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C4.1.17—Top-down vs. bottom-up control

Top-down versus bottom-up control explains how ecological evidence links organisms, resources and interactions to population size, distribution or community structure in a habitat.

Syllabus
First assessment 2025
Objective
C4.1.17
Level
SL

Top-down and bottom-up control start at different levels

Top-down control begins with consumers at higher trophic levels; bottom-up control begins with resource supply or primary producers at lower levels.

Top-down effects cascade downward when predators alter herbivores and therefore plants. Bottom-up effects propagate upward when nutrients or primary production limit herbivores and predators.

Removing a top predator can release herbivores and reduce plant biomass; low nitrogen can independently limit plant production and therefore all higher levels.

Both pathways can operate in one community, but evidence often shows one is dominant in a particular place and time. Identify the direction and trace effects through trophic levels rather than assuming universal control.

Top-down vs. bottom-up control

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Describe / Identify.

Command terms

Explain / Describe / Identify / Outline

What earns marks

Build the answer around this relationship: Top-down versus bottom-up control must be linked to the correct ecological unit, method or species interaction.

Watch for

Confusing a population with a community or with all organisms in an ecosystem.

Representative question

Question 1

[Maximum number: 3]

Explain how nutrients can have a positive or negative bottom-up effect on seagrass.

Populations and Communities

  • A population is one species in an area; a community is all interacting populations there.
  • Estimate abundance with unbiased sampling: quadrats for sessile organisms and capture–mark–release–recapture for mobile animals, checking each method’s assumptions.
  • Density-dependent competition, predation, disease and waste create negative feedback around carrying capacity; exponential growth slows into a sigmoid curve as limits strengthen.
  • Classify interspecific relationships by costs and benefits: predation, herbivory, competition, mutualism, parasitism and pathogenicity.
  • Invasive species may escape controls and displace endemic species. Removal experiments can reveal competition and fundamental versus realized niches.
  • Chi-squared tests assess species association from observed and expected quadrat counts.
  • Predator peaks usually lag prey peaks; top-down control begins with consumers, while bottom-up control begins with resources or producers.

Concept essentials

  • Top-down versus bottom-up control must be linked to the correct ecological unit, method or species interaction.
  • The evidence for top-down versus bottom-up control depends on measurable abundance, distribution, survival or resource patterns.
  • Top-down versus bottom-up control is clearest when cause, ecological process and population outcome are kept distinct.
  • Field or graph evidence for top-down versus bottom-up control needs biological interpretation, not values alone.