Factors
Limiting
Distribution:
Dispersal
– Chapter 4
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
are.
• 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
Example: Zebra Mussel
Probably introduced
from ship ballast
water.
Spread throughout
the majority of the
Mississippi drainage
within 10 years.
Example: Gypsy Moth
Accidentally
introduced by a
French astronomer in
1868.
Control programs
ceased around 1900.
Began to spread
again, and
accidentally
transported to
Michigan.
Spread 21 km per
year between 1966
and 1989.
Example: Chestnut Blight
Fungus that is lethal
to Chestnuts.
First noticed in 1900,
and apparently
introduced on
nursery stocks from
Asia.
Search began in 1927
to find blight
resistant trees.
Oak-chestnut forests
have been replaced
with oak or oakhickory forests.
Example: California Sea Otter
Thought to be extinct in
1911.
Small population found
in 1914 – was now
protected.
Northern range spread
about 1.4 km/year and
southern range spread
about 3.1 km/year.
Southern otters move
more as individuals and
northern suffering higher
mortality?
Three Modes of Dispersal:
• Diffusion – Gradual movement of a population
across a hospitable terrain for a period of several
generations.
• Jump Dispersal – Movement of individual
organisms across large distances of inhospitable
habitat followed by the successful establishment
of a population in the new area.
• Secular Dispersal – Diffusion occurring in
evolutionary time.
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.
– Not of immediate interest for ecologists working
in ecological time
Simple Diffusion Mathematically:
Distance moved = DnlogeR0
Where:D = average dispersal distance
n = number of generations
R0 = reproductive rate per generation
Can this explain the spread of oak trees since the last
ice age?
Oak Tree Dispersal Following Last Ice Age:
Distance moved = DnlogeR0
If a tree produces 107 (R0)seeds per generation for 300 (n)
generations and seeds disperse 30 m(D) with each
generation then:
Dispersal distance = (30m)(300yr)(loge(107seeds/yr) = 36 km
However, actual migration distance since the last ice age is
about 1,000 km – known as Reid’s paradox.
Measuring Tree Seed Dispersal
Seed traps can be use to determine seed dispersal
Distances are too small to account for dispersal of trees
after the ice age.
Answer to Reid’s
paradox seems to lie in
haphazard, long range
dispersal of seeds.
Colonization not driven
by mean seed dispersal,
but extreme dispersal
events.
Wind, animals, Johnny
Appleseed!
Dispersal Can Be Affected by Barriers
• Freshwater organisms are 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
– Isn’t it ironic that immobile trees colonized
offshore islands that flight capable birds didn’t!
Is Dispersal The Only Limit To Distribution?
• Humans have moved many species around
the globe – often with disastrous
consequences (think locally: nutria, hydrilla
and water hyacinth).
– Humans have allowed several species to bypass
traditional geographic barriers
• However, many times it is not just
inaccessibility that determines whether or not
a species is found in a particular habitat.
Terms for Introduced Nonnative Species
States
Transition
Definition
Escaping
Brought into the Country
Transition from imported to
introduced
Imported
Introduced
Found in the wild; feral
Establishing Transition from introduced to
established
Established
Becoming a
Pest
Pest
Has a self-sustaining population
Transition from established to
pest
Has a negative economic
(ecological?) impact
How Successful Are Introductions?
-actually, they are usually failures
Overall, continental bird introductions are successful
about 10 – 30% of the time.
Statistical Generalizations: Tens Rule
• Williams and Fitter (1996) predicted that
– 1 species in 10 imported to a country becomes
introduced
– 1 in 10 of introduced species becomes
established
– 1 in 10 of the established species becomes a
pest
• Of course exceptions occur
– Much of Hawaii has been cleared, so habitats
have become unsuitable to native birds but not
introduced ones
Local Scale Dispersal
• Transport is rarely a limiting factor in plant
dispersal
– Seeds/spores carried primarily by wind or
animals
– Rumex crispus var. littoreus not limited by
dispersion or seed-germination
– Wind dispersed seeds and spores usually
colonize disturbed areas first
• Small animal are often dispersed by wind
– Spiders
– Mosquitos
• Salt marsh mosquitoes from LA found 74-106
km offshore!
Colonization and Extinction: Krakatau
• August 26, 1883 – Krakatau exploded (25 km3)
• Two islands a few kilometers away were
completely covered in ash
• The nearest island not destroyed by the
explosion was 40 kilometers away
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
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.
Continental Drift – Disjunct Distributions
Antarctic beech modern distribution
Dispersal before or after barriers were formed?
Continental drift takes some continents farther apart,
while bringing others closer together.
Explanation Of Disjunct Distributions
• Dispersal explanations – assumes organism
dispersed across preexisting barriers (e.g.,
mountains and rivers).
• Vicariance explanantions – assume that a
species was present on the entire area and
subsequently was fragmented by the
formation of barriers.
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.
Examples of Dispersal Abandonment
• Flightless birds on islands
• Insect Species
– Subantarctic islands ~ 76% of the insects are
flightless
– Ecologogical islands (alpine zone of tropical
mountains)
Fugitive Species
• Devote most of their effort to dispersal
• ‘Weeds’ of plant (dandelion) and animal (water
boatman) kingdoms
– Colonize temporary habitats
– Grow almost predominately on disturbed areas
Species of
water boatman
Corixa
nigrolineata
Pond sp.
Corixa falleni
Large lake
species
No. immigrants to
temp. pond
% of total numbers
collected in temporary
habitats
209
81
11
23
Summary
• Three methods of dispersal: diffusion, jump,
secular
• Transplant experiment – inaccessibility
• Ten’s rule (most introduced sp. die out)
• Dispersal rarely limits local distribution of
plants and animals
• Dispersal is adaptive if it allows successful
colonization
– Fugitive species