The big idea: Species living in the same community do not exist in isolation — they interact.
An interspecific relationship is any close interaction between two different species (inter = between, specific = species).
A simple way to describe each relationship is to ask, for each of the two species: does it benefit (+), is it harmed (–), or is it unaffected (0)?
There are six relationships you need to know: herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity.
The six interspecific relationships, each shown by how it affects the two species involved — a green + means that species benefits, a red – means it is harmed.
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- Interspecific relationship
- A close interaction between two different species living in the same community.
- Herbivory
- An interaction in which an animal feeds on a plant. The herbivore benefits (+); the plant is harmed (–).
- Predation
- An interaction in which one animal (the predator) kills and eats another animal (the prey). Predator +, prey –.
- Interspecific competition
- An interaction in which two different species compete for the same limited resource, so both are harmed (– / –).
- Mutualism
- An interaction in which two species live together and both benefit (+ / +).
- Parasitism
- An interaction in which a parasite lives on or in a host, gaining nutrients (+) while harming the host (–).
- Pathogenicity
- An interaction in which a pathogen (a disease-causing organism) infects a host, benefiting itself (+) while causing disease in the host (–).
The shortcut: read it as a pair of signs: Every relationship boils down to two signs — one for each species.
Mutualism is the only + / + (both win).
Competition is the only – / – (both lose).
Predation, herbivory, parasitism and pathogenicity are all + / – (one wins, one loses).
Several relationships share the same + / – pattern, so the exam expects you to tell them apart by what the benefiting species is doing and how much harm is done.
Work through the table below — the effect signs narrow it down, then the mechanism tells you exactly which relationship it is.
| Relationship | Effect on each species (+ / – / 0) | What happens |
|---|---|---|
| Herbivory | Herbivore + / Plant – | An animal eats a plant (or part of it). The herbivore gains food; the plant is damaged. |
| Predation | Predator + / Prey – | One animal (the predator) kills and eats another animal (the prey). |
| Interspecific competition | Species 1 – / Species 2 – | Two different species need the same limited resource (food, light, space, water), so both are harmed. |
| Mutualism | Species 1 + / Species 2 + | Two species live together and both benefit from the relationship. |
| Parasitism | Parasite + / Host – | A parasite lives on or in a host, taking nutrients from it and harming it (usually without killing it quickly). |
| Pathogenicity | Pathogen + / Host – | A pathogen (a disease-causing organism) lives in a host and causes disease, benefiting itself while harming the host. |
Predator vs parasite vs pathogen — all + / –: These three look alike on the signs, so use the mechanism:
A predator kills its prey quickly and usually eats more than one in its life.
A parasite lives on or in one host, feeding off it over time and harming it but usually not killing it quickly (e.g. a tick or a tapeworm).
A pathogen is a parasite that specifically causes disease in its host (e.g. a bacterium or virus that makes the host ill).
Competition harms BOTH — that's the giveaway: If a question says both organisms are harmed, the answer is almost always interspecific competition (– / –).
It happens when two species need the same limited resource — light, food, water, space or mineral ions — so each one reduces the amount available to the other.
No species is feeding on the other here; they are simply getting in each other's way.
| Sign | Meaning | Relationships that use it |
|---|---|---|
| + | That species benefits (gains food, nutrients or protection) | predator, herbivore, both mutualists, parasite, pathogen |
| – | That species is harmed (loses energy, is damaged or made ill) | prey, the plant, both competitors, host |
| + / + | Both species benefit | mutualism only |
| – / – | Both species are harmed | interspecific competition only |
| + / – | One benefits, one is harmed | predation, herbivory, parasitism, pathogenicity |
Both benefit, or both harmed
- Mutualism (+ / +) — both species gain
- Example: a bee gets nectar, a flower gets pollinated
- Competition (– / –) — both species lose
- Example: two plant species shading each other out for light
One benefits, one harmed (+ / –)
- Predation — predator eats prey (kills it)
- Herbivory — animal eats a plant
- Parasitism — parasite feeds on a living host
- Pathogenicity — pathogen causes disease in a host
A memory hook: Mutual = mutual benefit (+ / +). Compete and you both come off worse (– / –).
For the + / – group: ask 'is it eating, living-on, or sickening' the other species — that's predation/herbivory, parasitism, or pathogenicity.
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How this is tested: On Paper 1A a 1-mark multiple-choice question often describes an interaction and asks you to identify the relationship — a common version gives an interaction that harms both organisms, and the answer is competition.
On Paper 2 a 2-mark Explain question may name two organisms (such as a hummingbird and the flower it feeds on) and ask you to explain the type of relationship — you must name it and justify it using the benefit to each species.
Other Paper 2 versions ask you to describe the mutual benefits to each organism in a mutualism, or to deduce the knock-on effects when one partner declines.
IB-style question — explain a relationship between two organisms
A hummingbird feeds on the sugary nectar inside a flower. As it feeds, pollen sticks to its head and is carried to the next flower, pollinating it. Explain the type of interspecific relationship between the hummingbird and the flower. [2]
How to score both marks
- Name the relationship. This is mutualism — a relationship in which both species benefit.
- Justify the benefit to each species. The hummingbird benefits by getting nectar (a food/energy source), and the flower benefits because the bird carries its pollen to other flowers (pollination / reproduction). Because both gain, it is mutualism. (Mark 1: mutualism. Mark 2: states the benefit to each organism.)
Final answer
Mutualism — the hummingbird gains nectar (food) and the flower gains pollination (so it can reproduce); both species benefit, so the relationship is + / +.
✓ Why this scores full marks: An Explain mark is not just the label — you must say why.
Here the second mark comes from spelling out the benefit to both the bird (nectar) and the flower (pollination). Naming 'mutualism' alone would score only the first mark.
The six interspecific relationships, each shown by how it affects the two species involved — a green + means that species benefits, a red – means it is harmed.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.