The big idea: No species lives alone. Where two different species meet, their lives are linked, and we describe each link by one simple question: does this interaction help or harm each species?
Naming and sorting the links
- An interaction between two different species is called an interspecific relationship ('inter' = between, 'specific' = species).
- We sort these relationships by the effect on each partner.
- A + means that species benefits.
- A – means that species is harmed.
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.
- Interspecific relationship
- An interaction between two different species (compare with intraspecific, which is within one species).
- Predation
- One organism (the predator) kills and eats another organism (the prey). The predator benefits (+); the prey is harmed (–).
- Herbivory
- An animal eats a plant or part of a plant. The herbivore benefits (+); the plant is harmed (–) but is often not killed.
- Competition
- Two species use the same limited resource (food, light, space, water). Both species do worse, so both are harmed (– / –).
- Mutualism
- Two species live closely together and BOTH benefit from the interaction (+ / +).
- Parasitism
- A parasite lives on or inside a host, taking nutrients from it. The parasite benefits (+); the host is harmed (–).
- Pathogenicity
- A pathogen (a disease-causing microbe, such as a bacterium, virus or fungus) infects a host and causes disease. The pathogen benefits (+); the host is harmed (–).
The key trick — read the two signs: Every relationship is just a pair of signs, one for each species:
+ / + = both benefit (mutualism), – / – = both harmed (competition), and + / – = one wins, one loses (predation, herbivory, parasitism, pathogenicity).
Get the two signs right first, then name the relationship — that is exactly how the exam is marked.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Start Studying Free Full access to Aimnova Pro · cancel anytime
Several relationships share the + / – pattern, so you cannot stop at the signs alone. You also have to look at how the benefit and harm happen.
Work through it in two steps
1
Who gains, who loses
First decide who benefits and who is harmed.
2
What kind
Then ask what kind of interaction it is.
Signs first, then the kind of interaction.
| Relationship | Effect on species 1 / species 2 | What happens |
|---|---|---|
| Predation | + / – | One organism (the predator) kills and eats another (the prey). |
| Herbivory | + / – | An animal eats a plant (or part of one); the plant is damaged but often not killed. |
| Competition | – / – | Two species need the same limited resource, so each does worse — both are harmed. |
| Mutualism | + / + | Two species live closely together and BOTH benefit from the interaction. |
| Parasitism | + / – | A parasite lives on or in a host, gaining nutrients while harming the host. |
| Pathogenicity | + / – | A pathogen (a disease-causing microbe) infects a host, benefiting itself while causing disease. |
Both benefit → mutualism: If both species gain something, the relationship is mutualism.
The test is simple: can you name a benefit for each side? If yes, it is mutualism.
Classic mutualism examples
- A bee gets nectar while it pollinates a flower.
- Nitrogen-fixing bacteria in the root nodules of a legume get sugars and a home, while the plant gets usable nitrogen.
- Gut bacteria make vitamins for a mammal and gain a warm, food-rich habitat in return.
Both harmed → competition: If both species are worse off, the relationship is competition, the only one where both partners are harmed (– / –).
It happens because the two species need the same limited resource (light, water, food, space), so neither gets as much as it would alone.
Example of competition: Two plant species growing side by side, each shading the other and slowing its growth.
One wins, one loses → look at HOW: When it is + / –, decide how the loser is harmed.
Killed and eaten
- Predation: animal prey is killed and eaten
- Herbivory: a plant is eaten
Lived on or inside
- One organism lives on or inside the other, feeding off it over time
- Parasitism: a parasite, e.g. a tick
- Pathogenicity: a microbe that causes disease, e.g. a fungus infecting a crop
Both partners benefit
- Mutualism ( + / + )
- Each species gains something
- Bee + flower; legume + N-fixing bacteria
- Test: name a benefit for each side
At least one partner is harmed
- Competition ( – / – ) — both harmed
- Predation / herbivory ( + / – ) — one is eaten
- Parasitism / pathogenicity ( + / – ) — one is lived on / infected
- Test: who is harmed, and how?
A memory hook: Mutualism = mutual (both gain). Competition is the odd one out where both lose. For the rest, one + and one –: eaten = predation/herbivory, lived-on or infected = parasitism/pathogenicity.
Learn what examiners really want
See exactly what to write to score full marks. Our AI shows you model answers and the key phrases examiners look for.
Try AI Feedback FreeYour first topic is free to keep • No credit card required
How this is tested: Paper 1A asks you to identify the relationship. Paper 2 asks you to explain it, with the effect on each species.
| Paper | What you are asked | What earns the mark |
|---|---|---|
| Paper 1A | A 1-mark question describes an interaction and asks you to identify the relationship. | The right label: gut bacteria that make vitamins for a human (and gain a habitat) is mutualism; an interaction that harms both organisms is competition. |
| Paper 2 | An Explain question gives two named species. | Name the relationship AND say how each species is affected (+ / –), not just the label. |
Watch for the trap: Several relationships are + / –, so you must say how the harm happens to choose between predation, herbivory, parasitism and pathogenicity.
IB-style question — explain the relationship between legume and bacteria
Nitrogen-fixing bacteria live inside the root nodules of a bean plant (a legume). The bacteria convert nitrogen gas into a form the plant can use, and the plant supplies the bacteria with sugars. Explain the interspecific relationship between the bean plant and the bacteria. [2]
How to score both marks
- Name the relationship. This is mutualism, because both species benefit from living together.
- Say how each species benefits. The bacteria gain sugars (and a protected habitat) from the plant; the plant gains usable nitrogen from the bacteria. Because there is a clear benefit on each side, the relationship is mutualistic. (Mark 1: mutualism. Mark 2: a benefit named for EACH species.)
Final answer
Mutualism — the bacteria gain sugars and a habitat, and the plant gains usable nitrogen, so both species benefit.
Why this scores full marks: An Explain of a relationship needs two things: the name of the relationship and how each species is affected.
Writing only 'mutualism' would score the first mark but miss the second — the examiner wants a benefit (or harm) stated for both partners.
| Pattern of effects | Relationship it points to | Example |
|---|---|---|
| Both benefit ( + / + ) | Mutualism | bee pollinates a flower while collecting nectar |
| Both harmed ( – / – ) | Competition | two plants shading each other out for light |
| One benefits, one harmed ( + / – ) — by being eaten | Predation (animal prey) or herbivory (plant) | owl eats a mouse; deer eats a shrub |
| One benefits, one harmed ( + / – ) — by living on / in the host | Parasitism or pathogenicity | tick feeds on a deer; a fungus infects a crop |