Seed dispersal is the movement or transport of seeds away from the parent plant. Plants
have limited mobility and consequently rely upon a variety of dispersal vectors to transport
their propagules, including both abiotic and biotic vectors. Seeds can be dispersed away
from the parent plant individually or collectively, as well as dispersed in both space and time.
The patterns of seed dispersal are determined in large part by the dispersal mechanism and
this has important implications for the demographic and genetic structure of plant populations,
as well as migration patterns and species interactions. There are five main modes of
seed dispersal: gravity, wind, ballistic, water and by animals. Some plants are serotinous
and only disperse their seeds in response to an environmental stimulus. It can be measured
using seed traps.
Benefits Seed dispersal is likely to have several benefits
for plant species. First, seed survival is often higher away from the parent plant. This
higher survival may result from the actions of density-dependent seed and seedling predators
and pathogens, which often target the high concentrations of seeds beneath adults. Competition
with adult plants may also be lower when seeds are transported away from their parent.
Seed dispersal also allows plants to reach specific habitats that are favorable for survival,
a hypothesis known as directed dispersal. For example, Ocotea endresiana is a tree species
from Latin America which is dispersed by several species of birds, including the three-wattled
bellbird. Male bellbirds perch on dead trees in order to attract mates, and often defecate
seeds beneath these perches where the seeds have a high chance of survival because of
high light conditions and escape from fungal pathogens. In the case of fleshy-fruited plants,
seed-dispersal in animal guts often enhances the amount, the speed, and the asynchrony
of germination, which can have important plant benefits.
Seeds dispersed by ants are not only dispersed to short distances but are also buried underground
by the ants. These seeds can thus avoid adverse environmental effects such as fire or drought,
reach nutrient-rich microsites and survive longer than other seeds. These features are
peculiar to myrmecochory, which may thus provide additional benefits not present in other dispersal
modes. Finally, at another scale, seed dispersal
may allow plants to colonize vacant habitats and even new geographic regions.
Types Seed dispersal is sometimes split into 'autochory
and allochory. Autochory
Gravity Barochory or the plant use of gravity for
dispersal is a simple means of achieving seed dispersal. The effect of gravity on heavier
fruits causes them to fall from the plant when ripe. Fruits exhibiting this type of
dispersal include apples, coconuts and passionfruit and those with harder shells. Gravity dispersal
also allows for later transmission by water or animal.
Two other types of autochory are ballochory and herpochory.
Allochory Wind
Wind dispersal is one of the more primitive means of dispersal. Wind dispersal can take
on one of two primary forms: seeds can float on the breeze or alternatively, they can flutter
to the ground. The classic examples of these dispersal mechanisms include dandelions, which
have a feathery pappus attached to their seeds and can be dispersed long distances, and maples,
which have winged seeds and flutter to the ground. An important constraint on wind dispersal
is the need for abundant seed production to maximise the likelihood of a seed landing
in a site suitable for germination. There are also strong evolutionary constraints on
this dispersal mechanism. For instance, Cody and Overton found that species in the Asteraceae
on islands tended to have reduced dispersal capabilities relative to the same species
on the mainland. Reliance on wind dispersal is common among many weedy or ruderal species.
Unusual mechanisms of wind dispersal include tumbleweeds.
Water Many aquatic and some terrestrial plant species
use hydrochory, or seed dispersal through water. Seeds can travel for extremely long
distances, depending on the specific mode of water dispersal.
The water lily is an example of such a plant. Water lilies' flowers make a fruit that floats
in the water for a while and then drops down to the bottom to take root on the floor of
the pond. The seeds of palm trees can also be dispersed by water. If they grow near oceans,
the seeds can be transported by ocean currents over long distances, allowing the seeds to
be dispersed as far as other continents. Mangrove trees live right in the water. Their
seeds fall from the tree and grow roots as soon as they touch any kind of soil. During
low tide, they might fall in soil instead of water and start growing right where they
fell. If the water level is high, however, they can be carried far away from where they
fell. Mangrove trees often make little islands as dirt and other things collect in their
roots, making little bodies of land. A special review for oceanic waters hydrochory
can be seen at oceanic dispersal.
By animals
Animals can disperse plant seeds in several ways, all named zoochory. Seeds can be transported
on the outside of vertebrate animals, a process known as epizoochory. Plant species transported
externally by animals can have a variety of adaptations for dispersal, including adhesive
mucus, and a variety of hooks, spines and barbs. A typical example of an epizoochorous
plant is Trifolium angustifolium, a species of Old World clover which adheres to animal
fur by means of stiff hairs covering the seed. Epizoochorous plants tend to be herbaceous
plants, with many representative species in the families Apiaceae and Asteraceae. However,
epizoochory is a relatively rare dispersal syndrome for plants as a whole; the percentage
of plant species with seeds adapted for transport on the outside of animals is estimated to
be below 5%. Nevertheless, epizoochorous transport can be highly effective if seeds attach to
wide-ranging animals. This form of seed dispersal has been implicated in rapid plant migration
and the spread of invasive species. Seed dispersal via ingestion by vertebrate
animals, or endozoochory, is the dispersal mechanism for most tree species. Endozoochory
is generally a coevolved mutualistic relationship in which a plant surrounds seeds with an edible,
nutritious fruit as a good food for animals that consume it. Birds and mammals are the
most important seed dispersers, but a wide variety of other animals, including turtles
and fish, can transport viable seeds. The exact percentage of tree species dispersed
by endozoochory varies between habitats, but can range to over 90% in some tropical rainforests.
Seed dispersal by animals in tropical rainforests has received much attention, and this interaction
is considered an important force shaping the ecology and evolution of vertebrate and tree
populations. In the tropics, large animal seed dispersers may disperse large seeds with
few other seed dispersal agents. The extinction of these large frugivores from poaching and
habitat loss may have negative effects on the tree populations that depend on them for
seed dispersal. Seed dispersal by ants is a dispersal mechanism
of many shrubs of the southern hemisphere or understorey herbs of the northern hemisphere.
Seeds of myrmecochorous plants have a lipid-rich attachment called the elaiosome, which attracts
ants. Ants carry such seeds into their colonies, feed the elaiosome to their larvae and discard
the otherwise intact seed in an underground chamber. Myrmecochory is thus a coevolved
mutualistic relationship between plants and seed-disperser ants. Myrmecochory has independently
evolved at least 100 times in flowering plants and is estimated to be present in at least
11 000 species, but likely up to 23 000 or 9% of all species of flowering plants. Myrmecochorous
plants are most frequent in the fynbos vegetation of the Cape Floristic Region of South Africa,
the kwongan vegetation and other dry habitat types of Australia, dry forests and grasslands
of the Mediterranean region and northern temperate forests of western Eurasia and eastern North
America, where up to 30–40% of understorey herbs are myrmecochorous.
Seed predators, which include many rodents and some birds may also disperse seeds by
hoarding the seeds in hidden caches. The seeds in caches are usually well-protected from
other seed predators and if left uneaten will grow into new plants. In addition, rodents
may also disperse seeds via seed spitting due to the presence of secondary metabolites
in ripe fruits. Finally, seeds may be secondarily dispersed from seeds deposited by primary
animal dispersers. For example, dung beetles are known to disperse seeds from clumps of
feces in the process of collecting dung to feed their larvae.
Other types of zoochory are chiropterochory, malacochory or ornithochory.
By humans
Dispersal by humans used to be seen as a form of dispersal by animals. Recent research points
out that human dispersers differ from animal dispersers by a much higher mobility based
on the technical means of human transport. Dispersal by humans on the one hand may act
on large geographical scales and lead to invasive species. On the other hand, dispersal by humans
also acts on smaller, regional scales and drives the dynamics of existing biological
populations. Humans may disperse seeds by many various means and some surprisingly high
distances have been repeatedly measured. Examples are: dispersal on human clothes, on shoes
or by cars. Deliberate seed dispersal also occurs as seed
bombing. This has risks as unsuitable provenance may introduce genetically unsuitable plants
to new environments. Consequences
Seed dispersal has many consequences for the ecology and evolution of plants. Dispersal
is necessary for species migrations, and in recent times dispersal ability is an important
factor in whether or not a species transported to a new habitat by humans will become an
invasive species. Dispersal is also predicted to play a major role in the origin and maintenance
of species diversity. For example, myrmecochory increased the rate of diversification more
than twofold in plant groups in which it has evolved because myrmecochorous lineages contain
more than twice as many species as their non-myrmecochorous sister groups. Dispersal of seeds away from
the parent organism has a central role in two major theories for how biodiversity is
maintained in natural ecosystems, the Janzen-Connell hypothesis and recruitment limitation. Seed
dispersal is essential in allowing forest migration of flowering plants
See also
References
Further reading Ridley, Henry N. The Dispersal of Plants Throughout
the World. Ashford, Kent: L. Reeve & Co. ISBN 0-85393-004-X.
External links Fruit and seed dispersal images at bioimages.vanderbilt.edu
Interactive model of movement of plant species induced by climate change
Secondary metabolites promote seed dispersal