3. Dispersal ali.pptx

1. DISPERSAL
Dr
. Muhammad Babar Khawar

2. There are several fundamental processes in biogeography:
Evolution is change in the heritable characteristics of biological populations
over successive generations. These characteristics are the expressions of
genes that are passed on from parent to offspring during reproduction.
Speciation is the evolutionary process by which populations evolve to
become distinct species.
Extinction is the termination of a kind of organism or of a group of kinds
(taxon), usually a species. The moment of extinction is generally considered
to be the death of the last individual of the species, although the capacity to
breed and recover may have been lost before this point.
Biological dispersal refers to both the movement of individuals from their
birth site to their breeding site, as well as the movement from one breeding
site to another.
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.

3. 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

4. The dispersalists were opposed
by the “extensionists”, 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

5. 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).

6. 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.

7. DISPERSAL
• The transport of animals to geographical
areas not currently inhabited by that
species.
• Simplest explanation as to why a particular species may
not be located in a geographical area.
• If colonization is successful, dispersal will
result in gene flow and thus affect the
genetic structure of a population.
• Also result in a founder effect.

8. 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.

9. 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.

10. THREE MODES OF DISPERSAL
• Jump Dispersal –Movement of individual
organisms across large distances of
inhospitable habitat followed by the successful
establishment of a population in the new area.
• Diffusion –Gradual movement of a population
across a hospitable terrain (specific physical
features of an area of land) for a period of
several generations.
• Secular Dispersal –Diffusion occurring in
evolutionary time.

11. Jump dispersal
Simply the colonization 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.

12. GROUPS COLONIZING KRAKATAU
Birds dependent on
plant colonization.
Most plants and
animals probably
colonized by wind.
Large vertebrates may
have arrived by floating
on driftwood rafts or
possibly swam.

13. 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.

14. 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.

15. Diffusion
The gradual spread 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.

16. Many other examples of range expansions include:
European starlings in NorthAmerica
House sparrows in NorthAmerica
American muskrat in Europe
Nine-banded armadillo in NorthAmerica
European rabbit in Australia
Red fox in Australia

17. House sparrow
Passer domesticus

18. 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.

19. JUMP AND DIFFUSION VS. SECULAR
• Most colonization's involve Jump dispersal
first, followed by Diffusion.
• Secular diffusion occurs over geologic
time. Although the geographic range is
expanding, natural selection is causing
migrants to diverge from the ancestral
population.

20. 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 theArctic and
winter in southern South America,
southernAsia, Australia, and the Pacific
islands. Migrating individuals regularly fly
nonstop fromAlaska to Hawaii, a distance
of 4000 km.

21. MECHANISMS
Active dispersal
Movement under own power =active: e.g., horse,
hippopotamus.
Passive dispersal
Most organisms disperse by passive means rather
than active means. Plants, animals, fungi and microbes
can disperse passively =slow.

22. ACTIVE DISPERSAL
Active dispersal involves movementof the entire
organismthrough its own ability.
It is common in both adult and juvenile animals.
Disperse by using their own energy.

23. Monarch butterflies migrate
great distances, flying from
Canada to the
U.S. and central
southern
southern
Mexico.
Individuals may fly as far as
375 km in four days and
4000 km during their
overwintering
lifetimes.
Most
individuals of the eastern
populations gather in winter
congregations in Mexico.

24. Many insects, spiders,
and mites disperse
through the
atmosphere, forming
an aerial plankton.

25. PASSIVE DISPERSAL
Passive dispersal involves both plants and animals
that cannot themselves move but use dispersal units
called disseminules to aid in reproduction or the
exploitation of new habitats.
Use energy fromthe environment.
(e.g., dispersal of ballooning spiderlings).

26. 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.

27. Many organisms employ other organisms for long
distance transport. This process is known as phoresy.
Parasites are a good example.
Brachiopods, corals, sponges, sedentary echinoderms
etc.
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.

28. WHY DISPERSE?
• Pro: Natural selection will favor those that
disperse from a crowded area to an ‘empty’
area.
• Con: Most individuals that disperse die.
Two choices: Stay at home and produce a few
descendants or take a chance to colonize a new area
and leave many descendants.

29. DISPERSAL CAN BE AFFECTED BY BARRIERS
• Freshwater organismsare prevented from
dispersing by land and saltwater.
• Local populations strongly affected by jump
dispersal.
• Water can be a barrier to some terrestrial animals
• Ruffed Grouse found only on three Michigan
islands of the great lakes, all within 800 m of
the mainland.
• Palmer (1962) showed that these birds could
not fly for more than 800 m –can’t colonize far
islands by jump dispersal.
• Artificial stockings have been successful.

30. THANK YOU