AP Syllabus focus:
‘Trophic cascades and niche partitioning illustrate how species interactions shape community structure and resource use.’
Species interactions can reorganize entire communities. Two key patterns are trophic cascades, where effects ripple across feeding levels, and niche partitioning, where species reduce overlap to coexist while using shared resources.
Trophic cascades: interaction “ripples” through food webs
Food webs link producers, consumers, and predators; changes at one level can propagate to others. A trophic cascade is typically described as top-down control, where predators regulate the abundance or behavior of prey, indirectly affecting lower trophic levels.
Diagram of a top-down trophic cascade showing how reducing an apex predator (wolves) releases herbivores (deer), which increases grazing pressure and reduces producer biomass (trees/grass). The side-by-side layout makes the indirect effect on producers visually explicit, emphasizing how changes at one trophic level propagate through the food web. Source
Trophic cascade: A chain of indirect effects across trophic levels triggered by changes in consumer or predator abundance/behavior, often producing alternating impacts (e.g., predator ↓ herbivore ↓ plant ↑).
Trophic cascades are often strongest when a predator has a large effect on a dominant prey species, especially in relatively simple food webs. Cascades can be driven by either abundance (how many individuals are eaten) or behavior (how prey alter habitat use, feeding times, or vigilance to avoid predation).
Common cascade patterns AP Biology expects
Predator increase → herbivore decrease → producer increase
Predator removal → herbivore increase → producer decline
Predator presence → prey shifts microhabitat/foraging → producers recover in “riskier” areas (a behavior-mediated cascade)
Why cascades matter for community structure and resource use
Trophic cascades change community composition by altering which species can persist and which resources dominate:
Producer biomass and species composition can change when herbivory pressure shifts.
Habitat structure can change (e.g., more plant cover) when producers rebound, which can create new refuges or alter microclimates, indirectly affecting other species’ survival and reproduction.
Resource use can shift as consumers move to alternative foods or habitats when their preferred resources decline or become risky to exploit.
Niche partitioning: coexistence by reducing overlap
Species using similar resources can coexist when they divide resource use along one or more niche axes. This reduces the intensity of direct competition and stabilizes community membership over time.
Niche partitioning: The division of limiting resources among similar species by differences in space, time, or resource type, reducing niche overlap and allowing coexistence.
Partitioning is best understood in terms of resource use rather than simple location. Even in the same habitat, species can avoid strong competition by specializing on different prey sizes, feeding heights, microhabitats, or activity periods.
Stylized conifer diagrams showing five warbler species concentrating feeding activity in different parts of the same tree canopy. The blackened zones illustrate spatial microhabitat partitioning (different foraging heights and branch positions), a mechanism that reduces direct competition and supports coexistence. Source
Major modes of niche partitioning
Spatial partitioning: using different microhabitats (e.g., canopy vs understory; shoreline vs open water)
Temporal partitioning: using the same resource at different times (day vs night; seasonal breeding/foraging)
Dietary/resource-type partitioning: consuming different food types or sizes (seeds vs insects; small vs large prey)
Behavioural/foraging-strategy partitioning: different hunting methods or search patterns that reduce direct overlap
How niche partitioning shapes community structure
Partitioning can increase local species richness by lowering competitive exclusion. It also shapes “who eats what,” which determines pathways of energy and matter transfer:
When consumers specialize, energy flow can be distributed across more links in the food web.
When species overlap strongly, a small change (e.g., a new competitor) can displace one species and simplify the community.
Connecting trophic cascades and niche partitioning
Trophic cascades and niche partitioning interact because predation and competition occur simultaneously:
Predators can prevent competitive dominance by keeping a strong competitor at lower density, indirectly allowing other species to persist.
If a shared resource becomes scarce due to a cascade (e.g., reduced plant biomass), competitors may intensify partitioning, shift niches, or one may be excluded.
Partitioning among prey species can weaken or redirect cascades by spreading predation pressure across multiple prey types, changing which producers are released from herbivory.
FAQ
They often use short-term manipulations such as exclosures, enclosures, or predator “risk” cues.
Common approaches include:
Predator-exclusion cages to isolate herbivory effects
Before–after control–impact (BACI) designs
Measuring behavioural shifts (e.g., foraging time) alongside abundance
Omnivory and many alternative pathways can dilute indirect effects.
If consumers switch diets, or multiple predators share prey, changes at one node may be buffered, producing smaller or inconsistent responses at producer levels.
You would compare resource use to availability and test whether overlap is lower than expected by chance.
Evidence strengthens if patterns persist over time, match performance differences (e.g., feeding efficiency), and reduce direct competitive interactions.
Researchers may quantify overlap using diet proportions, microhabitat use, or activity timing, then apply overlap indices.
Stable isotope data (e.g., $\delta^{13}C$, $\delta^{15}N$) can provide integrated evidence of long-term resource use rather than a single observation.
Yes. Rapid shifts can follow resource depletion, predator introductions, or environmental change.
Plastic behaviour (learning, altered foraging times) can produce short-term partitioning, which may later be reinforced by selection if differences increase fitness.
Practice Questions
Describe what is meant by a trophic cascade. (2 marks)
States that it involves indirect effects across multiple trophic levels (1)
Links change in predator/consumer to changes in lower levels (e.g., herbivores and producers) (1)
In a coastal food web, a large predatory fish declines due to overfishing. Explain how this could (i) produce a trophic cascade and (ii) alter niche partitioning among remaining consumer species. (6 marks)
(i) Predator decline leads to increase in prey/mesopredator abundance and/or activity (1)
Increased prey causes decreased abundance/biomass of its food (e.g., herbivores increase → algae/plant decrease) or equivalent alternating pattern (1)
Identifies this as an indirect effect across trophic levels (1)
(ii) Increased competition among consumers for shared limiting resources after prey release or resource decline (1)
Explains a valid partitioning axis change (space/time/diet/foraging strategy) among consumers (1)
Links partitioning to reduced niche overlap/coexistence (1)
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