Dispersal and Immigration

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Dispersal and Immigration
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There are several fundamental processes in
biogeography Evolution Speciation Extinction Disp
ersal These are the processes by which organisms
respond to changes in the geographic template.
The relative importance of movement, or
dispersal, has been the subject of great debate.
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The early dispersalists included Darwin, Alfred
Russel Wallace and Asa Gray. They argued that
disjunctions (a situation in which two closely
related populations are separated by a wide
geographic distance) could be best explained as
the result of long distance dispersal.
Wallace
Darwin
Gray
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The dispersalists were opposed by the
extremists, who believed that disjunctions had
resulted from movement along ancient corridors
that had disappeared. Among the leaders of the
extensionist movement were Charles Lyell and
Joseph Hooker.
Lyell
Hooker
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No evidence was ever discovered for the lost
corridors proposed by the extensionists. However,
evidence surfaced in the 20th Century for a new,
powerful means of dispersal continental drift.
The movement of continents could raft populations
away from each other and separate them in
vicariant events (tectonic, climatic, or
oceanographic occurrences that isolate previously
connected populations). As a result, the debate
between dispersalists and extensionists has been
replaced by a debate between dispersalists and
vicariance biogeographers.
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What is dispersal? Simply, the movement of
organisms away from their birthplace. Often,
confined to a particular life history
stage. Dont confuse with dispersion, which
refers to the position of individual organisms
with respect to others in the population.
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Dispersal is an ecological process that plays an
adaptive role in the life history of the organism
involved. In other words, the fitness of the
organism is increased in some way through the
process of dispersal. Why? Theres always a
trade-off. Dispersing individuals probably face
reduced interspecific competition, but theres
always the chance of finding a less suitable
environment. Look at it this way. The parental
environment was obviously good enough to allow
the parent organisms to reproduce. Leaving that
environment is risky, but it must be worth the
risk.
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The role of dispersal in biogeography is
different. Biogeographers are interested in those
dispersal events in which species change their
range by dispersing over long distances. These
events are rare, and largely random. They are,
however, critical to understanding the
distribution of organisms.
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• Dispersal and Range Expansion
• In order to expand its range through dispersal,
  an organism must be able to
• Reach a new area.
• Survive the potentially harsh conditions
  occurring during the passage.
• Survive and reproduce in the new area to the
  extent that a new population is established.

• Biogeographers often distinguish three types of
  dispersal events that can accomplish this
• Jump dispersal (sweepstakes)
• Diffusion.
• Secular migration.

How do they operate?
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Jump dispersal is simply the colonziation of new
areas over long distance.
An example can be seen in the rapid
recolonization of Krakatau after all life was
wiped out by the volcanic explosion of 1883.
Read the account in your text.
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We can see the same thing over longer distances
and greater time periods for many other
archipelagoes. The Galapagos lie 800 km west of
Ecuador in the Pacific Ocean. The Hawaiian
Islands lie 4000 km west of Mexico. In both
cases, there is indisputable evidence of many
groups of organisms reaching the islands by
dispersal.
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Long-distance dispersal likely has a selective
component. Certain organisms, possessing certain
traits, are more likely to be successful. Bats
are often common island inhabitants. Nonvolant
(non-flying) mammals, amphibians, freshwater
fish, and other forms are typically absent from
island populations.
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• Long-distance dispersal offers three
  important consequences for biogeographers
• It offers a way to explain wide, and often
  discontinuous, distribution patterns.
• It helps to account for the similarities and
  differences among biotas inhabiting widely
  separated, but similar, habitats.
• It emphasizes the importance of anthropogenic
  (human-induced) long-distance transport of
  species.

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Diffusion is the gradual spread of of individuals
outward from the margins of a species range. It
is a slower form of range expansion involving not
just individuals, but populations.
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Many other examples of range expansions
include European starlings in North
America House sparrows in North America American
muskrat in Europe Nine-banded armadillo in North
America European rabbit in Australia Red fox in
Australia
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European starling Sturnus vulgaris
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House sparrow Passer domesticus
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American muskrat Ondatra zibethica
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Nine-banded armadillo Dasypus novemcinctus
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European rabbit Oryctolagus cuniculus
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Red fox Vulpes vulpes
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Imported fire ant Solenopsis invicta
There are many examples of range expansions among
insects, as well. Some of the most notable
include imported fire ant
Read the abstract of this paper
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Africanized honeybee Apis mellifera
and the Africanized killer honey bee
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We have also seen range expansion through
diffision in many plants. Examples include the
spread of Fertile Crescent crops across Western
Europe.
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the spread of oaks (Quercus spp. in Great
Britain..
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the spread of elm (Ulmus spp.) in Great
Britain,.
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and the spread of purple loosestrife (Lythrium
salicaria) in North America.
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Secular migration occurs much more slowly. So
slowly, in fact, that organisms can evolve during
the process.
An example can be seen in the evolutionary
divergence of the camel family during its spread
across the world following its origins in North
America.
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Organisms can disperse either actively or
passively. The terms vagility and pagility refer
to the ability of organisms to disperse actively
or passively. Some animals have the capacity to
disperse great distances by flight.
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Monarch butterflies migrate great distances,
flying from southern Canada to the southern U.S.
and central Mexico. Individuals may fly as far
as 375 km in four days and 4000 km during their
lifetimes. Most overwintering individuals of the
eastern populations gather in winter
congregations in Mexico. The northern limits
correspond with the northern extent of its host
plant, milkweed.
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North American plants use a variety of diaspores
to enable dispersal from the mother plant.
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Many insects, spiders, and mites disperse through
the atmosphere, forming an aerial plankton.
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The cnidarians Velella and Physalia have sails or
floats that allow them to drift across the
surface of the ocean. In both cases, the
orientation of the sail causes them to drift
either to the right or left. This may enable
them to remain within a restricted area.
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Many organisms employ other organisms for long
distance transport. This process is known as
phoresy. Parasites are a good example. Many
plants have seeds that adhere to the coats of
animals. Wading birds often carry seeds or eggs
in the mud on their feet. Seeds of fruits may be
carried in the digestive tracts of animals.
Cockle burr
Seeds in coyote scat
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Barriers The nature of long-distance dispersal
means that organisms often have to survive for
periods of time in environments that are hostile
to them. These environments constitute physical
and biological barriers to dispersal. The
effectiveness of such barriers in preventing
dispersal depends not only on the nature of the
barrier, but also on the organism dispersing.
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Barriers are species-specific phenomena. Most
freshwater zooplankton have resistant stages that
help facilitate long-distance dispersal. As a
result, the same species are found in widely
separated locations in North America, Europe, and
Asia. Fish, on the other hand, lack such
dispersal mechanisms. As a result, similar fish
species are only found in bodies of water that
have, at some point, been connected.
Ostracod
Walleye
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The sheepshead minnow (Cyprinodon variegatus) is
found in estuaries and mangrove swamps throughout
the Caribbean. It has been able to colonize
these habitats by dispersing many hundreds of
miles across ocean water. Its ability to
tolerate wide ranges of salinity makes this
possible.
The many species of sunfish (Lepomis spp.) of the
southeastern United States, lacking this
tolerance, were unable to disperse to Caribbean
islands.
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The vegetation zones of the American Southwest
have changed dramatically since the last glacial
maximum (18,000 years ago). Today, the cool,
moist, mountainous regions are isolated by large
distances of inhospitable desert. Small
terrestrial mammals, reptiles, and amphibians
probably colonized the isolated mountains of the
region during the Pleistocene when they were
largely connected.
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Dan Janzen made the point that mountain passes in
the tropics are effectively higher than those
in temperate regions since temperate organisms
must deal with a greater temperature range over
the course of the year.
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Physiological barriers are created by
environmental conditions which organisms (or
their propagules) are unable to survive long
enough for dispersal. Such barriers can be
presented by salt (or fresh) water or unfavorable
temperatures,
The bird family Alcidae (auks, puffins, and
murres) is restricted to cool areas of the
Northern Hemisphere even though they are strong
flyers. The tropics apparently represent a
strong physiological barrier.
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The nature of barriers may change with the
season. In temperaate regions of North America,
large bodies of water serve as barriers to the
movement of many terrestrial species. However,
during winter these waters may freeze and allow
movement across them. Many species of terrestrial
mammals move across the ice of the St. Lawrence
River in New York state.
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• Biotic Exchange and Dispersal Routes
• Biogeographers often distinguish three kinds of
  dispersal routes based on how they effect biotic
  interchange. They are
• Corridors.
• Filters.
• Sweepstakes routes.

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Corridors are dispersal routes that allow
movement of most taxa from one region to another.
They do not selectively discriminate against one
form, but rather allow a balanced assemblage of
plants and animals to cross them. The areas at
the two ends of a corridor should contain a
fairly similar assemblage of organisms.
The Bering Land Bridge which existed some 20,000
years ago likely functioned as a corridor which
allowed organisms to pass from northern Eurasia
to North America with very little selection of
the types that could pass. Conditions along the
corridor would have differed little from those on
either end.
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A filter is a dispersal route that exercises some
selection over the types of organisms that can
pass through it. As a result, the colonists are a
somewhat biased subsets of those that could
potentially pass. The Arabian subcontinent acts
as a filter in that only certain mammals,
reptiles and ground birds can disperse between
northern Africa and central Asia. The Lesser
Sunda Islands form a two-way filter for the
reptilian fauna of southeastern Asia and
Australia. This region is sometimes known as
Wallacea, and is bisected by Wallaces Line.
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The deserts of the American southwest may act as
a two-way filter between the Rocky Mountains and
the Sierra Madre to the south. The desert
separating the two mountain ranges have allowed
limited mixing of the two biotas. This mixing was
greatest during the glacial maximum, when the
forest regions were most greatly expanded and
desert climates reduced.
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Sweepstakes Routes Sweepstakes dispersal refers
to the crossing of barriers by rare, chance
events. Such events, while highly unlikely in
the short term, are likely, even probable, over
the long term. Your text tells of 15 green
iguanas that were rafted 200 miles across the
Caribbean as the result of Hurricanes Luis and
Marilyn in 1995. In essence, they got lucky.
They won the sweepstakes.
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Another example of a sweepstakes route is
revealed in the limits of the distributions of
eight different families of land snails in the
South Pacific. Each of the groups has its origin
in Southeast Asia and then spread, by
sweepstakes, southward and eastward.