Biological dispersal refers to a
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual
fitnessFitness is a central concept in evolutionary theory. It describes the capability of an individual of certain genotype to reproduce, and usually is equal to the proportion of the individual's genes in all the genes of the next generation...
, but also for
population dynamicsPopulation dynamics is the branch of life sciences that studies short- and long-term changes in the size and age composition of populations, and the biological and environmental processes influencing those changes...
,
population geneticsPopulation genetics is the study of the allele frequency distribution and change under the influence of the four evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes account of population subdivision and population structure in space. As such, it attempts...
, and
species distributionSpecies distribution is the manner in which a biological taxon is spatially arranged. Species distribution is not to be confused with dispersal, which is the movement of individuals away from their area of origin or from centers of high population density. A similar concept is the species range,...
. Understanding dispersal and the consequences on an ecosystem level requires understanding on the type of dispersal, the dispersal range of a given species, and the dispersal mechanisms involved.
Types of dispersal
In general there are two basic types of dispersal:
Density independent dispersal:Organisms have evolved adaptations for dispersal that take advantage of various forms of kinetic energy occurring naturally in the environment. This is referred to as density independent or passive dispersal and operates on many groups of organisms (some invertebrates,
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
, insects and
sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a rock, or the hull of a ship in the case of barnacles. Corals lay down their own substrate....
organisms such as plants) that depend on animal vectors, wind, gravity or current for dispersal.
Density dependent dispersal:Density dependent or active dispersal for many animals largely depends on factors such as local
populationIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings. Individuals within a population share a factor may be reduced by statistical means, but such a generalization may be too vague to imply anything...
size, resource
competitionCompetition is a contest between individuals, groups, nations, animals, etc. for territory, a niche, or allocation of resources. It arises whenever two or more parties strive for a goal which cannot be shared. Competition occurs naturally between living organisms which co-exist in the same...
,
habitatThe term habitat has a number of meanings:* Habitat , a place where a species lives and grows** Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
quality, and habitat size. Due to population density, dispersal may relieve pressure for resources in an ecosystem, and competition for these resources may be a selection factor for dispersal mechanisms.
Dispersal of organisms is a critical process for understanding both geographic isolation in evolution through
gene flowIn population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
and the broad patterns of current geographic distributions (
biogeographyBiogeography is the study of the distribution of biodiversity over space and time. It aims to reveal where organisms live, and at what abundance....
).
At some time during its life, an animal moves, or is moved, so that it does not die exactly where it was born. Such movement is called dispersal.
Dispersal range
"Dispersal range" refers to the distance a species can move from an existing population or the parent organism. An
ecosystemAn ecosystem is a system of interdependent organisms which share the same habitat, in an area functioning together with all of the physical factors of the environment. Ecosystems can be permanent or temporary. Ecosystems usually form a number of food webs...
depends critically on the ability of individuals and populations to disperse from one habitat patch to another. Therefore, biological dispersal is critical to the stability of ecosystems.
Environmental constraints
Few species are ever evenly or randomly distributed within or across landscapes. In general,
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
significantly vary across the landscape in association with environmental features that influence their reproductive success and population persistence. Spatial patterns in environmental features (e.g. resources) permit individuals to escape unfavorable conditions and seek out new locations. This allows the organism to "test" new environments for their suitability, provided they are within animal's geographic
range-In mathematics:* Range - Interval, the range between minimum and maximum* Range , the set of all output values produced by a function**Column space, the range space of a matrix...
. In addition, the ability of a species to disperse over a gradually changing environment could enable a population to survive extreme conditions. (i.e.
climate changeClimate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average...
).
Dispersal barriers
A dispersal barrier may mean that the dispersal range of a species is much smaller than the species distribution (e.g.
habitat fragmentationHabitat fragmentation is a process of environmental change important in evolution and conservation biology. As the name implies, it describes the emergence of discontinuities in an organism's preferred environment...
due to human land use). However, there are also natural barriers to dispersal that limit species distribution (e.g. mountain ranges and rivers).
On the other hand, human activities may also expand the dispersal range of a species by providing new dispersal routes (e.g. vehicles).
Dispersal mechanisms
Most (but not all)
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
s are capable of
locomotionAnimal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...
and the basic mechanism of dispersal is movement from one place to another. Locomotion allows the organism to "test" new environments for their suitability, provided they are within animal's range. Movements are usually guided by inherited
behaviorBehavior or behaviour refers to the actions or reactions of an object or organism, usually in relation to the environment. Behavior can be conscious or subconscious, overt or covert, and voluntary or involuntary....
s.
Plant dispersal mechanisms
Seed dispersalSeed 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...
is the movement or transport of
seedsSEEDS is a voluntary organisation registered under the Societies Act of India....
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
bioticBiotic means relating to, produced by, or caused by living organisms.The term biotic may also refer to:*Life, or ecosystem, the condition of living organisms,*Biology, the study of life*Biotic component in ecology,...
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
migrationMigration refers to directed, regular, or systematic movement of a group of objects, organisms, or people, including:In ecology and anthropology:* Human migration* Bird migration** Reverse migration, a phenomenon in bird migration...
patterns and
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
interactions. There are five main modes of seed dispersal: gravity, wind, ballistic, water and by animals.
Non-motile animals
There are numerous animal forms that are non-motile, such as
spongesSponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells that can transform into...
, bryozoans,
tunicateTunicates, also known as urochordates, are members of the subphylum Tunicata or Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata...
s,
sea anemoneSea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower...
s,
coralCorals are marine organisms from the class Anthozoa and exist as small sea anemone-like polyps, typically in colonies of many identical individuals...
s, and
oysterThe word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....
s. In common, they are all either
marineAn ocean is a large body of saline water, and a principal component of the hydrosphere. Approximately 75% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
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 dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
—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.
All of the marine and aquatic
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
s 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
cnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
.
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
zygoteA zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development...
develops quickly into a multicellula
planulaA planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. In all cases, the planula forms directly from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...
. 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
The majority of all animals are motile. Although motile animals can, in theory, disperse themselves by their spontaneous and independent locomotive powers, a great many species utilize the existing kinetic energies in the environment, resulting in passive movement. Dispersal by water currents is especially associated with the physically small inhabitants of marine waters known as
zooplanktonPlankton consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their phylogenetic or taxonomic classification...
. The term plankton comes from the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
, πλαγκτον, meaning "wanderer" or "drifter".
Dispersal by dormant stages
A lot of animal species, especially frashwater invertebrates, are able to disperse by wind or by transfere with an aid of larger animals (birds, mammals or fishes) as dormant eggs? dormant embrios or, in some cases, dormant adult stages. Tardigrades, some rotifers and some copepods are able to withstand dissecation as adult dormant stages. Many other taxa (
CladoceraCladocera or cladocerans are small crustaceans commonly called water fleas, part of the Class Branchiopoda. They form a monophyletic group, which is currently divided into four suborders, 14 families, 87 genera, and about 400 species...
,
BryozoaThe Bryozoa, also known as Ectoprocta, are a phylum of aquatic animals, typically about long, that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia. Most marine species live in tropical waters, but a few occur in oceanic trenches and...
,
HydraHydra may refer to:* Lernaean Hydra, a mythological many-headed serpent* Hydra, Saronic Islands, one of the Saronic Islands of Greece* Hydra , the largest of the modern star constellations* Hydra , a satellite of Pluto...
, Copepoda and so on) can disperse as dormant eggs or embryos. Freshwater sponges usually have special dormant propagules called gemmulae for such a dispersal. Many kinds of dispersal dormant stages are able to withstand not only desiication and low and high temperature, but also aт action of digestive enzymes during their transfere througn digestive tracts of birds and other animals, high concentracion of salts and many kinds of toxicants. Such dormant resistant stages made possible the long-distane dispersal from one water body to another and зкщмшву broad distribution ranges of many freshwater animals.
See also
- Competition (biology)
Competition can be defined as an Biological interaction structure. Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition...
- Disturbance
In ecology, a disturbance is a temporary change in average environmental conditions that causes a pronounced change in an ecosystem. Outside disturbance forces often act quickly and with great effect, sometimes resulting in the removal of large amounts of biomass...
- Dormancy
Dormancy is a period in an organism's life cycle when growth, development, and physical activity is temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be closely associated with environmental conditions...
('dispersal in time')
- Gene flow
In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
- Habitat fragmentation
Habitat fragmentation is a process of environmental change important in evolution and conservation biology. As the name implies, it describes the emergence of discontinuities in an organism's preferred environment...
- Island hopping
Island hopping is a term that has several different definitions as it is applied in various fields. Generally, the term refers to the means of crossing an ocean by a series of shorter journeys between islands, as opposed to a single journey directly across the ocean to the destination.-World War...
- Landscape ecology
Landscape ecology is the science of studying and improving the relationship between spatial pattern and ecological processes on a multitude of landscape scales and organizational levels. As a highly interdisciplinary enterprise, landscape ecology integrates biophysical and analytical approaches...
- Metapopulation
A metapopulation consists of a group of spatially separated populations of the same species which interact at some level. The term metapopulation was coined by Richard Levins in 1969 to describe a model of population dynamics of insect pests in agricultural fields, but the idea has been most...
- Phoresy
- Population modeling
Population modeling is an application of differential equations to the study of changes in populations.Models allow us to better understand how complex interactions and processes work. Modeling of dynamic interactions in nature can provide a manageable way of understanding how numbers change...
- Population distribution
- Population ecology
Population ecology is a major sub-field of ecology that deals with the dynamics of species populations and how these populations interact with the environment....
- Rafting event
Rafting events occur when organisms transfer from one land mass to another by way of a sea crossing on large clumps of floating vegetation. Such matted clumps of vegetation are often seen floating down major rivers in the tropics and washing out to sea, occasionally with animals trapped on...
- Species distribution
Species distribution is the manner in which a biological taxon is spatially arranged. Species distribution is not to be confused with dispersal, which is the movement of individuals away from their area of origin or from centers of high population density. A similar concept is the species range,...
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)
- Turner MG, Gardner RH, O’Neill. 2001. Landscape Ecology: In theory and practice, pattern and process. New York (NY): Springer Science.
- Groom MJ, Meffe GK, Carroll CR. 2005. Principles of Conservation Biology. Sunderland (MA): Sinauer Associates, Inc. 793 p.
- Hanski, I. (1999). Metapopulation Ecology, Oxford University Press, Oxford.
- Hanski, I. & Gilpin, M. E. (eds.) (1997). Metapopulation biology : ecology, genetics and evolution, Academic Press, San Diego.
- Levin, S.A. 1974. Dispersion and Population Interactions. The American Naturalist. 108:960, p.207.
- Clobert, J., E. Danchin , A.A. Dhondt , and J.D. Nichols (eds.)(2001) "dispersal", Oxford University Press, Oxford.
- Cousens, R., C. Dytham, R. Law (2008), Dispersal in plants: a population perspective, Oxford University Press, Oxford
External links