Why seeds dispersed

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.

Seed dormancy, which was described earlier, allows plants to disperse their progeny through time: something animals cannot do. Dormant seeds can wait months, years, or even decades for the proper conditions for germination and propagation of the species.

Improve this page Learn More. Skip to main content. 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. Temperate hickory Carya tomentosa seedlings in abandoned fields have both light and freedom from the rodents, rabbits, and deer that prefer not to forage in the open Myster and McCarthy Synergisms are obvious when large areas are denuded by agriculture and abandoned or are exposed by fire, earthquake, or rain-induced landslides.

At first, the colonization advantage is pronounced, as the land is populated by rapidly growing bird-, bat-, and wind-dispersed pioneers Finegan , Guariguata et al. In extensive regrowing pastures in Brazil, a few small-seeded pioneer species e. Dispersal limitation slows the successional process for larger-seeded species from deep forest into large openings. In disturbances of a few hectares ha or less, however, dispersal of larger-seeded trees from nearby forests does occur figure 5.

Spatial demography may show that seedlings that establish under the canopy of pioneer trees fare better than members of cohorts in the forest itself. Escape from density-dependent mortality near parents may also help maintain forest diversity. Janzen argued that density-dependent seed and seedling mortality is severe enough to leave openings for other species if seed predators are species-specific.

It follows that seeds escaping from parent trees are safer if they land under a tree of another species rather than one of their own.

Much as a pattern of seed distribution is a template for what may follow for a particular population, a community of seeds or seedlings is a community template of possibilities for the future Howe and Miriti The community of established plants that actually develops is the result of an array of density-dependent and density-independent processes that influence later juvenile distributions in different ways.

Spatial demography is one way to tease apart these effects. As dense patches of sibling seeds or seedlings under parents are decimated by insects or pathogens, one community-level result is the release of seedlings of different species, an effect that turns out to be general. In rain forest, density-dependent mortality may occur in all stages of growth. Harms and colleagues recorded the densities of almost , seeds of 53 species that fell into fruit traps over 4 years in the Panamanian rain forest, and compared the distributions of species in each trap with samples from more than 13, seedlings growing nearby.

Not only were abundances of emerging seedlings inversely correlated with the number of seeds that fell into traps, the species abundance distributions of seedlings were more even, and therefore more diverse, than those of seeds in the traps. Peters further shows that density-dependent mortality occurs in sapling and adult stages in the vast majority of species that are not acutely rare in both Panamanian and Malayan rain forests; rain-forest trees survive better when neighbors are of other species.

The effect of neighbor identity might be expected to be reflected in lower elasticities for seedling-to-sapling and sapling-to-adult transitions in common trees when the trees are close to neighbors of their own species than when they are close to other species. Peters shows that the escape advantage, and the enhancement of diversity through release of less common species when local dominance is suppressed, extends well beyond seed and seedling transitions.

The escape advantage may be magnified in diverse forests, where it actually helps maintain tree diversity. Genetic variation is not random in seed and seedling cohorts.

Microsatellites are hypervariable, noncoding regions of chromosomes, with several or many alleles at a locus, that behave like Mendelian genes without selection Dow and Ashley Godoy and Jordano use microsatellites to measure dispersal distance and mosaics of seedling dispositions for the well-studied European cherry P.

Microsatellite techniques are still new to the field of spatial demographics, but the results for P. Microsatellites confirm that most P. More surprising is the finding that deposition sites at some distance from fruiting trees have offspring of only one or a few parents, while those under fruiting trees, though dominated by full or half siblings, include a number of other parentages as well. A demographic question that should be addressed is whether these outsiders have a better or worse chance of growth and survival than closely related members of the cohort.

Hypervariable markers will alter the questions that can be asked of dispersal processes. For instance, does chance immigration of plants from outside a stand contribute genes that have unusual fitness in a new site? The apparent paradox of the profuse adaptations for seed dispersal and the inconsequential prospects of any given seed lies in the confounding of averaged with partitioned demographic effects.

Spatially defined demographies exist, but the benefits of isolation are subtle Miriti et al. Between the extremes of totally closed and open habitats lies most of nature, where a thoughtful application of this distinction can be enlightening. Ecological restoration reestablishes ecological patterns and processes where they have been destroyed by humans, bypassing the slow stages of natural succession.

Intensive agriculture and long-term grazing not only destroy complex and species-rich aboveground communities, they also destroy seed banks that might permit revegetation. A realistic goal of directed succession is to reestablish processes that accelerate the development of community complexity. Demographic projections, using estimates parameterized from field or garden studies, may offer a much better chance of predicting success of key species than the usual trial-and-error approach.

Reforestation of large areas disturbed by humans is often dispersal limited; most forest species are very slow to arrive. Dispersal may be encouraged within a matrix of disturbed and remnant communities.

Janzen argues that effective ecological restoration creates habitat buffers around ecological remnants and connects these habitats with corridors and stepping-stone patches, thereby increasing the movements of pollinators and dispersal agents on which plants depend Tewksbury et al.

In the tropics, a dispersal pattern that promotes reforestation may be established by encouraging seed dissemination where many species are strongly dispersal limited. Managers might place perches in fields to attract birds and their loads of seeds Miriti , Holl or plant cover of short-lived trees to encourage shade-tolerant tree seedlings and suppress competition from grasses Hooper et al.

The goal of these methods is to overcome dispersal limitation by promoting seed arrival through birds and plant survival though tree cover , thereby accelerating the growth of buffers and corridors and increasing their effectiveness. In these cases, demography could be a tool, but not a critical one.

A complementary approach is to actively establish a diverse matrix to encourage processes of dispersal by and for diverse assemblages of animals and plants Martinez-Garza and Howe Properties of the matrix between habitat patches determine both recruitment in ecological remnants and migration of remnant plants into the matrix.

One way to accelerate succession in tropical situations is to plant late-successional trees capable of handling the rigors of open pasture, but to choose species whose fruits will attract large-bodied mammals and birds Wunderle Small-seeded pioneers that are less dispersal limited will occupy such habitat anyway Ingle By creating food patches attractive to dispersal agents, animal-borne seeds of many species enrich community templates and ultimately change forest composition.

Tucker and Murphy provide an example in their study of reforestation in northern Australia. Trees bearing fleshy fruits planted among pioneers draw a variety of birds and mammals that appear to accelerate the succession of complex forest structure. A restoration scheme grounded in demographic thinking would revegetate agricultural land with late-successional species that have low variance in seedling-to-juvenile transitions, thereby permitting predictable recruitment of species that provide resources for fruit-eating, seed-dispersing animals.

Projections of populations in subsets of communities parameterized by different planting circumstances may permit predictions of successful and unsuccessful species decades before forests mature.

Unreliable seed dispersal has consequences for plants. Less directly, large fruit-eating animals are the first to disappear from small habitat fragments when continuous forests are subdivided into smaller patches Laurance and Bierregaard Subsistence hunting in the American tropics exterminates animals ranging from 1-kilogram kg agoutis Dasyprocta punctata to hefty kg tapirs Tapirus bairdii.

With no thinning of dominant plants by mammals, these aggressive species take over. In forests where defaunation threatens, demographic projections of the same species in hunted and unhunted areas may predict quite different forest communities 20 or 30 years into the future. Forest fragmentation occurs when continuous forests are divided into smaller patches of varying sizes and degrees of isolation; intervening water or cropland is a barrier for forest plants and animals.

Because the number of species is related to habitat area, one expects habitat fragments to lose species randomly as area declines. Where plants depend on dispersal agents, consequences of habitat fragmentation are anything but random. In the East Usambara Mountains of Tanzania, fragments have been isolated from forest by tea plantations for 60 to 80 years Cordeiro and Howe Primates and large fruit-eating birds disappear quickly from isolated forest fragments.

Seedlings and juveniles of 31 animal-dispersed tree species are three times more common in continuous forest and large forest fragments more than 30 ha than in small fragments less than 10 ha , whereas the recruitment of eight wind- and gravity-dispersed trees of the forest interior is unaffected.

Recruitment of 10 endemic, animal-dispersed tree species is 40 times lower in small fragments than in larger patches. A study of one endemic tree dispersed by birds, Leptonychia usambarensis Sterculiaceae , finds that the understory birds that commonly eat its fruits in continuous rain forest are rare or absent in small forest fragments Cordeiro and Howe Moreover, fewer seeds are taken, more seedlings accumulate under parents, and fewer seedlings or juveniles occur away from parents in small fragments than in rain forest.

Overall, seedling and juvenile recruitment near parents in small fragments is about half that in extensive forest. One might expect that an elasticity analysis would show lower elasticities for the seedling-to-juvenile transition in forest fragments, perhaps accounting for local attrition of this species from small habitat patches. In this tropical forest, dispersal matters. As in the defaunation example, projections of the same species in continuous forest and in forest fragments might help a manager gauge the consequences of loss of dispersal agents or other factors that affect tree recruitment.

At what distance, or in what local circumstances, do variances in survival decline, thereby indicating where seed dispersal is predictably effective? Spatial demography shows that average variations in seed and seedling transitions do not reflect the overall population dynamics of a species. Where long-lived adults saturate species-poor communities, the advantages of dispersal are subtle: Dispersal of the vast majority of seeds is demographically inconsequential in any given generation, although even these subtle impacts may matter over large enough scales of time and space.

In successional populations, or among long-lived trees in diverse forests, dispersal can save seeds and seedlings from intense competition or from foci of pathogen or insect infestation under parents or others of the same species. Here the impact of slightly biased survival of dispersed versus undispersed seeds is pervasive.

A challenge will be to discover how spatially contingent consequences of seed dispersal influence the fortunes of species in areas where human activities have reset successional clocks, hunted out dispersal agents, or fragmented once continuous habitats. In such circumstances, spatially contingent demography may be used to show where seed dispersal and seedling recruitment matter most in the past, in the present, and in the projected future.

Sullivan, Roman Yukilevich, Barbara Zorn-Arnold, and two anonymous reviewers for comments on the manuscript, and to Lana Gits for figure 3. Wind dispersal of artificial fruits varying in mass, area, and morphology. Beattie AJ. New York: Cambridge University Press. Bleher B Bohning-Gaese K. Consequences of frugivore diversity for seed dispersal, seedling establishment and the spatial pattern of seedlings and trees. Clark JS.

Reid's paradox of rapid plant migration. Connell JH. On the role of natural enemies in preventing competitive exclusion in some marine mammals and in rain forest trees. Dynamics of Populations. Waginengin The Netherlands : Pudoc. Cordeiro N Howe HF.

Low recruitment of trees dispersed by animals in African forest fragments. Conservation Biology. Forest fragmentation severs mutualism between seed dispersers and an endemic African tree. Proceedings of the National Academy of Sciences. Davidar P. Similarity between flowers and fruits in some flowerpecker-pollinated mistletoes.

Dirzo R Miranda A. Altered patterns of herbivory and diversity in the forest understory: A case study of the possible consequences of contemporary defaunation. New York: Wiley. Microsatellite analysis of seed dispersal and parentage of saplings in bur oak, Quercus macrocarpa. Molecular Ecology. Finegan B. Pattern and process in neotropical secondary rain forests: The first years of succession.

Trends in Ecology and Evolution. Forget P-M Sabatier D. Dynamics of the seedling shadow of a frugivore-dispersed tree species in French Guiana. Journal of Tropical Ecology. Foster RB. Tachigalia versicolor is a suicidal Neotropical tree. Effect of nurse plants on the microhabitat and growth of cacti. Journal of Ecology. Earthquake-caused landslides: A major disturbance to tropical forests. The role of seed dispersers in the population dynamics of the columnar cactus Neobuxbaumia tetezo.

Godoy JA Jordano P. Seed dispersal by animals: Exact identification of source trees with endocarp DNA microsatellites. Structure and floristics of secondary and old-growth forest stands in lowland Costa Rica. Birds often fly far away from the parent plant and disperse the seeds in their droppings. Plants like pittosporum have sticky seeds that can be carried away by birds. Humans can also spread seeds if they get stuck to our clothing or shoes — and if we throw fruit pips and stones out of the car window!

Some plants, like peas, gorse and flax, have seedpods that dry out once the seeds are ripe. When dry, the pods split open and the seeds scatter. Plants cannot run away from a fire so some plants have developed a way to help their seeds survive.

There are some species of pine tree that require the heat from a fire before their cones will open and release seeds. Banksias, eucalypts and other Australian plants also rely on fire. The intensity and timing of the fire is important. It needs to be hot enough to trigger the cones to open, but if fires are too frequent, there is not enough time for the plants to grow big enough to make new seeds. Adaptation is an evolutionary process that helps an organism make the most of its habitat.

Seed dispersal is an example of adaptation. Fires are common in Australia, so some plants have adapted and become well suited to make the most of it.