Transcript File

CHAPTER 3
Factors Affecting Distribution and Abundance :
1)DISPERSAL
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Learning Outcome
1. To define and describe the concepts of
dispersal
2. To describe the colonization process
3. To acquire knowledge on fugitive
species
4. To explain the Lidicker’s Hypothesize
- pre-saturation and saturation dispersal
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Dispersal
• Dispersal = Movement of individuals away
from their place of birth
• Ecological process that affects distribution
• Disperse and colonize new area
• Dispersal into or out of some area
• Increase/decrease population density
• Expanding population/response to climate
change/changing in food supply
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If dispersal is successful it results in gene
flow (Darwinian fitness)
• If colonization is successful, dispersal
will result in gene flow and thus affect the
genetic structure of a population.
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Example of dispersal:
Gypsy Moth (Lymantria dispar)
•
an introduced sp. - silk
industry (purposely
introduced)
•
Purpose: to make a
caterpillar hybrid, that could
resist diseases.
•
Escaped - found suitable
habitat - multiply
•
Negative impact: initially in
Europe only but now in US –
destroy timber trees.
•
Overcome the spread: use of
pheromone traps
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Example of dispersal :
Chestnut
blight
fungus
(Cryphonectria parasitica)
-
Attack American chestnut tree
(Castanea dentate)
-
Eliminated chestnut in 40 years
only. The fungus kills only the
above ground biomass - roots of
trunks killed after 60 years still
send out shoots.
-
Resulted in replacement by
codominant trees such as oak,
beech, red maple
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Example of Dispersal:
Sea Otter (Enhydra lutris )
- Hunted to near extinction by
fur traders.
Protective laws since
1914: Range of the otters
widened
-
Other example of Dispersal :
African honey bee & zebra
mussel
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Modes of Dispersal
1) Diffusion
2) Jump Dispersal
3) Secular Dispersal
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Modes of Dispersal
1) Diffusion
• gradual/slow spread or movement of
populations across hospitable terrains for a
period of several generations
• Sea otter, Gypsy moth & Chestnut blight
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Modes of Dispersal
2) Jump Dispersal
• quick movement of individual organisms across large
distances - successful establishment in the new area
• occurs in a short time during the life span
• usually across unsuitable terrain
• E.g. island colonization – Bird Bonasa umbellus,
African Honey Bees and Fire Ant
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Modes of Dispersal
3) Secular Dispersal
• Occurs over geologic time.
• Although the geographic range is expanding,
natural selection is causing migrants to
diverge from the ancestral population.
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All forms of dispersal are
affected by barriers such
as……..
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Water as barrier
• Example of barriers in jump
dispersal – water as a barrier for
birds
• Islands closer to mainland are
colonized but further islands are
not.
• The
ruffed
grouse
(Bonasa
umbellus) are found on islands that
are 800 m or closer to the
mainland shore of the great lakes
in US because this birds cannot fly
for more than 800m.
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• Impact on native species :
dispersal may not limit distribution
because introduced sp may not be able
to survive local environments
– Only 4 out of 50 introduced bird species have survived in US
– Only 13 out of 85 introduced bird sp were successful in Europe
– Only 15 out of 50 bird sp. introduced in Sydney survived
• Generally the survival of introduced sp. is about 10 –
30%
• 1 in 10 imported species to a country becomes
introduced, 1 in 10 of introduced species becomes
established, 1 in 10 of the established species becomes a
pest
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Dispersal :
Who?
• Juvenile – to find or colonize new areas
• Adult - Escape from deteriorating situation
Why?
• Natural selection will favor individuals that leave a relatively
crowded habitat and colonize an empty one in which it can leave
many descendents – to maximize fitness
• Dispersers increase chances of encountering new individuals to
mate, thus reduces in-breeding, and increases fitness of offspring
due to heterogeneity
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When do they disperse?


the pre-reproductive period
when population increases
How far?

Depends on density of surrounding population
availability of suitable unoccupied habitats (new area)
and
The rule:
1.
2.
3.
move to the first uncontested (open) available site
disperse in a straight line from their natal area or make
exploratory surveys
settle in the first empty site
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Monk parakeets
1.
2.
3.
move to the first uncontested (open)
available site
disperse in a straight line from their natal
area or make exploratory surveys
settle in the first empty site
Red squirrels
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• Nearly all studies show very local
dispersal
– Most tree seeds fall within a few meters
of parent.
– Only a few seeds make it to further
distance.
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Deermouse
Proportion of the dispersing
Expected dispersal
Observed dispersal
Expected dispersal
White crowned
sparrow
Observed dispersal
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What are the motivation to disperse?
1) Intra-specific competition – population increase,
aggression increase, resources are less, breeding
partners decreases
2) Aggressive individual force subdominants to
move
3) Dispersal usually occurs before peak density of
populations/ before reproductive period
4) Asocial members disperse – have no ties.
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•
2 options :
(1) Stay at home and be happy
– living in crowded conditions (very dense, scarce food stock,
low quality of habitat, less individual to mate - leave few
offsprings.
OR
(2) Disperse & take a chance
– find a new place – colonize it – and leave many descendents
(good habitat, ample food resources, meeting new individuals
to mate, reduces inbreeding - increases fitness of offspring due
to heterogeneity.
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Colonization
• Colonization occurs
whenever any one or
more species populate
an area
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Colonization
E.g Krakatau Explosion
– 1883 Krakatau erupted and caused total
extinction of all species on the island and the
remaining land became sterile.
– 2 nearby islands were completely covered by
ash and this caused total extinction of species.
– Nearest island not destroyed was 40 km away.
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….one or more species populate an area
Colonization process…
Time Since
Explosion
Species found
9 months
Single Spider
3 years
Blue-green algae; 11 fern species; 15
flowering plant species
10 years
Coconut trees growing
25 years
263 species of animals; dense forest
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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.
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Colonization milestone
• Where there is vacant space
organisms will find it – mostly
by chance (exploratory survey)
and adaptive ability
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Fugitive Species
• Devote most of their effort to dispersal
(extreme disperser)
• ‘Weeds’ of plant (dandelion) and animal
(water boatman) kingdoms
– Colonize temporary habitats.
– Grow
almost
predominately
on
disturbed areas.
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Characteristics of fugitive species:
-
Colonize temporary habitats.
-
Reproduce and leave before the
habitat disappear or competition
with other organisms increases.
-
These fugitive sp usually found
in disturbed habitats.
-
They produce large number of
seeds.
-
Adapted for long distance
dispersal by wind or animals –
Taraxacum officinale a fugitive
weedy sp.
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• Dispersal is advantageous when
habitats are patchy and unstable.
• Organisms living in stable or uniform
habitats also disperse as long as there
are potential sites away from parents
• Species that live in more permanent
habitats generally disperse less.
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Lidicker’s (1975)* Hypothesis
• Presaturation Dispersal
– During the increase phase of pop before reaching
peak/maximum or before resources get depleted
– Dispersers are in good condition and are of any sex or
age group- enough energy/fit
– Have a good chance of survival and settling into a
new area
*Lidicker, W.Z (1975). The role of dispersal in the demography of small mammals. In: F.B.
Golley, E. Petrusewicz & L. Ryszkowski (eds.). Small Mammals, Their Productivity and
Population Dynamics. Cam bridge Univ. Press, Cambridge, England. p. 103-128.
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Lidicker’s Hypothesis
• Saturation dispersal
– When population reach/exceed its carrying
capacity
– Mostly juveniles
• Have 2 choices
– Stay, perish and do not breed
– Or leave
– Survival is low (less energy/ unfit)
– Not in good condition.
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Conclusion
• Dispersal ability has been molded by
natural selection to maximize the
chance of colonizing vacant areas
• Dispersal requires:
(1) a source
(2) a sink (empty and unfilled, marginal
and unsuitable habitat)
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