I. Dispersal - Transport of Organisms
1. Interest Ecologists – limit distribution
2. Interest Biogeographers – historical geographic differentiation
3. Modes of Dispersal:
Diffusion – gradual movement several generations suitable habitat
Jump – large distances life of individual barrier
Secular – evol. time > imp. to biogeographers
4. Function of Medium (marine vs terrestrial)
5. Successful dispersal results:
Gene flow
Modify the genetic structure of population
Founder Effect: Individuals not random dist. pop.
6. Problems studying dispersal org.:
Know little present distrib. of org.-Habitat loss
May disperse not colonize
7. Examples of Dispersal: Most important introduced by humans - Exotics
1. Zebra mussel (Dreissena polymorpha)
Native Caspian Sea (Asia) – Lake St. Claire Detroit
Potentially from ballast water
Efficient filter feeders – through siphons intake water
Hudson river – 80-90% phytoplankton – > food web effects
Increasing water quality, light benthic vegetation (some good)
Egg to Adult in 3mths – Disperse readily through water
Adhere not only biotic but abiotic surfaces –pipes & intakes
Significantly affecting industry, power plant intakes etc.
Geographic Distribution – High dispersal rate of zebra mussels
2.Gypsy moth (Lymantria dispar)
Harvard Univ. French astronomer 1850 eggs European moth Mass.
Few caterpillars escaped 1868
Initiated devastating caterpillar plagues in New England
Control program eradicate moth 1889 to 1900
Timber loss is high: US Forest Service program 90’s pheromone traps
3. Chestnut blight (Cryphonectria parasitica)
Fungal disease attacks trees – nursery stock Asia
Distribution chestnut 1990
Was 40% overstory trees climax forest
Now replaced by subdominant oak-hickory
. Not all introductions Survive Chinese & hybrids successful some areas
Bioengineering 2 genes blight resistance Asia may incorporated Amer. genome
4. More examples: Starlings and Florida exotics
Exotics More problematic for Island Nations (Hawaii, Tasmania)
The 10% rule.
9. Not all increases in distributions are unwanted: Sea Otters California
California Sea Otter (Enhydra lutris)
Hunted to close to extinction 1900s.
International treaty 1911 come back
Calculating rates of increase (linear)
10. Modes of Dispersal:
Diffusion (slow steady movement)
Hospitable terrain – gypsy moth, otter expansion
Use equations to determine if this type of dispersal
Simple Diffusion Model
Example: Reid 1899 asked question:
How trees recolonized British Isles 1,000 km after Ice Age 10,000 ya?
This simple model predicts: 36 km (36,000 m) extension
However: Reid (1899) calculated 1,000 km in 10,000 yrs??
This discrepancy now called: Reid’s Paradox
Paleoecologists calculated repopulate Britain & N America-Post Ice Age
Trees migrate 100 to 1000 m/yr vs 3.6 m that calculated diffusion model
Explanation lies in the dispersal patterns of trees – the outliers
Colonization rates driven by the extreme dispersal events!!
11. Local Distributions - Dispersal not limited by barriers
a. Life history > org. disperse widely (insects)
b. Space limitations - Primary limitation
12. Global Distributions - Barriers more important vs local
a. Terrestrial bats most animals do not cross sw barriers
b. Faunal divergence Islands; Hawaii & Galapagos
c. Continental Drift - Imp. isolating fauna & flora
i. Disjunct distribution
ii. Two Hypotheses:
1. Vicariance: spp. present entire area & fragmented
If new spp. Allopatric Speciation (reprod. isolation)
2. Dispersal: spp. dispersed across preexisting barriers
13. Why Disperse?
a. Evolutionary problem - most dispersing org. die only few successful
b. Strategies:
i. Abandon dispersal - energetic cost
ii. Fugitive spp. - devote > energy dispersal (weedy spp.)
-predominate in disturbed areas
-get in & get out
-lg. # seeds; long-distance dispersal
iii. Bimorphic - wing & wingless forms
-Aphids - env. poor develop wings
c. Conclusions:
Natural selection molded anatomy, physiology, behavior of organism’s ability to disperse to perpetuate the species.
Many adaptations for dispersal in plants & animals illustrate the importance of this aspect of survival.