Lyell, E. Forbes, J. Hooker Vicariance. Ecological Adaptive Dispersal "why it happens" Movement to new, unused resources away from parents Within same habitat Across barrier to new habitat within range. Dispersal as a biogeographic Event Rare and infrequent, seldom observed Fundamental process of biogeography Common in the fossil record minimum age. Successful Dispersal depends on "long distance" transport withstanding unfavorable conditions during travel and upon early arrival establishing a viable population.
Dispersal and Range Expansion Jump Dispersal : long distances over inhospitable habitat example: oceanic islands mostly by volant organisms flight ex. Dispersal and Range Expansion Secular Migration : geological times scale Plants: diversification and spread of flowering plants Animals: spread of camels from North America to S. America and Asia. The male gametes are generally motile, and eggs are moved passively via ocean currents. Other sessile animals exemplify natal dispersal in that they have a free-living, aquatic juvenile stage, wherein larvae drift near the surface and are passively carried by water currents to other locations.
In plants, disseminules include seeds, spores, and fruits, all of which have modifications for movement away from the parent plant via available environmental kinetic energy. Distance traveled by a disseminule is a result of the velocity and direction of movement by the dispersal agent. Winds, flying animals, or water currents are some of the most successful agents of long-distance passive dispersal.
Seeds and fruits that have wings, hairs, or inflated processes are carried efficiently by wind. For example, modifications in Hypochaeris radicata Asteraceae seeds have allowed it to successfully disperse in a fragmented landscape in the Netherlands and counteract the negative effects of population isolation with substantial levels of gene flow Mix et al.
Furthermore, some plants have sticky or barbed seeds, or fruits, that adhere to the feathers or fur of mobile animals Figure 2. Some disseminules are explosively released over short distances whereas others fall to the ground at the base of the parent plant.
On the ground, invertebrates, mammals, and birds compete for fallen seeds and fruits. Seeds and fruits are scattered during feeding and after ingestion are distributed in feces. These seeds are adapted to resist digestive juices and, consequently, can pass through the digestive tract while remaining viable. The distance a disseminule travels by animal transportation, either via ingestion or attachment, is indefinite and depends on the dispersal behavior of their host.
For example, some animals may follow a nomadic or brief dispersal trajectory, resulting in variance in the distances traveled. Multiple processes influence juvenile and adult dispersal. Proximate causes vary but include local population conditions such as crowding and food availability. Environmental stochasticity e.
Individuals that emigrate as a result of environmental conditions may experience more favorable conditions in the new location. Additionally, climate change will impact dispersal. Since climate typically influences the distributions of species, the general warming trend that will occur as a result of global climate change will cause species' ranges to shift. As a result, many areas outside of current distributions may become climatically suitable.
However, these areas may be beyond the dispersal capacity of many species. Ultimate causes of dispersal can be explained by avoidance of inbreeding and inbreeding depression. Small, isolated populations can become inbred and result in decreased fitness, but dispersal can counteract these negative effects. Additionally, dispersal can reduce competition for resources and mates, thereby increasing individual fitness.
In some situations, these ultimate causes will result in sex-biased dispersal. For example, mammals typically exhibit male-biased dispersal, and birds typically exhibit female-biased dispersal. These dispersal strategies result mostly from males attempting to increase their access to females male-biased dispersal and in female-biased dispersal systems in birds from male resource defense female-biased dispersal in birds results Greenwood Despite the perceived benefits of dispersal, there can be costs.
First and foremost, there is a greater mortality risk during dispersal due to increased energy expenditure, unfamiliar habitat, or predation risk e. Second, dispersers may suffer reduced survival or reproductive success because of unfamiliarity with the new environment and the inability to acquire sufficient resources, resulting in decreased adaptive ability to the new habitat.
Dispersal affects organisms at individual, population, and species levels. Survival, growth, and reproduction at the level of individuals are intimately tied to both the distance and frequency of dispersal, factors which are typically mediated by aspects of local resource availability. At the population level, patterns of emigration and immigration within and among habitat patches associated with local population density, among other factors, drive temporal and spatial cycles of colonization and extinction.
The form of such movements, such as stepping-stone versus one-way migration, ultimately determines the genetic structure of populations, wherein genetic differentiation is directly proportional to the amount of gene flow among populations. For populations exhibiting frequent dispersal, ongoing gene flow within and among populations results in those populations becoming genetically similar to one another and ultimately evolving as a single unit.
Finally, over evolutionary time frames, a lack of dispersal among populations impacts organisms at the species level. If dispersal between populations ceases, these newly isolated populations accumulate novel genetic attributes via genetic drift or natural selection potentially leading to local adaptation.
Insurmountable landscape features, such as mountains and rivers, typically drive such processes, and in cases where genetic differentiation persists even after dispersal between formerly isolated populations could resume, such entities can then be designated as separate species Figure 3. Figure 3: Phylogenetic relationships of hypothetical populations that became isolated via dispersal Uppercase letters represent taxa, roman numerals represent geographic areas, black arrows represent dispersal events.
All rights reserved. Species exhibit geographic distributions that are constrained by a range of environmental variables — outside of which individuals may experience reduced survival and reproduction due to physical and physiological constraints. For example, species are often accustomed to particular temperature ranges, and dispersal to regions with temperatures outside those ranges reduces fitness.
Additionally, resources necessary for population persistence may be insufficient at range edges and outside the range. Physical barriers to dispersal consist of landscape features that prevent organisms from relocating.
Mountains, rivers, and lakes are examples of physical barriers that can limit a species' distribution. Anthropogenic barriers, like roads, farming, and river dams, also function as impediments to movement. It has been suggested that anthropogenic barriers are the most serious threats to dispersal. These barriers can effectively divide up a species' range into isolated fragments, and dispersal from one habitat patch to another can prove difficult.
Creating dispersal corridors has been suggested as a means to maintain connectivity between habitat patches. For example, Banff National Park in Alberta, Canada, contains 22 underpasses and 2 overpasses to facilitate wildlife dispersal within the park across a busy four-lane highway the Trans-Canada Highway.
Similarly, wildlife crossings, specifically designed for Florida panthers, were constructed along a forty-mile stretch of Interstate 75 in Florida. Corridors are not just for large mammals either: Salamanders have also benefited from miniature underpasses to facilitate dispersal. Additionally, recent research has focused on using modeling techniques to analyze available habitat to designate potential dispersal pathways for species whose ranges have been fragmented Figure 4.
Source populations in the West were as follows: A. Badlands, ND; B. Black Hills, SD; C. Kimble County, TX. Anabrus simplex with radio transmitters attached see Lorch et al. Direct methods can be somewhat easier to use in larger animals simply because tracking the smallest organisms e. However, tracking devices are becoming increasingly more advanced and useful in small organisms Figure 5.
Interpretation of results from direct measurement can sometimes prove difficult though. Low accuracy of spatial position, disproportionate mortality of marked individuals, labor intensity, and high costs are all deterrents to using direct measurement methods. In contrast to direct methods, indirect methods infer the degree of dispersal without actually having to observe the dispersal movement.
Typically, indirect methods involve utilizing molecular markers to measure gene flow and deduce dispersal patterns based on within and among population genetic differences.
Specifically, the differences in allele or genotype frequencies resulting from gene flow between populations reveal patterns and levels of dispersal. Indirect methods are increasingly being used to infer dispersal because of the difficulties involved with direct measurement. Human activities have facilitated and impeded dispersal in many ways. As stated previously, anthropogenic barriers in the form of human development have disrupted natural dispersal patterns in a variety of species.
Conversely, humans have also facilitated dispersal, both deliberately and accidentally. A common inadvertent way organisms have been dispersed is through their transport in the ballast water of ships.
Ships emptying ballast water may release foreign organisms. For example, zebra mussels, a freshwater mollusk native to the lakes of southeast Russia, were accidentally introduced into the Great Lakes of North America where they have caused major economic problems by clogging water treatment and power plants through ballast water discharge. As a result of the potential for introduction of non-native organisms via ballast water, new standards have been proposed for ballast tank cleaning.
Humans have also transported organisms to areas outside their native ranges for deliberate reasons. The seeds of attractive plants native to areas outside North America are routinely used in gardens and have the capacity to disperse to wild areas if conditions are suitable e. Also, bighead and silver carp originating from China were introduced to catfish farm ponds in the United States to control algal growth. Fish accidentally escaped from these ponds and have subsequently colonized the Mississippi, Missouri, Illinois and Ohio rivers where they have had significant negative impact on the native fauna Figure 6.
Dispersal is a common process undertaken by individuals at different stages of the life cycle and in response to various factors. Morphological adaptations make dispersal achievable but with varying degrees of success due to anthropogenic and natural barriers. These barriers modify the level of dispersal and consequently exert effects on population dynamics and genetic structure. As environments are altered, through stochastic events and global climate change, it will become increasingly important to assess how such changes will affect dispersal at the individual, population, and species levels.
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Density-dependent dispersal in birds and mammals. Ecography 28 , — doi: Penrod, K. Sorensen, A. Seed dispersal by adhesion. Annual Review of Ecology and Systematics 17 , — Sword, G. Insect behaviour: Migratory bands protect insects from predation. Nature , doi Wright, S. The genetical structure of populations. Annals of Eugenics 15 , — doi
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