Seed Dispersal

Why disperse?

Three pressures drive the evolution of dispersal:

1. Escape from the parent. A seedling that germinates in its mother's shade competes with her for light, water and nutrients. The mother almost always wins.
2. Escape from enemies. Herbivores and pathogens accumulate around adult plants — the Janzen–Connell effect, one of the best-tested explanations for tropical tree diversity.
3. Colonization. A few rare long-distance dispersal events drive the entire global distribution of plants. Hawaii's flora descends mostly from seeds that arrived once, on a bird's foot or on driftwood.

Different syndromes are different bets across these pressures. A heavy nut bets on survival; a feather-light wind seed gambles on luck.

Mechanisms of dispersal

	Anemochory	Hydrochory	Zoochory (epi-)	Zoochory (endo-)	Autochory	Myrmecochory
Vector	Wind	Water	Outside of animal (fur, feathers)	Animal gut	Plant itself (explosion / drop)	Ants
Seed feature	Wing, plume, dust-fine	Buoyant husk, low-density tissue	Hook, barb, sticky mucilage	Fleshy fruit around hard-coated seed	Tension or turgor in fruit wall	Elaiosome (lipid-rich body)
Typical distance	Tens of m to many km	Cm (rain) to thousands of km (ocean)	1–1000 m	10 m – 100 km (birds)	1–70 m	1–10 m
Examples	Dandelion, maple, milkweed, orchid dust	Coconut, water-lily, mangrove	Burdock, foxtail, beggarticks	Cherry, raspberry, fig, mistletoe	Squirting cucumber, sandbox tree, Impatiens	Trillium, bloodroot, many Acacia
Energy cost to plant	Low per seed	Moderate (heavy fruit)	Moderate (hooks, mucilage)	High (fleshy fruit pulp)	Moderate (mechanical tension)	Low–moderate (small reward)
Reliability	Low — most seeds land near	Variable — depends on water	High when fur present	Very high in fruiting season	High — local but reliable	High in temperate forests
Notable trick	Maple samara autorotates	Coconut fibre traps air	Burdock inspired Velcro (1948)	Acid scarification can boost germination	Witch hazel ejects 11 m	Ants discard seed in nutrient-rich nest

Wind — anemochory

Wind dispersal is cheap per seed but slot-machine in outcome — most seeds land within metres, but a tiny fraction ride storm thermals far further. Three architectures dominate:

• Plumes — dandelion, milkweed, willow. Fine bristles form a parachute. Fluid-dynamics work shows the dandelion pappus sustains a stable ring vortex above the seed, slowing fall and allowing transport over kilometres in moderate wind.
• Wings (samaras) — maple, ash, elm, tipu. The wing is offset from the seed's centre of mass, and as the samara falls it autorotates like a helicopter rotor, more than doubling its time aloft.
• Dust seeds — orchids, ericaceous shrubs. Seeds are reduced to micrograms; the cost is that they carry no food reserve and must germinate with a fungal partner.

Water — hydrochory

Most aquatic plants and many coastal trees disperse by water. The recipe is buoyancy plus a watertight inner seed coat. The coconut is the celebrity case: a fibrous mesocarp packed with air, encasing a hard endocarp, encasing the seed and its store of liquid endosperm. Coconuts have remained viable after more than 100 days adrift in the open Pacific — long enough to colonize beaches across thousands of kilometres. Mangroves do something stranger: their seeds germinate on the parent tree and drop as pencil-shaped propagules ready to root the moment they snag in mud.

Animals — zoochory

• Endozoochory (gut transport). The plant pays a calorie tax — a fleshy, sugary, brightly coloured fruit — and a vertebrate eats it. The hard seed coat protects the embryo through the gut, and acid scarification often improves germination. Birds (cherries, juniper, mistletoe), bats (figs), bears (blueberries) and elephants (marula, baobab) are major vectors. Around 90% of tropical tree species rely on this route.
• Epizoochory (sticking on the outside). The plant pays no calories — it just makes the seed sticky or hooked. Burdock burrs caught on his dog's coat after a 1941 hunting walk inspired George de Mestral to invent Velcro, patented 1955. Foxtail awns and beggarticks travel the same way.

The plant itself — autochory

Some plants pack tension into their fruit walls and release it explosively. Witch hazel (Hamamelis) ejects seeds up to 11 metres. Squirting cucumber (Ecballium elaterium) builds turgor pressure inside the fruit until it pops off its stalk, shooting seeds in a jet up to 6 metres. The sandbox tree (Hura crepitans) detonates its dry fruits with a rifle-like crack at exit speeds near 70 m s⁻¹. Even Impatiens (touch-me-not) coils its fruit walls under tension; the lightest brush triggers them.

Ants — myrmecochory

Roughly 11 000 plant species attach a small lipid- and protein-rich body called an elaiosome to each seed. Foraging ants carry the seed home, eat the elaiosome and discard the seed in the nest's waste pile — a predator-safe, nutrient-rich micro-site. Trillium, bloodroot and many forest-floor herbs depend almost entirely on ants, as do many Australian acacias and Cape Proteaceae.

Fire-dependent dispersal

Many fire-prone ecosystems hold their seeds back until heat releases them. Jack pine cones are sealed with resin and open only after a fire melts it. Manzanita seeds (Arctostaphylos) have seed coats so hard they germinate poorly without fire scarification — Mediterranean chaparral and California fire ecology depend on them. Fire is not a disturbance to these communities; it is a recruitment cue.

Common misconceptions and pitfalls

• "Most seeds end up far from the parent." The opposite. Even strong dispersers leave the median seed within tens of metres; the rare long-distance event matters most ecologically.
• "Fleshy fruits exist to feed animals." They exist to recruit animals as transport. The pulp is the toll, not the goal.
• "Wind dispersal is for tiny seeds; animals carry the heavy ones." True on average, but heavy dipterocarp seeds whirl on giant wings, and tiny fig seeds travel hundreds of kilometres in bird guts.
• "Native plants disperse better than introduced ones." Often false — many invasives (kudzu, garlic mustard, knotweed) spread aggressively because they escape the enemies that limited them at home.
• "Once a seed lands, dispersal is over." Secondary dispersal — by ants, dung beetles, surface water — moves seeds metres further and significantly reshapes seedling distributions.