What Is Seed Dispersal? Plants Moving When They Cannot Move

Several pressures push seeds away from the parent. Competition is the most direct, since a seedling under an established plant faces shade, depleted soil and a root system it cannot outcompete. Density-dependent mortality is the second and is better supported than intuition suggests, since the pests, fungi and herbivores specialising on a plant concentrate around it, so seed and seedling survival rises steeply with distance, a pattern documented across many tropical tree species and which helps explain why tropical forests contain so many species at low density. Colonising new ground requires reaching it. And avoiding inbreeding requires that offspring establish where they may encounter unrelated individuals. Against all this sits the cost, since every dispersal mechanism consumes resources the plant could have spent on more seeds.

The solutions are varied and each carries a recognisable signature in the seed:

• •Wind, using wings, plumes or very small size, which is cheap and imprecise
• •Water, using buoyant seeds with waterproof coats, which is how coconuts and many riverside plants travel
• •Animals eating fruit, where the plant pays with edible flesh and the seed passes through intact
• •Animals caching, where nuts are buried for later and the forgotten ones germinate
• •Attachment, using hooks and barbs that catch on fur and clothing, which costs the animal and pays the plant nothing
• •Explosive release, where a drying or turgid structure flings seeds several metres, which needs no agent at all

Fruit is an arrangement in which the plant produces expensive edible tissue to buy transport, and the details of the arrangement are tuned. Ripening changes colour, softness and sugar content simultaneously, which signals to the disperser that the seed is ready and withholds the reward until it is. Unripe fruit is frequently defended by bitterness or toxins for exactly that reason. Seed size and gut passage are matched, since a seed must be swallowable and must survive digestion, and seeds are frequently protected by a hard coat that the gut scarifies usefully. Some plants target specific dispersers, with fruit colour, height and scent suited to birds, to mammals or to bats. And where the disperser has gone extinct, the plant is left with fruit nothing eats, which is a documented situation for several species with large fruit that appear adapted to animals no longer present.

Establishing dispersal distances is technically awkward and the methods are ingenious. Seed traps placed in arrays around a parent catch what falls and give the shape of the distribution close in, and they miss the rare long-distance events that matter most. Marking seeds with dyes, with fluorescent powder or with tiny tags allows individuals to be recovered. Genetic methods have transformed the field, since matching a seedling to its parent by genotype establishes the distance directly regardless of how the seed travelled, and that has revealed dispersal far longer than trap data suggested. Following animals with tracking devices and combining their movement with gut passage times produces modelled distributions. The consistent finding is that the distribution has a long tail, with most seeds falling close and a small number travelling very far, and that the rare events dominate the ecological consequences.

Dispersal failure is a quiet and serious conservation problem. Habitat fragmentation leaves populations without a route to new ground, so a species can persist in a patch while being unable to colonise anywhere, which makes it vulnerable to local loss with no replacement. Loss of large fruit-eating animals, through hunting and habitat loss, removes the only dispersers for large-seeded trees, and studies in defaunated forests find measurable reductions in the dispersal distances and regeneration of those species even where the trees themselves are untouched. Climate change adds a further demand, since species tracking shifting conditions must move at rates that many dispersal mechanisms cannot achieve, and the gap between required and observed rates is a recognised concern. Assisted movement of seeds by people is being considered and argued over for exactly this reason.

Seeds beneath a parent face shade, depleted soil and the pests concentrated around it, so survival rises steeply with distance. Wind, water, gut passage, caching, attachment and explosive release are the mechanisms, each leaving a signature in the seed. Fruit buys transport with edible tissue, and losing large fruit-eating animals measurably reduces the regeneration of large-seeded trees.