Dispersal and Immigration
There are several fundamental processes in biogeography:
Evolution
Speciation
Extinction
Dispersal
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.
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
Gray
Darwin
The dispersalists were
opposed by the
“extremists”, who
believed that
disjunctions had
resulted from movement
along ancient corridors
that had disappeared.
Lyell
Among the leaders of
the extensionist
movement were Charles
Lyell and Joseph
Hooker.
Hooker
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been replaced by a debate between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.
What is dispersal?
Simply, the movement of
organisms away from their
birthplace.
Often, confined to a
particular life history stage.
Don’t confuse with
dispersion, which refers to
the position of individual
organisms with respect to
others in the population.
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?
There’s always a trade-off. Dispersing individuals probably face
reduced interspecific competition, but there’s 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.
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.
Dispersal and Range Expansion
In order to expand its range through dispersal, an organism must be
able to:
1. Reach a new area.
2. Survive the potentially harsh conditions occurring during the
passage.
3. 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:
1. Jump dispersal (“sweepstakes”)
2. Diffusion.
3. Secular migration.
How do they operate?
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.
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.
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.
Long-distance dispersal offers three important consequences for
biogeographers:
1. It offers a way to explain wide, and often discontinuous, distribution
patterns.
2. It helps to account for the similarities and differences among biotas
inhabiting widely separated, but similar, habitats.
3. It emphasizes the importance of anthropogenic (human-induced)
long-distance transport of species.
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.
An example is
provided by the
cattle egret,
Bubuculus ibis.
Read your texts’
account of the
spread of the cattle
egret over the last
century.
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
European starling
Sturnus vulgaris
House sparrow
Passer domesticus
American muskrat
Ondatra zibethica
Nine-banded armadillo
Dasypus novemcinctus
European rabbit
Oryctolagus cuniculus
Red fox
Vulpes vulpes
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
“Africanized honeybee”
Apis mellifera
…and the
Africanized
“killer” honey bee
We have also seen
range expansion
through diffision in
many plants.
Examples include
the spread of “Fertile
Crescent” crops
across Western
Europe….
…the spread of oaks (Quercus
spp. in Great Britain…..
…the spread of elm (Ulmus spp.) in Great Britain,….
…and the spread of purple loosestrife (Lythrium salicaria) in North America.
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.
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.
Golden plovers breed in the Arctic and
winter in southern South America,
southern Asia, Australia, and the Pacific
islands. Migrating individuals regularly fly
nonstop from Alaska to Hawaii, a distance
of 4000 km.
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.
North American plants use a
variety of diaspores to
enable dispersal from the
mother plant.
Many insects, spiders,
and mites disperse
through the
atmosphere, forming
an aerial plankton.
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.
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.
Cockle burr
Seeds of fruits may be carried in the
digestive tracts of animals.
Seeds in
coyote scat
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.
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.
Ostracod
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.
Walleye
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. It’s 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.
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.
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.
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.
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.
Biotic Exchange and Dispersal Routes
Biogeographers often distinguish three kinds of dispersal
routes based on how they effect biotic interchange.
They are:
1. Corridors.
2. Filters.
3. Sweepstakes routes.
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.
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
Wallace’s Line.
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.
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”.
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.