Biological dispersal
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.[1] The act of dispersal involves three phases: departure, transfer, and settlement. There are different fitness costs and benefits associated with each of these phases.[2] 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.[3][4][5] 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. Biological dispersal can be correlated to population density. The range of variations of a species' location determines the expansion range.[6]
This article is about biological dispersal in ecosystems. For other forms of dispersion, see Dispersion (disambiguation).
Biological dispersal may be contrasted with geodispersal, which is the mixing of previously isolated populations (or whole biotas) following the erosion of geographic barriers to dispersal or gene flow.[7][8][9]
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.[10] 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.
Quantifying dispersal[edit]
Dispersal is most commonly quantified either in terms of rate or distance.
Dispersal rate (also called migration rate in the population genetics literature) or probability describes the probability that any individual leaves an area or, equivalently, the expected proportion of individual to leave an area.
The dispersal distance is usually described by a dispersal kernel which gives the probability distribution of the distance traveled by any individual. A number of different functions are used for dispersal kernels in theoretical models of dispersal including the negative exponential distribution,[24] extended negative exponential distribution,[24] normal distribution,[24] exponential power distribution,[25] inverse power distribution,[24] and the two-sided power distribution.[26] The inverse power distribution and distributions with 'fat tails' representing long-distance dispersal events (called leptokurtic distributions) are thought to best match empirical dispersal data.[24][27]
Consequences of dispersal[edit]
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.[28]
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 occurrences 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.[11] However, human-created habitats such as urban environments have allowed certain migrated species to become urbanophiles or synanthropes. [29]
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. [30]
Dispersal observation methods[edit]
Biological dispersal can be observed using different methods. To study the effects of dispersal, observers use the methods of landscape genetics.[33] This allows scientists to observe the difference between population variation, climate and well as the size and shape of the landscape. An example of the use of landscape genetics as a means to study seed dispersal, for example, involves studying the effects of traffic using motorway tunnels between inner cities and suburban area.[34]
Genome wide SNP dataset and species distribution modelling are examples of computational methods used to examine different dispersal modes.[33] A genome-wide SNP dataset can be used to determine the genomic and demographic history within the range of collection or observation [Reference needed]. Species distribution models are used when scientists wish to determine which region is best suited for the species under observation [Reference needed]. 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, an example of an anthropogenic effect, can contribute to biological dispersal ranges and variations.[35]
Informed dispersal is a way to observe the cues of biological dispersal suggesting the reasoning behind the placement.[36] This concept implies that the movement between species also involve information transfer. Methods such as GPS location are used to monitor the social cues and mobility of species regarding habitat selection.[37] GPS radio-collars can be used when collecting data on social animals such a meerkats.[38] Consensus data 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 [Reference needed].
Direct tracking or visual tracking allows scientists to monitor the movement of seed dispersal by color coding.[14] Scientists and observers can track the migration of individuals through the landscape. The pattern of transportation can then be visualized to reflect the range in which the organism expands.