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Biological dispersal
refers to those processes by which
a
species maintains or expands
the distribution of a
population. Dispersal implies
movement—movement away from an
existing population (population
expansion) or away from the parent
organisms (population
maintenance). In the latter case,
dispersal may simply involve
replacement of the parent
generation by the new generation,
with only minor changes in
geographic area occupied. In
either case, dispersal is
important because new
life must replace old, and the
two
generations cannot easily
occupy the same physical space
during the transition. More
significantly, dispersal enables
the species population to occupy
much of the available
habitat, thereby maximizing
resources in its favor and
providing a hedge against local
adverse events.
In most cases, organisms
(plants and especially sedentary
animals) have evolved adaptations
for dispersal that take advantage
of various forms of kinetic energy
occurring naturally in the
environment: water flow, wind,
falling (response to gravity).
Dispersal in plants
Unlike animals,
plants are limited in their
ability to seek out favorable
conditions for life and growth.
Consequently, plants have evolved
many ways to disperse and
spread a population through their
seeds or
spores (see also
vegetative reproduction).
Those properties or attributes
that promote the movement of the
next generation away from the
parent plant may involve the
fruit more so than the seeds
themselves.
Dispersal is a universal
biological need, and it is to be
expected that most higher plants
have solved the problem in one way
or another through adaptations
involving their fruit or seed.
Examine the fruit of any species
and it is likely, with perhaps a
bit more knowledge about the
ecosystem, to at least
intelligently speculate on what
these adptations are in that
plant. However, realize that
particularly where plant-animal
interactions are central to the
dispersal mechanism, seeing a
plant outside of its native
ecosystem may not reveal so much
about the adaptations present in
the fruit and seed. Indeed, in
many instances of plants
introduced into areas where they
are not native, it is the failure
of the dispersal mechanism that
accounts for the species not
becoming established beyond the
garden.
Gravity
The effect of
gravity on the dispersal of
seeds and spores is straight
forward. Heavier seeds will tend
to drop downward from the parent
plant, and not by themselves
travel very far. Spores, being
much lighter, are more easily
impacted by motions in the
environment, especially those
caused by
wind and
water, and therefore less
strictly subject to the law of
gravity (see examples below).
Gravity may be sufficient agent
for plants growing on steep
slopes, but upslope movement of a
population can be a problem. The
naked seeds of
gymnosperms are largely
dependent upon gravity for
dispersal. Most extant
conifers are long-lived large
shrubs or tall trees, thus taking
full advantage of gravitational
dispersal and allowing for gradual
upslope movement of a population.
Dispersal of seeds "strictly" by
gravity should not overlook storm
effects: seeds from a
deteriorating
cone placed high on a tall,
narrow tree will get spread widely
during a wind storm (see "Wind"
below).
Encasing seeds in a rounded
fruit promotes gravity driven
movement away from the parent.
Mechanical dispersal
Numerous species have evolved
mechanical means to overcome the
tendency of a seed to drop close
to its parent. Seedpods are often
shaped so that the seeds are flung
away from the parent plant with
considerable force as the seedpod
matures.
Examples of fruit with
mechanical dispersal mechanisms:
Dandelion "clock,"
showing brown achenes and
attached pappuses
Wind
For non-aquatic, terrestrial
plants, the wind is an obvious
supplier of energy of movement,
and many plant adaptations exist
that clearly take advantage of
this fact. Perhaps most familiar
are the feather-light fibre
parachutes with attached achenes
that are produced by a number of
species of
Asteraceae, a well-known
example being the
dandelion (see right).
Water
Plants that grow in water
(aquatic and obligate wetland
species) are likely to utilize
water to disperse their seeds. For
example, all
mangroves disperse their
offspring by water.
Rhizophora demonstrates an
unorthodox method of propagation
called vivipary: the
embryo is retained on the plant
until after
germination; in essence, a dry
seed is not produced. The
hypocotyl of the germinating
seedling (now called a
propagule) bursts through
the fruit and hangs, poised for
continued growth. In R. mangle,
the hypocotyl can reach a length
of 20 to 25 cm; and in R.
mucronata lengths up to 1 m
have been recorded. Eventually,
the seedling separates from the
fruit, leaving its
cotyledons behind,
and—floating horizontally on the
water surface—is carried away by
tidal or river flow. After a month
or two, the propagule turns
vertical in the water. Once the
hypocotyl of a propagule "feels"
bottom or strands, roots start to
develop and leaves appear at the
upper end (Hogarth, 1999).
Adaptations commonly seen in
littoral plants are those that
promote floatation of the fruit,
allowing the seed to be carried
away on the tide or ocean
currents. Examples would be:
- Cocos nucifera – the
coconut produces a large,
dry, fiber-filled fruit (a
fibrous
drupe) capable of a long
survival adrift at sea.
- Calophyllum inophyllum
– Alexandrian laurel or
kamani produces a globose
fruit that is almost cork-like.
Animals
The co-evolution of plants and
animals is a fascinating story in
itself. And a very significant
aspect of this co-evolution
involves plant adaptations that
take advantage of animal abilities
to locomote. Some fruit have
prickly burrs or spikes that
attach themselves to a passing
animal's fur or feathers so that
the animal will carry them away.
Seeds are contained within a soft
fruit that "invites" animals to
consume it. These seeds have a
tough protective outer-coating so
that while the fruit is digested,
the seeds will pass through their
host's digestive tract intact, and
grow wherever they fall. Such
fruit attractive to birds is
perhaps the most successful of
fruit adaptations related to plant
dispersal.
Examples of fruits with
attachment hairs and structures:
- Bidens spp. – Many
species of this
beggartick genus produce
achenes with awns that are
barbed.
Dispersal in animals
Most (but not all)
animals are capable of
locomotion and the basic
mechanism of dispersal is movement
from one place to another.
Locomotion allows the organism to
"test" new environments for their
suitability, although movement is
usually guided by inherited
behaviors.
Non-motile animals
There are numerous animal forms
that are non-motile, such as
sponges,
bryozoans,
tunicates,
sea anemones,
corals, and
oysters. In common, they are
all either
marine or
aquatic. It may seem curious
that plants have been so
successful at stationary life on
land, while animals have not, but
the answer lies in the food
supply. Plants produce their own
food from sunlight and
carbon dioxide—both generally
more abundant on land than in
water. Animals fixed in place must
rely on the surrounding medium to
bring food at least close enough
to grab, and this occurs in the
three-dimensional water
environment, but with much less
abundance in the atmosphere.
However, that such a life form
might be possible is at least
suggested by the
orb-weaver spiders.
All of the marine and aquatic
invertebrates whose lives are
spent fixed to the bottom (more or
less; anemones are capable of
getting up and moving to a new
location if conditions warrant)
produce dispersal units. These may
be specialized "buds", or motile
sexual reproduction products, or
even a sort of alteration of
generations as in certain
cnidaria.
Corals provide a good example
of how sedentary species achieve
dispersion. Corals reproduce by
releasing sperm and eggs directly
into the water. These release
events are coordinated by lunar
phase in certain warms months,
such that all corals of one or
many species on a given reef will
release on the same single or
several consecutive nights. The
released eggs are fertilized, and
the resulting
zygote develops quickly into a
multicellular
planula. This motile
stage then attempts to find a
suitable substratum for
settlement. Most are unsuccessful
and die or are fed upon by
zooplankton and bottom dwelling
predators such as anemones and
other corals. However, untold
millions are produced, and a few
do succeed in locating spots of
bare limestone, where they settle
and transform by growth into a polyp. All things being
favorable, the single poplyp grows
into a coral head by budding off
new polyps to form a colony.
Motile animals
Although motile animals can, in
theory, disperse themselves by
their locomotive powers, a great
many species utilize the existing
kinetic energies in the
environment. Dispersal by water
currents is especially associated
with the physically small
inhabitants of marine waters known
as
zooplankton. The term plankton
comes from the
Greek, πλαγκτoν, meaning
"wanderer" or "drifter".
Reference
- Hogarth, Peter J. 1999.
The Biology of Mangroves.
Oxford Univ. Press. 228 p.
ISBN 0198502222
