Read Interaction Signs and Examples
Interaction signs show whether each species benefits, is harmed or is unaffected.
Species Interactions is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Use the sign pairs
Mutualism is +/+; commensalism +/0; amensalism −/0; competition −/−; predation and parasitism +/−. Not every interaction requires close contact, and not every positive interaction involves shared feeding.
Check examples and definitions
Predation includes herbivory and carnivory. Plants use physical and chemical defences. Competitive exclusion can follow introduction of a strong competitor; resource partitioning reduces overlap. Mycorrhiza and lichen are mutualistic; protocooperation is non-obligatory mutual benefit.
Compare interaction effects with a baseline
When a table gives each species’ growth with and without an interaction, classify the interaction by the change caused by the partner—not by the raw growth sign alone. The patterns include amensalism (−/0), commensalism (+/0), protocooperation (+/+ from a neutral baseline) and mutualism (+/+).
Trace indirect effects and removal experiments
In a food web, two prey can experience apparent competition by sharing a predator. Indirect effects can also be positive across a consumer chain. In grazer-removal experiments, compare each removal treatment with the control: the stronger rise in seaweed after sea-urchin removal indicates stronger grazing control by urchins than limpets.
Kin selection and inclusive fitness
Kin selection can favour helping close relatives because their reproductive success shares genes with the helper. Resource allocation in eusocial colonies can therefore favour brood with high relatedness, rather than equal or random allocation.
Locate interaction signs in a matrix
In a sign matrix, read the row and column together: +/− or −/+ represents an exploitative interaction such as predation or parasitism, while −/− is competition. The direction merely identifies which partner benefits; it does not change the interaction class.
Compare relatedness in haplodiploids
Human sisters share, on average, half their genes. In haplodiploid ants, sisters receive the same haploid paternal genome and share half of the variable maternal contribution, giving relatedness of three quarters. The difference is 0.25, so relatedness is greater by a factor of 0.25 in ants. Choose C.
Classify an interaction against its baseline
Compare each species with and without the interaction. One harmed while the other is unchanged is amensalism; one benefits while the other is unchanged is commensalism; mutual benefit without dependence is protocooperation; mutual benefit where both were otherwise worse off is mutualism. Choose B.
OlympIQ exam lens
- ✓Read both partners’ signs.
- ✓Predation includes plant–herbivore interactions.
- ✓Protocooperation is beneficial but not obligatory.
- ✓Parasite benefits while host is harmed.
- ✓Classify effects relative to the no-interaction baseline.
- ✓Shared predators can create apparent competition between prey.
- ✓Compare grazer-removal treatments to the control to infer relative effects.
- ✓Full sisters in haplodiploid insects are related by 0.75, compared with 0.50 in humans.
- ✓Classify interactions by the change caused relative to the no-interaction baseline.
Positive interaction means no partner is harmed; it does not mean both partners necessarily help one another feed.
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