Biological dispersal
Biological dispersal refers to both the movement of individuals from their birth site to their breeding site and the movement from one breeding site to another. The term also encompasses the movement of propagules such as seeds and spores. Technically, dispersal is defined as any movement that has the potential to lead to gene flow. The act of dispersal involves three phases: departure, transfer, and settlement. Each phase is associated with distinct fitness costs and benefits. By simply moving from one habitat patch to another, an individual's dispersal can influence not only its own fitness but also broader processes such as population dynamics, population genetics, and species distribution. Understanding dispersal and its consequences, both for evolutionary strategies at a species level and for processes at an ecosystem level, requires understanding on the type of dispersal, the dispersal range of a given species, and the dispersal mechanisms involved. Biological dispersal can be correlated to population density. The range of variations of a species' location determines the expansion range.
Biological dispersal may be contrasted with geodispersal, which refers to the mixing of previously isolated populations following the erosion of geographic barriers to dispersal or gene flow.
Dispersal can be distinguished from animal migration, although within population genetics, the terms 'migration' and 'dispersal' are often used interchangeably.
Furthermore, biological dispersal is impacted and limited by different environmental and individual conditions. This leads to a wide range of consequences on the organisms present in the environment and their ability to adapt their dispersal methods to that environment.
Types of dispersal
Some organisms are motile throughout their lives, while others are adapted to move—or be moved—only during specific, limited phases of their life cycles. This stage is commonly referred to as the dispersive phase. The strategies underlying an organism's full life cycle are often shaped by the nature and conditions of this dispersive phase.In general, there are two basic types:
; Passive Dispersal
; Active Dispersal
Due to population density, dispersal may relieve pressure on 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 flow and the broad patterns of current geographic distributions.
A distinction is often made between natal dispersal, where an individual moves away from the place it was born, and breeding dispersal, where an individual moves away from one breeding location to breed elsewhere.
Costs and benefits
In the broadest sense, dispersal occurs when the fitness benefits of movement outweigh its costs.There are a number of benefits to dispersal, such as locating new resources, escaping unfavorable conditions, avoiding competition with siblings, and avoiding breeding with closely related individuals, which could lead to inbreeding depression.
There are also a number of costs associated with dispersal, which can be thought of in terms of four main currencies: energy, risk, time, and opportunity. Energetic costs include the extra energy required for movement as well as energetic investment in movement machinery. Risks include increased injury and mortality during dispersal and the possibility of settling in an unfavorable environment.
Time spent dispersing is time that often cannot be spent on other activities, such as growth and reproduction.
Finally, dispersal can also lead to outbreeding depression if an individual is better adapted to its natal environment than the one it ends up in. In social animals a dispersing individual must find and join a new group, which can lead to a loss of social rank.
Dispersal range
"Dispersal range" refers to the distance a species can move from an existing population or its parent organism. An ecosystem depends critically on the ability of individuals and populations to disperse from one habitat patch to another. Therefore, biological dispersal is critical for the stability of ecosystems.Urban Environments and Dispersal Range
s can be seen to have their own unique effects on the dispersal range and dispersal abilities of different organisms. For plant species, urban environments largely provide novel dispersal vectors. While animals and physical factors have played a role in dispersal for centuries, motor vehicles have recently been considered major dispersal vectors. Tunnels that connect rural and urban environments have been shown to expedite a large number and diverse set of seeds from urban to rural environments. This could lead to potential sources of invasive species on the urban-rural gradient. Another example of the effects of urbanization can be seen next to rivers. Urbanization has led to the introduction of different invasive species through direct planting or wind dispersal. In turn, rivers next to these invasive plant species have become vital dispersal vectors. Rivers could be seen to connect urban centers to rural and natural environments. Seeds from the invasive species have been shown to be transported by the rivers to natural areas located downstream, thus building upon the already established dispersal distance of the plant.In contrast, urban environments can also provide limitations for certain dispersal strategies. Human influence through urbanization greatly affects the layout of landscapes, which can leads to the limitation of dispersal strategies for many organisms. These changes have largely been exhibited through pollinator-flowering plant relationships. As the pollinator's optimal range of survival is limited, it results in a limited supply of pollination sites. Subsequently, this leads to less gene flow between distantly separated populations, thereby decreasing the genetic diversity of each area. Likewise, urbanization has been shown to impact the gene flow of distinctly different species in similar ways. While these two species may have different ecological niches and living strategies, urbanization limits the dispersal strategies of both populations. This leads to genetic isolation between the populations, resulting in limited gene flow. While urbanization did have a greater effect on mouse dispersal, it also led to a slight increase in inbreeding among bat populations.
Environmental constraints
Few species are ever evenly or randomly distributed within or across landscapes. In general, species significantly vary across the landscape in association with environmental features that influence their reproductive success and population persistence. Spatial patterns in environmental features 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 an animal's geographic range. In addition, the ability of a species to disperse over a gradually changing environment could enable a population to survive extreme conditions..As the climate changes, prey and predators have to adapt to survive. This poses a problem for many animals, for example, the Southern Rockhopper Penguins. These penguins are able to live and thrive in a variety of climates due to their phenotypic plasticity. However, they are predicted to respond by dispersal, not adaptation in this case. This is explained by their long life spans and slow microevolution. Penguins in the subantarctic have very different foraging behavior from those of subtropical waters; it would be very hard to survive by keeping up with the fast-changing climate because these behaviors took years to shape.
Dispersal barriers
A dispersal barrier may result in a dispersal range of a species much smaller than the species' distribution. An artificial example is habitat fragmentation due to human land use. By contrast, natural barriers to dispersal that limit species distribution include mountain ranges and rivers. An example is the separation of the ranges of the two species of chimpanzee by the Congo River.On the other hand, human activities may also expand the dispersal range of a species by providing new dispersal methods. Many such dispersed species become invasive, like rats or stinkbugs, but some species also have a mildly positive impact on human settlers like honeybees and earthworms.
Dispersal mechanisms
Most 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, provided they are within its range. Movements are usually guided by inherited behaviors.The formation of barriers to dispersal or gene flow between adjacent areas can isolate populations on either side of the emerging divide. Geographic separation and subsequent genetic isolation of portions of an ancestral population can result in allopatric speciation.
Plant dispersal mechanisms
is the movement or transport of seeds away from the parent plant. Plants are limited by vegetative reproduction 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 specific dispersal mechanism, and this has important implications for the demographic and genetic structure of plant populations, as well as their migration patterns and species interactions. There are five main modes of seed dispersal: gravity, wind, ballistic, water, and animals.Animal dispersal mechanisms
Non-motile animals
There are numerous animal forms that are non-motile, such as sponges, bryozoans, tunicates, sea anemones, corals, and oysters. What they share in common is that 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.All of the marine and aquatic invertebrates whose lives are spent fixed to the bottom 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. Broadcast spawning corals reproduce by releasing sperm and eggs directly into the water. These release events are coordinated by the lunar phase in certain warm months, such that all corals of one or many species on a given reef release gametes 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 polyp grows into a coral head by budding off new polyps to form a colony.