Why are seeds scattered

In this project you will make your own artificial "seeds" from craft materials. Can you design seeds that will stay in the air for a long time? You should find that adding light materials to the "seed" can make it fall more slowly and blow farther—however, the shape of the materials is also very important. For example, a paper clip attached to a crumpled-up piece of paper will still fall very fast. A piece of paper with a "wing" design similar to that of a maple seed or a bunch of individual streamers like a dandelion seed , however, will fall more slowly and be blown farther by the fan.

Exactly how far the seeds blow will depend on the strength of your fan but you should definitely see a difference in the horizontal distance traveled between a "plain" seed and one with a dispersal mechanism. When you take your best designs and try to improve on them, you mimic the process of evolution—because the "best" seed designs in nature are the ones most likely to reproduce!

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See Subscription Options. Discover World-Changing Science. Key concepts Biology Plants Evolution Aerodynamics Introduction Have you ever looked outside on a windy day and seen "helicopter" seeds spinning through the air? Materials Examples of different seeds that are dispersed by the wind Depending on where you live, you may be able to find some of these seeds outside.

If you have access to the Internet, you can also do a Web search for maple seeds, dandelion seeds and other types of wind-dispersed seeds to help get ideas. Small, uniform, lightweight objects that you can use as "seeds" For example, you could use small paper clips or small binder clips; or purchase a bag of real seeds—such as sunflower seeds—at the supermarket.

Science is an attempt to explain the natural world. Evolution explores how groups of living things have changed over long periods of time, for example, how plants have developed different ways to disperse their seeds. Add to collection. Nature of science Science is an attempt to explain the natural world. Go to full glossary Add 0 items to collection. Download 0 items. Twitter Pinterest Facebook Instagram. Email Us. See our newsletters here. Would you like to take a short survey? His interests include seed dispersal by tropical birds and mammals and the effects of rodents on tallgrass prairie vegetation.

Maria N. Miriti e-mail: miriti. Her interests include spatial demography of plants, demographics of invasive species, and modeling the landscape—population interface. Howe, Maria N. A profusion of fruit forms implies that seed dispersal plays a central role in plant ecology, yet the chance that an individual seed will ultimately produce a reproductive adult is low to infinitesimal. Extremely high variance in survival implies that variations in fruit production or transitions from seed to seedling will contribute little to population growth.

The key issue is that variance in survival of plant life-history stages, and therefore the importance of dispersal, differs greatly among and within plant communities. In stable communities of a few species of long-lived plants, variances in seed and seedling survival are immense, so seed-to-seedling transitions have little influence on overall population dynamics. However, when seedlings in different circumstances have very different chances of survival—in ecological succession, for example, or when dispersed seeds escape density-dependent mortality near parent trees—the biased survival of dispersed seeds or seedlings in some places rather than others results in pervasive demographic impacts.

The profusion of fruit sizes, shapes, colors, and scents, and the variety of rewards for the agents that disperse plant seeds, imply a central role for seed dispersal in plant ecology figure 1. However, this leaves open the ecological questions of why seed dispersal is important and how understanding it can help predict population and community change.

Early studies of dispersal ecology emphasized natural history, asking which animals take such fruits as Virola or Ficus , or how the winged pods of the legumes Platypodium and Tachigalia ensure sufficient dispersal by wind. Such questions can still be iterated for tens of thousands of plant species from Cape Horn to the Arctic, but they no longer define the field.

The challenge now is to understand such natural-history lore in light of the demographic processes that shape plant populations and communities. The role of seed dispersal in plant reproduction presents curious contrasts. Despite the evident importance of their dispersal, most seeds fall near their parents. Genetic markers confirm that the vast majority of the offspring of one common European cherry species end up within 10 meters m of the tree that produced them, while a minority travel much further.

The consequences of seed distributions are therefore important to know. For a highly fecund tree such as the tropical fig Ficus insipida , which may produce 5 million seeds per year and hundreds of millions in a lifetime, the likelihood that a seed distribution reflects later stages is all but nil. For the less fecund Virola nobilis Myristicaceae , with , seeds in a lifetime, the chance that seed distributions reflect distributions of surviving offspring is orders of magnitude greater, but still minuscule.

Processes of survival and mortality determine plant fate in space and time. One way to overcome the challenges of calculating the effects of dispersal on populations with immense mortality is to calculate the likelihood of transitions among life stages, from seed to seedling, seedling to juvenile, juvenile to sapling, and sapling to adult.

Probabilities of transitions allow projections of subsets of populations into the future; variances in those transitions indicate which steps between life stages, under different ecological conditions, contribute most to population growth.

In this article, we use a demographic context to explore ways in which varying degrees of dispersal success may or may not influence plant populations. We conclude by discussing the implications of those insights for conservation and management of plant communities. Demography enumerates processes of population recruitment and mortality in populations over time.

An evolving insight of spatial demography is that patterns of recruitment and mortality in space are enlightening. Dispersal attributes, which offer insights into the ways in which plant species have adapted the dispersal process, provide the first glimpses into understanding the advantage of seed dispersal for different kinds of plants.

In the mature rain forests of Central and South America, the genus Virola produces fruits that appear specialized for a few large fruit-eating birds and monkeys, a plant tactic that makes use of large animals that track the ripening of especially favored foods Howe , Russo However, a limited set of bird or primate dispersal agents is not an advantage for a tree that colonizes after massive, unpredictable disturbances, such as mudslides.

Because no particular animal consistently frequents such places, a generalized strategy of animal or wind dispersal is more advantageous. So, for instance, small fleshy fruits of Trema Ulmaceae are eaten by many birds of a variety of sizes and behaviors, allowing opportunistic invasion of expanses of exposed earth by thousands of Trema saplings when an earthquake drops an entire watershed into the sea Garwood et al.

Tachigalia has a predictable target for its seeds and seedlings under a dying parent, whereas Platypodium offspring must occupy new ground in distant light gaps holes in the forest canopy.

For species that colonize nearby unoccupied ground, selection may act less on precision in dispersal ability than on the capacity of parent plants to saturate their environs with offspring. Quantitative thinking introduces different perspectives, relying less on speculation about the adaptive features of fruits and more on the consequences of numerical patterns of survival and mortality Harper Demographic matrix projections indicate whether incremental differences in the success with which seeds or young juveniles survive affect overall population growth Wang and Smith Estimates of survival, growth, and fecundity at different life-history stages, represented as transition probabilities a i,j , may be used to project future population size and structure.

An accounting of the survival of seeds, seedlings, or other stages at different distances from parent trees, or in distinct ecological circumstances of slope, light, or soil type, provides the first step in spatial demography.

Bur sage, Ambrosia dumosa Asteraceae , a long-lived and often dominant shrub of the desert Southwest of North America, illustrates both the power and the limitations of demographic thinking Miriti et al. In Joshua Tree National Park in southern California, these shrubs grow singly or in small clumps separated by unvegetated space figure 2. Seeds drop off plants and are blown along the ground by wind, accumulating under shrubs and other vegetation.

However, as seedlings grow, they compete with each other and with the former nurse. A switch from facilitation to antagonism occurs within 5 to 10 years, contributing to high variance and low elasticity in the transition from seedling to older juvenile.

Spatial demography, or partitioning recruitment of parts of a cohort in different microhabitats or zones of risk, shows that dispersal away from adult shrubs does matter, even in this saturated community of ancient plants.

Ambrosia dumosa seedlings that survive the first few years in the open have a better chance of survival and growth than seedlings that establish under the protection of adult nurse shrubs Miriti et al. In effect, isolation from nurse plants decreases the variance in the transition from seedling to less vulnerable juvenile, thereby increasing the likelihood that differences in mortality of plants at some distance from nurses will influence population growth. Seed dispersal matters if dispersed seedlings a escape from density-dependent mortality near parents, b colonize open habitats, or c find microsites critical for establishment Howe and Smallwood If any of these mechanisms apply, dispersed seedlings are more likely to survive to reproductive age than undispersed seedlings.

The advantage to a parent of occupying a new region or continent with its seeds are obvious, but demographic logic suggests that long-distance dispersal may not be the primary advantage to most species most of the time.

Subtle but consistent differences in dispersal may bias processes of establishment and survival, conferring a potentially strong advantage to local seed dispersal. Partitioned cohorts in space are likely to show reduced variance in seed-to-seedling or seedling-to-juvenile transitions for subsets of a population, thereby indicating which parts of seed or seedling distribution influence population growth and, ultimately, community composition. A population's seed template is not a faithful guide to seedling or adult distributions if mortality is density dependent or is influenced by spatially distinct light, soil, or drainage microhabitats.

The seed template defines what is possible for seedling recruitment in a given place. To predict what actually happens, studies of dispersal that discover where seeds go can be integrated with spatially partitioned demographic analyses, thereby predicting population trajectories for seeds or seedlings starting in different circumstances. Elasticities then show which transitions among stages influence population growth and, therefore, which transitions yield the best predictions of future population dynamics.

One plausible advantage to seed dispersal is escape from enemies that live near the parent plant or that seek concentrations of seeds or seedlings. Janzen and Connell hypothesized that, among tropical trees, seeds or seedlings near the parent plant suffer density-dependent mortality from insects, mammals, or pathogens, while those that are carried away end up at much lower densities and therefore are more likely to survive to adulthood.

Widely scattered seeds and seedlings are less likely to be destroyed or infested by enemies than those in clumps. Dispersal of V. A monkey Ateles geoffroyi and a number of small birds, such as motmots Baryphthengus martii and trogons Trogon massena , eat the fruits figure 1 and regurgitate or defecate seeds in viable condition, but these species leave most seeds under or near the parent tree.

There Conotrachelus weevils lay eggs on seeds as they germinate; the larvae kill virtually all seeds and young seedlings. Dispersal agents diffuse the seed distribution in space and thereby maximize the work of seed and seedling enemies. Larger toucans Ramphastos swainsonii and Ramphastos sulfuratus and turkey-sized guans Penelope purpurascens carry most seeds that they eat beyond the edge of crown, leaving at least half more than 45 m away, where weevils are much less likely to find them box 1, figure 3.

For instance, a motmot dropping 10 seeds between 5 and 10 m from a Virola trunk in a morning of foraging would scatter them over m 2 , also occupied by hundreds of other Virola seeds or seedlings under the crown of the parent tree. A toucan dropping 10 seeds 40 to 45 m from the parent plant would scatter them over , m 2 , more than a fold difference in density. In a forest of loose aggregations of two to five individuals of this tree, most seeds in a band 40 to 45 m away from their parent would be even further from other Virola trees.

Here seed dispersal matters, as does the identity and behavior of the animals that remove the fruits. One might ask how cohorts of Virola seeds 20 m away from their parents fare compared with seeds or seedlings at much lower densities or m away from trees of the same species.

Seeds dispersed far from adult trees on a tail of a long, skewed seed distribution might in fact have the highest chance of survival Nathan and Muller-Landau Spatially explicit demographic analysis could confirm or reject speculation, based on estimates of high seedling mortality close to V.

Such an analysis would indicate whether enough very sparsely distributed seedlings, perhaps at or m from parent plants, offset density-dependent loss of seeds and seedlings from weevils. The Virola example suggests other demographic tests that could clarify the forces acting on the dispersal process. The large birds that do most of the work for this tree in Panama favor individual trees that have small seeds and large ratios of edible aril to indigestible seed.

The trees must balance powerful selective forces: Small seeds are more likely to be dispersed and therefore to escape almost certain death under their crowns, but large seeds, if they are dispersed, produce large seedlings that are better equipped to establish and survive in the shaded understory.

A parent tree cannot maximize both dispersal and establishment by adjusting seed size alone, because a seed cannot be small and large at the same time. The optimization criteria for dispersal of a tree species may differ in different environments Forget and Sabatier For instance, an aril-to-seed ratio that is optimal for Virola in Panama might be inappropriate in Peru, where different birds could favor a different balance of pulp reward to seed ballast, or where other dispersal agents could choose fruits on the basis of other criteria.

The competitive environment for seed dispersal differs in the two sites; many more primates feed on fruits of tress in the forests of the Peruvian Amazon, and more of these trees are adapted for primate dispersal than in Panama Janson Russo found that birds in Peru, as in Panama, favor Virola calophylla fruits with high aril-to-seed ratios. However, these preferences matter little to the trees, because almost all the fruits of this species are dispersed by a spider monkey, Ateles paniscus , which visits trees with large fruit crops.

A challenging but potentially useful demographic analysis might evaluate the actual success of seeds taken by monkeys from large fruiting trees compared with those scattered by birds from all trees in the population. Comparative studies of dispersal between close relatives indicate that the questions suggested by Virola are more than academic. For instance, comparing bird dispersal of the tree Commiphora Burseraceae in Madagascar and in mainland Africa, Bleher and Bohning-Gaese find that dispersal for the island species Commiphora guillaumini involves far fewer potential frugivores, fewer effective dispersal agents, less seed dispersal, and more spatial aggregration of juveniles around adults and of the adults themselves than for the continental species Commiphora harveyi.

The island tree has larger seeds and a much smaller aril than the mainland species, indicating a strategy of reduced reliance on animals for dispersal.

It costs more seeds to make a seedling on Madagascar, and the chances are that the seedling will be under the parent plant. These contrasts suggest that there may be adaptive advantages to broad dispersal and recruitment in mainland Africa and to lowered dispersal and perhaps greater allocation to seed defense in the island flora. The advantage of escape from the parent plant appears to be general. Focal studies of particular tree species and their dispersal show that bird dispersal of a Spanish cherry, Prunus mahaleb Rosaceae , confers an immediate advantage to local seed dispersal Jordano and Schupp In North America, bird dispersal of black cherry Prunus serotina helps seedlings avoid rapid buildup of a pathogenic pseudo-fungus Pythium spp.

If the seed is in fruit, dissemination occurs when the fruit falls from the plant or when an animal carries the fruit away. Some of these seeds end up in the soil and grow into new plants. Springtime Soil is ideal for growing seeds. This soil is damp and warm both of which are important for seed survival. If a seed does not land in moist soil it will stay dormant until moisture is present, and then it will grow.

Seeds can be scattered in many ways. Some seeds are scattered by the wind, some are carried away by birds and insects, some float in water, others are forced away from the plant by exploding pods, and still others hitchhike on animals and people.