Biological dispersal refers to those processes by which a species maintains or expands the distribution of a population. In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. In Biology a population is the collection of inter-breeding organisms of a particular Species; in Sociology Dispersal implies movement—movement away from an existing population (population expansion) or away from the parent organism (population maintenance). This is necessary for members of a population because organisms of the same age require all of the same resources within an ecosystem. Dispersal relieves pressure for resources in an ecosystem, and competition for these resources may be a selection factor for dispersal mechanisms. Natural selection is the process by which favorable Heritable traits become more common in successive Generations of a Population of [1]
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. More significantly, dispersal enables the species population to occupy much of the available habitat, thereby maximizing resources in its favor and providing an edge against local adverse events. A habitat (which is Latin for "it inhabits" is an Ecological or environmental area that is inhabited by a particular Species.
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 of organisms is a critical process for understanding both geographic isolation in evolution and the broad patterns of current geographic distributions.
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Of plants
Unlike animals, plants are limited in their ability to seek out favorable conditions for life and growth. Plants are living Organisms belonging to the kingdom Plantae. Consequently, plants have evolved many ways to disperse and spread a population through their seeds or spores (see also vegetative reproduction). A seed (in some plants referred to as a kernel) is a small embryonic Plant enclosed in a covering called the seed coat usually with some stored In Biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions Vegetative reproduction is a type of Asexual reproduction found in plants and is also called vegetative propagation or vegetative multiplication. 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. The term fruit has different meanings dependent on context and the term is not synonymous in Food preparation and Biology.
Gravity
The effect of gravity on the dispersal of seeds and spores is simple: heavy seeds drop downward from the parent plant, though not very far by themselves. Gravitation is a natural Phenomenon by which objects with Mass attract one another Encasing seeds in a rounded fruit promotes gravity driven movement away from the parent. Spores, being much lighter, are more influenced by physical movements in the environment, especially those of wind and water, and therefore less strictly subject to gravity alone. Wind is the flow of Air or other Gases that compose an Atmosphere (including but not limited to the Earth's) Water is a common Chemical substance that is essential for the survival of all known forms of Life. Gravity may be sufficient agent for plants growing on steep slopes, but upslope movement of a population can be a problem. Plants such as gymnosperms that utilize gravity for dispersal often rely on additional dispersal adaptations favoring zoochory or anemochory; few rely on gravity alone. Gymnosperm (Gymnospermae are a group of Spermatophyte seed-bearing Plants with Ovules on the edge or blade of an open Sporophyll, which are
Mechanical
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:
- Oxalis corniculata – capsule, as it dries, becomes sensitive to disturbance, ejecting tiny seeds in an explosive discharge. Description Creeping Woodsorrel ( Oxalis corniculata) to be confused with '''[[Oxalis stricta|Yellow Woodsorrel], or Oxalis stricta In Botany a capsule is a type of simple dry Fruit produced by many species of Flowering plants A capsule is a dehiscent structure composed of two
Wind
Anemochory, or wind dispersal, is probably the most primitive form of dispersal besides gravity alone. Cardamine impatiens or Narrow-leaved Bittercress is a plant species of the genus Cardamine belonging tio Brassicaceae A legume is a Plant in the family Fabaceae (or Leguminosae or a Fruit of these specific plants Biological dispersal refers to those processes by which a Species maintains ecosystem Wind is reliable, but plants must produce seeds to ensure that a sufficient number will land by chance in a suitable location, making anemochory an inefficient means of dispersal. Wind dispersal is common among ruderal species, including some important agricultural and horticultural weeds such as hawkweed, horseweed, and dandelion. Like many members of the Asteraceae, or sunflower family, they produce seeds with a feathery "parachute" called a pappus that aids wind dispersal,allowing them to be carried over distances
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. Mangroves (generally are Trees and Shrubs that grow in saline coastal habitats in the Tropics and Subtropics. Hydrochory is the dispersal of seeds by water; a plant which uses this method in its life cycle is termed a hydrochore. 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. Rhizophoraceae is a family constituted by Tropical or Subtropical Flowering plants Among the better known members are Mangrove A viviparous Animal is an animal employing vivipary: the embryo develops inside the body of the mother as opposed to outside in an egg ( Ovipary Germination is the process whereby growth emerges from a period of dormancy The hypocotyl of the germinating seedling (now called a propagule) bursts through the fruit and hangs, poised for continued growth. Hypocotyl is a botanical term for a part of a germinating seedling of a Seed Plant. 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. A cotyledon ("seed leaf" from Greek: κοτυληδών kotylēdōn, from κοτύλη ''kotýlē'', "cup bowl" 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 flotation of the fruit, allowing the seed to be carried away on the tide or ocean currents. Littoral refers to the coast of an ocean or sea or to the banks of a river lake or estuary An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. 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. The Coconut Palm ( Cocos nucifera) is a member of the Family Arecaceae (palm family In Botany, a drupe is a Fruit in which an outer fleshy part ( Exocarp, or skin and Mesocarp, or flesh surrounds a shell (the pit
- Calophyllum inophyllum – Alexandrian laurel or kamani produces a globose fruit that is almost cork-like.
Terrestrial plants may also have their seeds dispersed by raindrops. Rain is Liquid precipitation. On Earth it is the condensation of atmospheric Water vapor into drops heavy enough to fall often making it to
By animals
Many plants rely on zoochory, dispersal by animals. Animal dispersal is broadly divided into two categories: endozoochory, transport internally, and epizoochory, external transport. Endozoochory is generally a coevolved mutualistic relationship in which a plant surrounds seeds with an edible, nutritious fruit as a reward to animals that consume it. A frugivore is an Animal that feeds primarily or in some cases exclusively on Fruit. Seeds within the fruit are often protected by tough outer coating; many seeds have such thick protection that they cannot germinate until they are scarified by digestion. Scarifying involves scratching etching or some sort of superficial cutting or incision This keeps the seed from sprouting while still in the fruit and reduces competition with the parent plant. Birds and mammals are the most important seed dispersers, but a wide variety of other animals, such as box turtles[3][4] and fish[5] can transport viable seeds. Plants with red berries dispersed by birds are one of the most widespread and successful examples of this mutualism. The word berry has two meanings one based on a botanical definition the other on common identification Some animals that disperse may also eat the seed. Seed predation includes any process inflicted on a Plant ’s seeds by an animal that results in the inviability of the Seed. Many rodents (such as squirrels) hoard seeds in hidden caches; those left uneaten can grow into a new plant. A squirrel is one of the many small or medium-sized Rodents in the family Sciuridae. Hoarding is the storing of food or other goods or money Hoarding of food is a natural behaviour in certain species of animals
Epizoochory, or transport on the outside of an animal, is often a commensalistic relationship in which the plant benefits but the animal does not. In Ecology, commensalism is a kind of symbiotic relationship between two organisms where one benefits and the other is not significantly harmed or helped (like a bird Plants such as burdock and cocklebur have fruits with recurving hairs or spikes that cling to fur or feathers so that passing animals will carry them away. Burdock is any of a group of biennial Thistles in the Genus Arctium, family Asteraceae Cockleburs ( Xanthium) are a Genus of Flowering plants in the family Asteraceae, native to the Americas and eastern Many species in the genus Bidens are called "beggar's ticks" because their achenes stick in clothes using special barbed awns. Bidens is a Genus in the family Asteraceae. It contains about 200 Species. An achene is a type of simple dry Fruit produced by many species of Flowering plants Achenes are "monocarpellate" (formed from one Carpel Many members of the Apiaceae (carrot family) such as Torilis, Caucalis, and Daucus have spikes or spiky hairs on their fruits.
Of animals
Most (but not all) animals are capable of locomotion and the basic mechanism of dispersal is movement from one place to another. In Biomechanics, animal locomotion is the study of how Animals move. Locomotion allows the organism to "test" new environments for their suitability, provided they are within animal's range. Movements are usually guided by inherited behaviors. Behavior or behaviour (see spelling differences) refers to the actions or Reactions of an object or Organism, usually
Non-motile animals
There are numerous animal forms that are non-motile, such as sponges, bryozoans, tunicates, sea anemones, corals, and oysters. The sponges or poriferans (from Latin porus "pore" and ferre "to bear" are Animals Bryozoans are tiny colonial Animals that generally build stony Skeletons of Calcium carbonate, superficially similar to Coral (although some Tunicate, also known as urochordata, tunicata (and by the common names of urochordates, sea squirts, and sea pork) is the Sea anemones are a group of water dwelling predatory animals of the order Actiniaria; they are named after the Anemone, a terrestrial Flower Corals are Marine organisms from the class Anthozoa and exist as small Sea anemone –like Polyps typically in colonies of many The common name oyster is used for a number of different groups of Bivalve Mollusks most of which live in marine habitats or Brackish water. In common, they are all either marine or aquatic. An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. 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. Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single 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. The orb-weaver spiders (family Araneidae) are the builders of spiral wheel-shaped webs often found in gardens fields and forests
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. An invertebrate is an Animal lacking a Vertebral column. The group includes 98% of all animal Species — all animals except those in the Chordate These may be specialized "buds", or motile sexual reproduction products, or even a sort of alteration of generations as in certain cnidaria. Cnidaria (naɪˈdɛəriə is a phylum containing some 9000 Species of Animals found exclusively in aquatic mostly marine, environments
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 warm 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. For other meanings see Zygote (disambiguation. A zygote (from Greek ζυγωτός zugōtos "joined" or "yoked" A planula is the free-swimming flattened ciliated, bilaterally symmetric Larva of a Jellyfish, a Hydrozoan Cnidarian or 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 polyp 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. Plankton consist of any drifting Organisms ( Animals Plants Archaea, or Bacteria) that inhabit the Pelagic zone of The term plankton comes from the Greek, πλαγκτον, meaning "wanderer" or "drifter". Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly
See also
References
- ^ Irwini, AJ; PD Taylor (2000). Dormancy is a period in an organism's life cycle when growth development and (in animals physical activity is temporarily suspended In Population genetics, gene flow (also known as gene migration) is the transfer of Alleles of Genes from one Population to another "Evolution of Dispersal in a Stepping-Stone Population with Overlapping Generations". Theoretical Population Biology 58: 321�328. Academic Press.
- ^ http://andromeda.cavehill.uwi.edu/Dispersal.htm#Mechanical%20Dispersal
- ^ Seed Production and Seedling Establishment in the Mayapple, Podophyllum peltatum L. , by R. W. Rust and R. R. Roth. 1909. The American Midland Naturalist, University of Notre Dame.
- ^ Hong Liu, Steven G. Platt, and Christopher K. Borg. 2004. Seed dispersal by the Florida box turtle (Terrapene carolina bauri) in pine rockland forests of the lower Florida Keys, United States. Oecologia Volume 138, Number 4
- ^ POLLUX, B. J. A. ; OUBORG, N. J. ; VAN GROENENDAEL, J. M. ; KLAASSEN, M. 2007. Consequences of intraspecific seed-size variation in Sparganium emersum for dispersal by fish. Functional Ecology, Volume 21, Number 6
- Hogarth, Peter J. 1999. The Biology of Mangroves. Oxford Univ. Press. 228 p. ISBN 0-19-850222-2
Further reading
- Ingold, C. T. (1971) Fungal spores: their liberation and dispersal Oxford, Clarendon Press 302 p. ISBN 0198541155
- Lidicker, W. Z. and R. L. Caldwell (1982) Dispersal and migration Stroudsburg, Pa. : Hutchinson Ross Pub. Co 311 p. ISBN 0879334355 (Dispersal of animals)
- Bullock, J. M. ; R. E. Kenward, and R. S. Hails (editors) (2002) Dispersal ecology : the 42nd symposium of the British Ecological Society. Oxford, UK : Blackwell Science 458 p. ISBN 0632058765 (Animals and plants)
External links
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