Consider a freshwater lake food web consisting of (i) phytoplankton as producers, (ii) zooplankton as primary consumers, (iii) small fish as secondary consumers, and (iv) large predatory fish as tertiary consumers. Suppose an invasive predator, the Nile perch, is introduced and starts preying heavily on the small fish. Describe in detail how this introduction could alter the structure and functioning of the lake ecosystem. Discuss at least three cascading effects on energy flow, nutrient cycling, and biodiversity, and explain the mechanisms behind each.

Step-by-step solution

1. 1. Immediate effect: Nile perch consumes large numbers of small fish, lowering secondary consumer biomass. This releases zooplankton from predation pressure, increasing their population.
2. 2. Energy flow shift: With more zooplankton, grazing pressure on phytoplankton intensifies, leading to a decline in phytoplankton biomass. The overall energy available to higher trophic levels is reduced, causing a trophic cascade.
3. 3. Nutrient cycling changes: Higher zooplankton populations enhance bioturbation and excretion rates, which release more dissolved nutrients (nitrogen, phosphorus) into the water column, potentially stimulating primary production or, if excessive, causing eutrophication.
4. 4. Biodiversity impact: Native small fish species decline or go locally extinct, reducing species richness. Competitive displacement by Nile perch also alters community composition, and the loss of prey species for other predators further reduces diversity.
5. 5. Long‑term ecosystem function: Persistent changes in species composition can shift the lake from a balanced, oligotrophic state to a nutrient‑rich, degraded state, altering habitat structure and ecological services.

Concept tested: Trophic cascade and invasive species

Published 28 September 2026 · How this solution was prepared and checked