Lesson focus: Seed and fruit structures influence dispersalHow seeds are spread away from the parent plant, for example by wind or animals. probability, reducing local competitionWhen living things try to get the same thing that is in short supply, such as food, light or space. and enabling colonisationWhen living things move into and start living in a new area. while carrying trade-offWhen getting a benefit means accepting a cost somewhere else, like a cheaper phone with a shorter battery life..
Infer dispersal mechanisms from evidence and explain how dispersal affects survival, competition and distribution.
Build on earlier learning
Investigating germinationGermination requires a suitable environment. Dispersal changes where seeds encounter those conditions relative to parent and siblings.
You will be successful when you can…
- Describe major dispersal mechanisms.
- Link seed or fruit structures to a mechanism.
- Explain benefits and trade-offs of dispersal.
- Use data rather than assuming every seed reaches a suitable site.
Words that carry the science
- dispersal
- Movement of seeds or fruits away from the parent plant.
- competition
- Interaction in which organisms require the same limited resource.
- colonisation
- Establishment of a population in a new area.
- buoyancy
- The upward force from a fluid that can keep an object afloat.
- dormancy
- Delayed growth of a viable seed until cues permit germination.
- trade-off
- A benefit gained alongside a cost or limitation.
Infer mechanism from structure and testable evidence
01Different vectors favour different structures
Wind-dispersed seeds or fruits may have wings, hairs or low mass that slow falling and increase travel time. Water-dispersed fruits may be buoyant and water-resistant.
Animal dispersalHow seeds are spread away from the parent plant, for example by wind or animals. may involve hooks attaching externally, fleshy fruits eaten with resistant seeds, or caching of nuts. Some dry fruits eject seeds through rapid tension release.
02Distance can reduce competition and correlated risk
Moving away can reduce competitionWhen living things try to get the same thing that is in short supply, such as food, light or space. with the parent and siblings for light, water, mineral ions and space. It can also reduce exposure to locally concentrated predators, pathogens or shading.
Dispersal can enable colonisationWhen living things move into and start living in a new area. and gene flow, but distant seeds may land in unsuitable places. Producing many seeds offsets low individual success in some species.
03Mechanism should be tested
A wing suggests wind dispersal, but evidence can include fall time, release height, air speed and distance distributions. Hooks suggest external animal transport, but attachment and retention tests provide stronger evidence.
Record distributions rather than only the farthest seed. A rare extreme distance does not describe typical dispersal.
Seed dispersal by wind
Plants spread their seeds so the young plants do not compete with the parent for light, water and minerals. Seeds with parachutes or wings fall slowly, so the wind carries them further.
D: dandelion. S: sycamore. A: acorn. Dashed lines show where each seed lands.
Controls
Falling speeds are approximate. Distance = wind speed × time to fall. Animals and explosions also disperse seeds.
Infer dispersal from a samara
A dry fruit has a broad wing and rotates slowly as it falls.
- The wing increases surface area relative to mass.
- Rotation and drag reduce downward speed.
- More time in the air allows horizontal wind to move the fruit farther.
- Greater distance can reduce local competition, although landing suitability remains uncertain.
How to make the reasoning visible: Structure supports a wind-dispersal hypothesis that can be tested with repeated fall and airflow measurements.
Compare dispersal evidence
Infer the most plausible main mechanism and name a measurement that could test it.
| Structure or observation | Likely vector | Mechanistic link | Testable measurement |
|---|---|---|---|
| Light fruit with papery wing | Wind | Slow rotating fall | Fall time and horizontal distance |
| Fibrous waterproof fruit that floats | Water | Buoyancy and water resistance | Float duration and germination after soaking |
| Hooked burr | Animal exterior | Hooks catch fur or feathers | Attachment and retention rate |
| Fleshy fruit with resistant seeds | Animal interior | Fruit eaten; seeds carried and released | Germination after simulated passage |
| Dry pod opens explosively | Self-dispersal | Stored tension ejects seeds | Launch-distance distribution |
Why should a dispersal investigation report a distribution rather than only maximum distance?
Explain one trade-off of animal-mediated dispersal.
Check your understanding
Answer from memory, then check against the suggested answer.
Why can dispersal reduce competition?
How can wings aid dispersal?
Name two animal dispersal routes.
A seed floats for two months but cannot germinate after salt-water exposure. Evaluate its water-dispersal adaptation.
Explain how dispersal and dormancy can interact to spread risk through space and time.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Some seeds and fruits, such as sycamore, are dispersed by the wind.
Explain how a sycamore’s wing helps it travel further.
(3)Explain why this helps the new plant, and give one limitation.
(2)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., build the chain from structure to movement to ecological consequence, then state the trade-off.