The big idea: No species lives alone. Wherever two different species meet, their lives are linked — and we can describe each link by asking one simple question:
does this interaction help or harm each species?
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 + if that species benefits, a – if it 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.
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: first decide who benefits and who is harmed, then ask what kind of interaction it is.
| 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.
Classic 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.
The test is simple: can you name a benefit for each side? If yes, it is mutualism.
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: 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:
if one organism is killed and eaten, that is predation (animal prey) or herbivory (a plant is eaten);
if one organism lives on or inside the other, feeding off it over time, that is parasitism (a parasite, e.g. a tick) or 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.
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How this is tested: On Paper 1A a 1-mark question describes an interaction and asks you to identify the relationship — for example, gut bacteria that make vitamins for a human (and gain a habitat) is mutualism, while an interaction that harms both organisms is competition.
On Paper 2 an Explain question gives two named species and expects you to 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 |