User:Michael3468/Biological dispersal

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Biological dispersal refers to both the movement of individuals (animals, plants, fungi, bacteria, etc.) from their birth site to their breeding site ('natal dispersal'), as well as the movement from one breeding site to another ('breeding dispersal'). Dispersal is also used to describe 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, settlement and there are different fitness costs and benefits associated with each of these phases. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population genetics, and species distribution. Understanding dispersal and the 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. Furthermore, biological dispersal is impacted and limited by different environmental 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. Biological dispersal can be correlated to population density. The range of variations of a species' location determines expansion range.

Types of Dispersal

Some organisms are motile throughout their lives, but others are adapted to move or be moved at precise, limited phases of their life cycles. This is commonly called the dispersive phase of the life cycle. The strategies of organisms' entire life cycles often are predicated on the nature and circumstances of their dispersive phases.

In general, there are two basic types:

Passive Dispersal (Density-Independent Dispersal)

In passive dispersal, the 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 themselves. 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 current for dispersal.

Invertebrates, like sea sponges and corals, pass gametes through water. In this way are able to successfully reproduce because the sperm move around, while the eggs are moved by currents. Plants act in similar ways as they can also use water currents, winds, or moving animals to transport their gametes. Seeds, spores, and fruits can have certain adaptations that aid in facilitation of movement.

Active Dispersal (Density-Dependent Dispersal)

In active dispersal, an organism will move locations by its own inherit capabilities. Age is not a restriction, as location change is common in both young and adult animals. The extent of dispersion is dependent 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-dependence, as density of the community plays a major role in the movement of animals. However, the effect is observed in age groups differently, which results in diverse levels of dispersion.

When it comes to active dispersal, animals that are capable to free movement of large distances are ideal achieved through flying swimming, or walking. Nonetheless, there are restrictions enforced 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 in comparison to flying or swimming, especially when passing through adverse conditions.

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 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.

Urban Environments and Dispersal Range

Image shows how rivers can be used as dispersal vectors

Image depicts how development in urban areas can limit dispersal. Here it can specifically be seen that the development of highways leads to the splitting of natural lands, thus limiting dispersal in the two areas

Urban areas can be seen to have their own unique affects 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 as major dispersal vectors. Tunnels that connect rural and urban environments have been shown to expedite a large amount of and diverse set of seeds from urban to rural environments. This could lead to possible sources of invasive species on the urban-rural gradient. Another example of the affects of urbanization could 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 were 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 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 leads to a limited supply of pollination sites. Subsequently, this leads to less gene flow between distantly separated populations, in turn decreasing the genetic diversity of each of the areas. 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 species. This leads to genetic isolation of both populations, resulting in limited gene flow. While the urbanization did have a greater affect on mice dispersal, it also led to a slight increase in inbreeding among bat populations.

Consequences of Dispersal

Dispersal not only has costs and benefits to the dispersing individual (as mentioned above), it also has consequences at the level of the population and species on both ecological and evolutionary timescales. Organisms can be dispersed through multiple methods. Carrying through animals is especially effective as it allows traveling of far distances. Many plants depend on this to be able to go to new locations, preferably with conditions ideal for precreation and germination. With this, dispersal has major influence in the determination of population and spread of plant species.

Many populations have patchy spatial distributions where separate yet interacting sub-populations occupy discrete habitat patches (see metapopulations). Dispersing individuals move between different sub-populations which increases the overall connectivity of the metapopulation and can lower the risk of stochastic extinction. If a sub-population goes extinct by chance, it is more likely to be recolonized if the dispersal rate is high. Increased connectivity can also decrease the degree of local adaptation.

Human interference with the environment has been seen to have an effect on dispersal. Some of these occurrence have been accidents, like in the case of zebra mussels, which are indigenous to Southeast Russia. A ship had accidentally released them into the North American Great Lakes and they became a major nuisance in the area, as they began to clog water treatment and power plants. Another case of this was seen in Chinese bighead and silver carp, which were brought in with the purpose of algae control in many catfish ponds across the U.S. Unfortunately, some had managed to escape into the neighboring rivers of Mississippi, Missouri, Illinois, and Ohio, eventually causing a negative impact for the surrounding ecosystems. However, human created habitats such as urban environments have allowed certain migrated species to become urbanophiles or synanthropes.

Dispersal has caused changes to many species on a genetic level. A positive correlation has been seen for differentiation and diversification of certain species of spiders in the Canary Islands. These spiders were residing in archipelagos and islands. Dispersion was identifying as a key factor in the rate of both occurrences.

Dispersal Range

"Dispersal range" refers to the distance a species can move from an existing population or the parent organism. An ecosystem depends critically on the ability of individuals and populations to disperse from one habitat patch to another. The pattern of transportation can affect the range in which the organism expands. Organisms living in areas such as Reef, or any uneven large plane can experience this phenomena. Therefore, biological dispersal is critical to the stability of ecosystems.

To investigate dispersal range, landscape genetics can be used to study the population expansion of species. Such was used to investigate the dispersal route of Aedes albopictus.

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 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 the penguins' phenotypic plasticity. However, they are predicted to respond by dispersal, not adaptation this time. This is explained due to 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.

Observational Methods

Biological dispersal be can observed using different methods. In order to observe the variations of biological dispersal, scientist and researchers have used different methods to observe such phenomena. To study the effects of dispersal the method using Landscape genetics. this allows scientist to observe the difference between population variation and the size, shape and climate of the landscape. An example of using the landscape genetics as a means is study seed dispersal is by the effects of traffic using motorway tunnels with inner cities and suburbia area.

Genome wide SNP dataset and species distribution modelling are another method used to exam different dispersal modes. Genome wide SNP dataset was used to determine the genomic and demographic history within the range of collection or observation. Species distribution model is used when scientists need make a prediction when determining which region is best suited for the species of observation. Methods such as these are used to understand the criteria the environment provides when migration and settlement occurs such as the cases in biological invasion.

Human aided dispersal also known as Anthropogenic effect, is a contribution to biological dispersal ranges and variations. This is when humans manipulate the ecosystem, natural resources and biodiversity. This applied as an experimental method allows scientist to understand the movement of dispersal followed by their own ecological or environmental alterations. Changing the physical surrounds such as population density, road density, vegetation augmentation or fragmentation can be ways to observe dispersal properties. This can include studying the effects of urbanization landscape or closed habitats

Informed dispersal is a way to observe the cues of biological dispersal suggesting the reasoning behind the placement. This concept implies that the movement between species also involve information transfer. Methods such as GPS location is used to monitor the social cues and mobility of species regarding habitat selection. Consensus such as detailed trip records and point of interest (POI) data can be used to predict the movement of humans from rural to urban areas are examples of informed dispersal.

Direct tracking or visual tracking allows scientists to monitor the movement of seed dispersal by color coding them. Scientists and observers can track the migration of each variation. The pattern of transportation can then be visual to reflect the range in which the organism expands. Organisms living in areas such as Reef, or any uneven large plane can experience this phenomena.  

Human-Mediated Dispersal

Human impact has had a major influence on the movement of animals through time. An environmental response occurs in due to this, as dispersal patterns are important for species to survive major changes. There are two forms of human-mediated dispersal:

Human-Vectored Dispersal (HVD)

In Human-Vectored Dispersal, humans directly move the organism. This can occur deliberately, like for the usage of the animal in an agricultural setting, hunting or more. However, it can also occur accidentally, if the organism attaches itself to a person or vehicle. For this process, the organism first has to come in contact with a human and then movement can start. This has become more common has the human population all over the world has increased and movement through the world has also become more prevalent. Dispersal through a human can be many times more successful in distance compared to movement by wild or other environmental means.

Human-Altered Dispersal (HAD)

Human-Altered Dispersal signifies the effects that have occurred due to human interference with landscapes and animals. Many of these interferences have caused negative consequences in the environment. For example, many areas have suffered habitat loss, which in turn can have a negative effect on dispersal. Researchers have found that due to this, animals have been reported to move further distances in an attempt to find isolated places. This can especially be seen through construction of roads and other infrastructures in remote areas.

Long distance dispersals are observed when seeds are carried through human vectors. A study conducted to test the effects of human-mediated dispersal of seeds over long distances in two species of Brassica in England. The main methods of dispersal compared with movement by wind versus movement by attachment to outerwear. It was concluded that shoes were able to transport seeds to further distances than what would be achievable through wind alone. It was noted that some seeds were able to stay on the shoes for long periods of time, about 8 hours of walking, but evenly came off. Due to this, the seeds were able to travel far distances and settle into new areas, where they were previously not inhabiting. However, it is also important that the seeds land in places where they are able to stick and grow. Specific shoe size did not seem to have an effect on prevalence.