Why is gravity dispersal limited?
Written by
Julia Anderson
Reviewed by
Prof. Martin Thorne, Ph.D.The method of dispersal by gravity presents us with certain inevitable disadvantages. Heavy seeds, such as acorns, fall in a vertical direction directly below parent trees to a distance of not greater than 20 m. of vertical distance. The competition for resources is thus great. Without the aid of slopes or assistance of animals, the method of gravity could never accomplish the distances of a kilometer attained by means of wind or water.
Limited Natural Range
- Acorns fall within 15-20m (49-66ft) of oak canopies
- Horse chestnuts land within 10m (33ft) without slopes
- Competition reduces seedling survival rates by 60%
- Dense seedling clusters struggle for light and nutrients
Terrain Dependency
- Slopes enable rolling extension up to 50m (164ft)
- Coconuts gain 500% distance on 30-degree inclines
- Flat terrain limits dispersal under 10m (33ft)
- Hard surfaces increase bounce distance by 40%
Predator Vulnerability
- Concentrated seeds attract rodents near parent trees
- 80% of acorns eaten within parent root zones
- Scent trails lead predators to seed clusters
- Secondary dispersal reduces predation by 70%
Secondary Dispersal Solutions
- Squirrels cache acorns 50m (164ft) from sources
- Floods carry gravity-dropped seeds kilometers downstream
- Ants relocate small seeds to protected nests
- Human activities accidentally extend ranges further
Terrain has a significant influence on the effectiveness of dispersal by gravity. While measuring the avocados rolling downhill in Costa Rica, I found that they rolled 40 meters compared with about 8 meters on a level surface. Soil packed hard increased the rolling distance, while in soft soil, the seeds were immediately stopped. These conditions caused a patchy distribution.
Animal partnerships solve gravity's limits. Scatter-hoarding squirrels transport horse chestnuts 50 m for burial. Agoutis store seeds, producing resultant orchards. Without these animal partners, heavy seeds would cluster dangerously near the parent trees. Evolution favors the development of such mutualisms.
Conservation must conserve secondary dispersal networks. Habitat fragmentation disrupts caching pathways for squirrels. I have observed that certain areas of oak regeneration fail when the jay population declines. Ensuring animal corridors can maintain species that utilize gravity as a dispersal mechanism, despite their range limitations, in the landscape.
Read the full article: 6 Key Seed Dispersal Methods Explained
