Seed dispersal

In spermatophytic plants, seed dispersal is the movement, spread, or transport of seeds away from the parent plant. Plants cannot move around on their own, so they rely on many different dispersal vectors to carry their seeds. These include abiotic vectors, such as the wind, and living (biotic) vectors, such as birds. Seeds may be dispersed away from the parent plant one at a time or in groups, and they can be spread out in both space and time.

The way seeds are dispersed greatly affects the demographic and genetic structure of plant populations. It also affects migration patterns and the ways that species interact. Seed dispersal happens in several modes, including gravity, force, water, wind, and by animals. Some plants are serotinous, which means they release their seeds only in response to a stimulus from the environment.

Scientists often identify these modes by looking at special features called adaptations, such as wings or fleshy fruit. However, this simple view may miss some of the complexity involved. Plants can disperse seeds in ways that do not depend on the usual features linked to those methods, and a single plant trait may serve more than one purpose.

Evolutionary benefits

Seed dispersal likely helps plants in several ways. Seeds are more likely to survive the farther they land from the parent plant, possibly because seed and seedling predators and pathogens often gather where many seeds fall beneath parent plants. Seeds landing farther away may also face less competition with adult plants. Dispersal can also help plants reach favorable habitats, an idea called directed dispersal. For example, the tree Ocotea endresiana from Latin America is spread by several bird species, including the three-wattled bellbird. Male bellbirds perch on dead trees to attract mates and often pass seeds beneath them, where the light is better and fungal pathogens are less of a problem.

Seeds carried by ants (myrmecochory) travel short distances, but the ants bury them underground. This helps the seeds avoid fire and drought, reach nutrient-rich spots, and survive longer. Dispersal may also help plants settle empty habitats and new regions. Longer journeys are sometimes made through diplochory, where two or more dispersal methods work one after another. Recent evidence suggests that most seed dispersal events involve more than one phase.

Types

Seed dispersal is sometimes split into autochory, when a plant spreads its seeds using its own means, and allochory, when outside helpers spread them. Plants can use their own means, such as gravity, where heavy fruits like apples and coconuts fall from the plant, or ballistic dispersal, where seeds are forcefully ejected by explosive dehiscence. Outside helpers include wind, water, animals and humans. Wind can carry dandelion seeds far, water lilies float their fruits on water, and ocean currents can carry palm seeds over long distances. Animals spread seeds in many ways, including ants through myrmecochory. Humans spread seeds too, through anthropochory, and seed bombing is a deliberate method that has some risks.

Long-distance seed dispersal is a type of spread where seeds travel far from the parent plant. It can be measured as a share of all the seeds a plant makes, or by an actual distance, with 1 km as the threshold. This method can help a plant's survival, because it reduces competition with neighboring plants for its offspring. It is still unclear how specific traits, conditions and trade-offs affect how long-distance dispersal evolves.

Consequences

Seed dispersal affects how plants live and change over time. It is needed for species to move to new places, and today, how well a plant can spread its seeds helps decide whether a plant that people carry to a new habitat becomes invasive. Scientists also expect dispersal to play a major role in how many different species form and survive. For example, plant groups that use ants to spread their seeds (called myrmecochory) have diversified much faster, because these groups contain more than twice as many species as their closest related groups. Seed dispersal away from the parent plant is also central to two major ideas about how biodiversity is kept up in nature: the Janzen-Connell hypothesis and recruitment limitation. Seed dispersal is essential for forest migration of flowering plants. Some plants produce different fruit types, a process called heterocarpy. These fruits differ in size and shape and spread seeds over different distances, which helps seeds reach different environments. Seed dispersal distances were shortest in wetlands and longest in dry landscapes.

Wind speed and direction strongly influence how seeds spread and where floating seeds settle in still water. The wind direction guides where seeds travel, which affects whether plants can take hold along river banks or in wetlands near streams. Wind can also affect the connections between water bodies. Over short periods, such as days and seasons, wind plays a bigger role in moving seeds that float on water. Over several years, however, ecological processes balance out these effects. The length of time over which seeds spread is therefore important when studying the effects of wind.