What if seeds could not travel?

A plant stays in one place all its life. It cannot walk.

But its seeds can go on a trip! What if seeds could not travel at all?

Blow on a fluffy dandelion and dozens of tiny seeds float away on the breeze. Plants can't walk, but their seeds can travel by wind, by water, inside animals and stuck to their fur.

So what if seeds couldn't travel at all, and every seed just dropped to the ground under its parent? In this thought experiment you can explore crowded seedlings, the wind and animal helpers, and the hooks and burrs that hitch a ride.

Seed dispersal is the movement of seeds away from the plant that made them. Because most plants are rooted in one place, their seeds are the only stage of life that can move, by wind, water, animals or even explosive fruits.

This lab asks what would change if dispersal stopped: if every seed fell beneath its parent. It is a thought experiment, but one with an unusual piece of real evidence from the island of Guam, where many seeds really did stop traveling.

Some seeds have fluffy parachutes. The wind carries them away.

Some seeds hide in berries. Birds eat the berries and drop the seeds somewhere new.

Some seeds have tiny hooks. They stick to fur and socks!

Seeds have many ways to travel. Each dandelion seed has a feathery parachute that catches the breeze, and maple seeds have wings that spin like little helicopters. Coconuts float across the sea.

Animals help too. Birds eat berries and leave the seeds somewhere else in their droppings. Squirrels bury acorns and forget some of them, and the forgotten acorns can sprout. Burrs have hooks that grab fur, feathers and even your socks, then drop off far from where they started.

Britannica describes a wide toolkit. Wind dispersers carry parachutes, like the dandelion's feathery pappus, or wings, like the maple's spinning samaras. Coconuts are buoyant and watertight, keeping the embryo alive for months at sea, and some mangrove propagules survive up to a year in salt water.

Animals carry many more. Fruit eaters swallow seeds and pass them elsewhere; blackberry seeds even need wearing down in a bird's gizzard before they can sprout. Squirrels forget buried acorns, and ants carry seeds with tasty attachments called elaiosomes to their nests. Some fruits explode, using water pressure (squirting cucumber) or tension in drying fruit (certain violets).

Now imagine every seed just falls down. Plop! Right under its parent plant.

Lots of baby plants would grow in one small spot. It would be very crowded.

In our imaginary world, no seed can fly, float, or ride on an animal. Every seed falls straight down under the plant that made it.

The ground beneath each tree or flower might fill up with seedlings, all squeezed together in the parent's shade. They would have to share the same water, the same soil and the same patch of light. Plants might also have a hard time reaching new places, like a fresh patch of ground on the other side of a meadow.

Removing dispersal in imagination means every seed lands in the parent's own patch. Britannica notes that both parent and offspring are usually at a disadvantage when they must compete for the same water, nutrients and sunlight, and a K-State Research and Extension article describes dispersal as keeping seedlings out of competition with the parent plant.

The likely consequences are ideas, not measurements: dense clumps of seedlings, fewer survivors per seed, and plants unable to reach newly opened ground. How strong each effect would be would depend on the species and the place.

A baby plant needs water, sunshine and good soil.

If it grows right next to its big parent, it has to share. Moving away gives it more room.

A seed's life is tough. To grow, it has to land somewhere with the right soil, water, warmth and light. Next to its parent, it has to compete with a big plant that is already there, and with all its brothers and sisters too.

That's why so many plants have ways to send their seeds away. A K-State extension article explains that moving seeds away from the parent keeps seedlings out of competition with it and increases their chance of survival.

A seedling needs a viable seed, a favorable place and the right moisture, temperature and light, as a K-State Research and Extension article lists. Dispersal improves the odds of the second step and reduces competition with the parent for water, nutrients and sunlight.

Research on Guam added another reason: seeds and seedlings were less likely to germinate or survive underneath parent trees. Seeds swallowed by birds also sprouted better, two to four times more often than seeds still inside the fruit flesh, so for some plants the journey itself prepares the seed.

On the island of Guam, most forest birds went away.

Now many seeds just fall under their trees. Fewer new little trees are growing there.

On the Pacific island of Guam, a brown treesnake that reached the island after World War II made 10 of the island's 12 kinds of forest birds disappear. Five of them ate fruit and spread the seeds of about 70 percent of Guam's forest trees.

Ecologist Haldre Rogers of Iowa State University put out seed baskets on Guam and nearby islands. On Guam, seeds now fall right under the parent tree. On islands with birds, the seeds are carried through the forest. Her team found far fewer seedlings on Guam for the two tree species they studied.

Guam is the closest thing to a real version of this lab. The brown treesnake, believed to have stowed away on military cargo ships after World War II, drove 10 of Guam's 12 forest bird species to extinction on the island; five were frugivores that dispersed seeds of around 70 percent of the island's forest trees.

Haldre Rogers (Iowa State University) collected seeds in baskets on Guam and three nearby islands in 2009 and 2010. Seeds on Guam fell directly under the parent tree, mostly still inside their fruit. Bird-swallowed seeds were two to four times more likely to germinate, and seeds and seedlings fared worse under parents. Together, these effects meant a 61 to 92 percent decline in seedlings for the two species studied, as Iowa State University reported the study (Nature Communications, 2017). Rogers describes Guam as becoming an increasingly different forest.