Biological dispersal refers to both the movement of individuals (animals, plants, fungi, bacteria, etc.) from their birth site to their breeding site ('natal dispersal') and the movement from one breeding site to another ('breeding dispersal'). 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 (or entire biotas) following the erosion of geographic barriers to dispersal or gene flow.
Dispersal can be distinguished from animal migration (typically round-trip seasonal movement), 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 (Density-Independent Dispersal) In passive dispersal, organisms cannot move on their own but use other methods to achieve successful reproduction or facilitation into new habitats. Organisms have evolved adaptations for dispersal that take advantage of various forms of kinetic energy occurring naturally in the environment. This can be done by taking advantage of water, wind, or an animal that is able to perform active dispersal itself. Some organisms are capable of movement while in their larval phase. This is common amongst some invertebrates, fish, insects and sessile organisms such as plants) that depend on animal vectors, wind, gravity or currents for dispersal. Some invertebrates, such as sea sponges and corals, release their gametes into the water. In this way, they can successfully reproduce: sperm move actively through the water, while eggs are carried by currents. Plants use similar strategies, relying on water currents, wind, or animals to transport their gametes. Seeds, spores, and fruits often possess specific adaptations that facilitate their movement. Active Dispersal (Density-Dependent Dispersal) In active dispersal, an organism moves between locations using its own inherent capabilities. Age is not a restriction, as changes in location are common in both young and adult animals. The extent of dispersion depends on multiple factors, such as local population, resource competition, habitat quality, and habitat size. Due to this, many consider active dispersal to also be density-dependent, as the density of the community plays a major role in the movement of animals. However, the effect is observed differently across age groups, which results in diverse levels of dispersion. When it comes to active dispersal, animals that are capable of free movement over large distances are ideal, typically moving through flying, swimming, or walking. Nonetheless, there are restrictions imposed by geographical location and habitat. Walking animals are at the biggest disadvantage when it comes to this, as they can be prone to being stopped by potential barriers. Although some terrestrial animals traveling by foot can travel great distances, walking uses more energy compared to flying or swimming, especially when passing through adverse conditions.
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 (biogeography).
A distinction is often made between natal dispersal, where an individual (often a juvenile) moves away from the place it was born, and breeding dispersal, where an individual (often an adult) 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 (e.g. wings). 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 (such as many birds and mammals) 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
Urban areas 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 (i.e. wind, water, etc.) 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 (ex. mice and bats) 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 (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 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. (i.e. climate change).
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 (e.g., ballast water from ships). 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