Ecological Relationships
Classify an interaction by its effect on both organisms.
Ecological Relationships
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Classify from measurable effects—and remember that indirect and changing effects matter
Start by asking how each organism’s survival or reproduction changes. Mutualism is +/+, commensalism +/0, parasitism and predation +/−, and competition −/−.
Then look deeper: a relationship can depend on resource availability, density, life stage, or the presence of other species. Indirect effects can travel through food webs.
+/+ and +/0
Mutualism benefits both; commensalism benefits one without significantly affecting the other.
+/− and −/−
Parasitism and predation benefit one while harming another; competition imposes costs on both.
Organism 1, then organism 2
Why it works
This prevents familiar examples from overriding the actual description in the question.
Five forms you should recognize
If Species A reproduces less when Species B is present, A is harmed. Compare B alone and together before deciding whether the pattern is competition, predation, or parasitism.
Both are +/−. Predators usually kill and consume prey relatively quickly; parasites commonly exploit a living host over time.
Species can reduce competition by feeding in different places, at different times, or on different resource sizes.
Wolves reduce deer; reduced browsing allows young trees to survive. Wolves therefore have an indirect positive effect on trees.
A partner that provides a major benefit under scarce nutrients may provide little benefit when nutrients are abundant. Relationship labels describe net effects under stated conditions.
Check before you commit
- Assuming every +/− interaction is parasitism
- Calling every close association mutualism
- Treating coexistence as commensalism
- Ignoring whether the host is harmed
- Forgetting competition costs both organisms
- Classifying by species rather than effects
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Ecological Relationships FAQ
How is predation different from parasitism?
Predation normally kills and consumes prey quickly; parasites usually live on or in a host over time.
Can a relationship change?
Yes. Effects can depend on environmental conditions.
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