Why disperse seeds

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.

Community-level analyses decisively confirm the advantages of seed escape see below ; it costs several times more to produce a seedling in dense than in sparse aggregations, and many times more to produce a reproductive adult offspring near the parent plant than farther away. The question is no longer whether the advantage to escape from parents predicted by Janzen and Connell exists.

The questions are, when and where is the effect important, and what is the relative importance of spatially defined components of seed or seedling distributions in contributing to population and community structure? Projections of plant population growth, based on the types of dispersal agents that take the seeds, may provide a window into the future in understanding how different animals bias recruitment of plants.

In a dispersal study of the giant cactus Neobuxbaumia tetezo in Mexico, Godinez-Alvarez and colleagues find that a bat Leptonycteris curasoae is probably the most effective dispersal agent for this species. Compared with other bats, birds, and coyotes Canis latrans , Leptonycteris eats many fruits and deposits a large number of seeds under bushes and trees that could serve as nurse plants.

Given the mismatch between initial recruitment and survival of Ambrosia discussed above Miriti et al. Seed dispersal has other advantages. Seeds can colonize not only distant sites from which a given species is absent but also vacant sites in the local ecological succession.

Once thought to be primarily a means of colonizing distant places, seed dispersal may also be locally important if some species' inability to get to a region or habitat affects community composition Clark et al. The advantages of local colonization are real. Open ground is occupied by small-seeded, fast-growing species, followed by successive invasions of larger-seeded trees that are slower to arrive but are capable of establishing under a dense secondary forest canopy.

In demographic terms, seed-to-seedling and seedling-to-juvenile elasticities change with the advancing front. For instance, Parker finds that the seed-to-seedling transition explains almost all population growth at the front of advancing populations of the invasive shrub Cytisus scoparius in California, while adult death explains population change behind the front.

Directed dispersal occurs when an animal preferentially carries seeds directly to situations that are critical for seedling establishment. Examples include bird dispersal of parasitic mistletoes to appropriate host trees Davidar , ant dispersal of seeds that establish better in and around ant nests than elsewhere Beattie , and jay or nutcracker dispersal of pine and oak seeds to forest edges or openings Vander Wall Seeds can also be dispersed to forest light gaps by male birds frequenting habitual display sites.

For example, bellbirds Procnias tricarunculata carry the seeds of a montane tree Ocotea endresiana , Lauraceae to clearings where males display to females Wenny and Levey Other birds take Ocotea fruits but do not consistently deliver them to suitable habitats figure 4.

Elaiosomes seed appendages, typically rich in fat, that attract ants or other animals and the sticky seeds of mistletoes are adaptations for directed dispersal, but many plants without these adaptations, including oaks, bird pines, and Ocotea , opportunistically use birds to disseminate seeds to a suitable environment.

Colonization and escape may be synergistic. Human activities have facilitated and impeded dispersal in many ways. As stated previously, anthropogenic barriers in the form of human development have disrupted natural dispersal patterns in a variety of species. Conversely, humans have also facilitated dispersal, both deliberately and accidentally. A common inadvertent way organisms have been dispersed is through their transport in the ballast water of ships. Ships emptying ballast water may release foreign organisms.

For example, zebra mussels, a freshwater mollusk native to the lakes of southeast Russia, were accidentally introduced into the Great Lakes of North America where they have caused major economic problems by clogging water treatment and power plants through ballast water discharge. As a result of the potential for introduction of non-native organisms via ballast water, new standards have been proposed for ballast tank cleaning.

Humans have also transported organisms to areas outside their native ranges for deliberate reasons. The seeds of attractive plants native to areas outside North America are routinely used in gardens and have the capacity to disperse to wild areas if conditions are suitable e. Also, bighead and silver carp originating from China were introduced to catfish farm ponds in the United States to control algal growth. Fish accidentally escaped from these ponds and have subsequently colonized the Mississippi, Missouri, Illinois and Ohio rivers where they have had significant negative impact on the native fauna Figure 6.

Dispersal is a common process undertaken by individuals at different stages of the life cycle and in response to various factors. Morphological adaptations make dispersal achievable but with varying degrees of success due to anthropogenic and natural barriers. These barriers modify the level of dispersal and consequently exert effects on population dynamics and genetic structure. As environments are altered, through stochastic events and global climate change, it will become increasingly important to assess how such changes will affect dispersal at the individual, population, and species levels.

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Seeds dispersed by water are contained in light and buoyant fruit, giving them the ability to float. Coconuts are well known for their ability to float on water to reach land where they can germinate. Similarly, willow and silver birches produce lightweight fruit that can float on water. Animals and birds eat fruits, and the seeds that are not digested are excreted in their droppings some distance away. Some animals, like squirrels, bury seed-containing fruits for later use; if the squirrel does not find its stash of fruit, and if conditions are favorable, the seeds germinate.

Humans also play a big role in dispersing seeds when they carry fruits to new places and throw away the inedible part that contains the seeds. Turn on the fan.

Standing in the same place, try dropping your seeds one at a time in front of the fan. Also try dropping a plain "seed" for example, a regular paper clip with nothing attached to see what happens.

How far do the seeds get blown by the fan? Do certain seeds take longer to reach the ground than others? Think about your results. Did some of your designs not work at all fall straight down, without blowing forward? Did some work better than others? What can you do to improve your designs? Can you make changes to your seeds to make them blow even farther? Extra: Have a friend use a stopwatch to time how long it takes the seeds to hit the ground. This might be easier if you drop the seeds from a higher location.

Have a tall adult drop them, carefully stand on a chair or drop them from the top of stairs. Extra: Use a tape measure to record how far the seeds travel horizontally from where you drop them to where they hit the ground. Which seeds go the farthest? Extra: How do your results change if you change the speed of the fan? Observations and results 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.

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