Infer direct and indirect interactions from controlled contrasts

Mechanism before a label

Competition reduces performance when organisms share a limiting resource; predation and herbivory transfer material through consumption. Mutualism can benefit both partners, while parasitism benefits one at another’s expense. Interaction signs depend on the outcome and context being measured.

A food web includes indirect effects. Reducing an herbivore can increase producer biomass; a predator may indirectly benefit producers by suppressing herbivores. An observed correlation between predator abundance and vegetation is not by itself a manipulation.

Keystone effects are disproportionate to a species’ abundance, not merely evidence that a species is common. Disturbance and succession change communities through colonization, environmental modification and species interactions; a fixed inevitable final community should not be assumed.

Niches, resource partitioning and context-dependent signs

An ecological niche describes a species’ use of conditions and resources, not just its physical address. Two species can reduce direct competition by using different food sizes, feeding times or spatial zones. Such resource partitioning can allow coexistence, but observing different niches alone does not prove that competition historically caused the difference.

Commensalism describes a benefit to one partner with no detectable effect on the other under the measured conditions. “No detectable effect” is not proof that all possible effects are exactly zero. Mutualism, commensalism and parasitism require specifying the outcome, timescale and environment; an interaction can shift category as conditions change.

Prediction: Two virtual consumers overlap strongly in food size but feed at different times. If their feeding times are forced to overlap while all other stated conditions remain fixed, competition may increase when food is limiting. Propose a factorial supplied-data comparison that separately changes timing overlap and food supply. Track each species’ reproductive or growth response, not only their joint total.

Predict, compare and calculate

The following record is an original synthetic teaching dataset. Its assigned conditions are not a claim that an actual organism was measured. Write your prediction before revealing the changed condition.

Virtual plot comparison	Value (values with counts for herbivores and grams for plants)
Predator-present herbivores	20
Predator-absent herbivores	60
Predator-present plant biomass	80
Predator-absent plant biomass	35

Synthetic matched plots; units differ by named response.

Your prediction before comparison

This scratch prediction is not saved automatically. Copy it to the lesson notebook to retain it.

Evidence conditionReference comparisonChanged condition / follow-up

Infer direct and indirect interactions from controlled contrasts

Supplied synthetic evidence: Synthetic matched plots; units differ by named response.

Virtual plot comparison: reference condition

Quantity	Value	Unit
Predator-present herbivores	20	values with counts for herbivores and grams for plants
Predator-absent herbivores	60	values with counts for herbivores and grams for plants
Predator-present plant biomass	80	values with counts for herbivores and grams for plants
Predator-absent plant biomass	35	values with counts for herbivores and grams for plants

Task: Design a supplied-data factorial comparison that separates predator presence from nutrient limitation.

Random assignment and replicated plots are needed for the causal claim.

Compare with the conditional interpretation

Predator presence associates with 40 fewer herbivores and 45 g more plant biomass.

The reference plant difference is80−35=45g, while the changed-condition difference is40−35=5g. A constant herbivore contrast does not imply a constant producer response.

Case index for this investigation

0. Distinguish directional responses, activity and fitness
1. Track energy flow and distinguish production from standing biomass
2. Estimate abundance with sampling assumptions and uncertainty
3. Compare exponential and density-limited growth models
4. Use per-capita evidence to identify density dependence
5. Infer direct and indirect interactions from controlled contrasts
6. Separate richness, evenness and sampling completeness
7. Connect disturbance, nutrient cycles and evidence-based intervention

No numerical confidence, laboratory credit or independent mastery is inferred from opening a supplied case. Save your first prediction and comparison in the notebook.

Reset this unsaved comparison

Worked reasoning

The reference plant difference is80−35=45g, while the changed-condition difference is40−35=5g. A constant herbivore contrast does not imply a constant producer response.

Predator presence associates with 40 fewer herbivores and 45 g more plant biomass.

Apply to a new case

Design a supplied-data factorial comparison that separates predator presence from nutrient limitation.

Compare your reasoning with the worked response

Use four groups crossing predator presence/absence and nutrient condition, with independently assigned replicate plots. Estimate the predator contrast in each nutrient condition and its uncertainty. Use virtual records, not unauthorized predator removal.

This public response supplies feedback, not a score for independent reasoning. Record which part of your prediction changed and why.

Evidence and investigation boundary

Random assignment and replicated plots are needed for the causal claim. Indirect effects can depend on another limiting factor.

For a supervised investigation, predeclare the question, biological unit of replication, treatment/comparison assignment, quantities and units, exclusion rules, graph and uncertainty method. Keep the raw records and the first prediction separate from later revisions. A simulated case does not supply laboratory hours or establish AP authorization. No physical manipulation is required by this lesson.

Source and continuation

Framework topics: 8.5. Current AP Biology framework; supporting scientific reference. Sources checked 30 October 2026; all teaching wording, questions and numbers here are original. Return to AP Biology to compare this mechanism with another scale of organization.