Dispersal is the spread of reproductive products (propagules, larvae, seeds) or individuals (juveniles or adults) from a single source. It is molded by natural selection.
Reproductive strategies: Frequency of reproduction during lifetime
- Iteroparous: organisms reproduce more than once
- Benefits: Many chances to increase fitness
- Costs: Continued allocation of energy throughout lifetime
- Mast seeding: episodic, synchronous production of large seed crops by a population of plants. Masting is geographically disperse.
- Maple trees
- Fruit trees
- Lowland rainforest trees
- Conifers
- Oaks
- Beech
- Semelparous: organisms reproduce only once
- Benefits: Delay the cost of reproduction and allocate energy to growth and maintenance.
- Advantages: Often associated with mortality (ultimate loss of fitness)
- The investment of reproduction is sometimes too great that the organism dies.
- Bamboo
- Squid
- Yucca century plant
Three major hypothesis for the origin of masting
- Predator satiation
- Reduce per capita seed mortality; swamp predator (especially specialists)
- Wind pollination
- Increase chances of fertilization; no animals
- Environmental facing (prediction)
- Species track weather
- Masting after El Niño rains
- Fires in Australia synced to masting
- New hypothesis: difference in temperatures between two and one years previous to masting
'r' vs. 'k' selection
Population growth curve for r-selected species is exponential.
r = intrinsic rate of increase
dN/dt = rN
Traits:
- High reproductive investment
- High dispersal
- Poor competitor
Examples:
- Weedy species
- Invasive species
- zebra mussels
- cheat grass
- purple loofstrife
Population growth curve for k-selected species is logistic.
k = carrying capacity; limit to population size set by resources.
dN/dt = rN(1-N/k)
Traits:
- Low/Moderate reproduction investment
- Lower dispersal
- Good competitors
Examples:
- Some tree species
- Some corals
Dispersal
All organisms can exhibit three subtypes of dispersal
- Fragmentation: organisms break and disperse
- Migration: adults and juveniles
- Individuals of new generation
Why should organisms disperse?
- Disperse away from habitat with poor resources or low amounts
- Increase survival by reducing competition
- Spread genotypes in place where they'll have high fitness
- Prevent local extinction by maintaining connectivity among populations
- Dispersal can save species from extinction
Escape hypothesis
"Get away from parents".
Survival increases with increased distance from parents.
Selective force for dispersal; escape high mortality close to parents
Sources of high mortality
- High competition for resources; low light, low nutrients
- High predation on seeds; seed shadow; predators aggregate near base of tree
- Pathogens; high risk from molds or fungi
Janzen-Connel Model
Hobbel Model
Exact Compensation Model
Dispersal distances
Short distance dispersal
- Seeds (maple seeds); propagules
- Sea palms: drip zoospores
- Ascidians: asexual fragmentation
Long distance dispersal
- Pacific trade winds
- Dominance of passive dispersal
Dispersal vectors
- Animals: seeds ingested and defecated
- Wind
- Seed burrs: hooks attach to animals
Behavior can influence dispersal. Example: bellbirds drop seeds away from canopy when showing off for females
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