decreases
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Biodiversity questions
- A conservation organisation is working to protect a rainforest ecosystem that co…
- A farmer converts a large area of species-rich grassland into a monoculture crop…
- Biodiversity is important for the stability of ecosystems. Explain why a reducti…
- Describe the importance of biodiversity and explain how human activities can red…