Density Dependent populations
Density Dependent
populations
population growth is a function of the
size of the population; intrinsically
controlled sometimes, biotic factors in
the environment typically limit the
population
Density Dependent populations
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Density Independent
populations
population growth is checked by
extrinsic forces, typically abiotic
factors
Andrewartha and Birch suggested that populations were
primarily controlled by density independent factors such as
rainfall
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Mountain Sheep reestablishment
Mountain Sheep
reestablishment
1. growth slow at first because of small
population
2. begin to form herds and live in all suitable
territories (exponential growth)
3. have hard winter and population shrinks
(density independent)
4. have string of mild winters and populations
get large (density dependent)
5. predator populations increase, K brought
down (density dependent)
Age Structure
Population Age Structure - the
partitioning of a population
into discrete groups by life
cycle stages (overlapping
generations are present)
• Prereproductive
• Reproductive
• Postreproductive
• larvae
• pupae
• adult
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Age structure and growth
Patterns in reproduction and
survivorship
….and sex ratios!
Semelparous Reproductive
Strategy
only reproduce once, the
reproductive stage is the final part
of the life cycle
Iteroparous Reproductive
Strategy
reproduce numerous times during
reproductive phase
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Life-history trade-offs
Size/number trade-off
• Life history- adaptations of an organism
that influence survival, reproduction,
size/age at maturity
– What is not a life history trait?
• Limited energy going to growth/survival or
reproduction
• Patterns of allocation
– Size/number of offspring trade-off
– Survival-reproduction timing
You've got to know when to hold 'em, know when to fold 'em
Know when to walk away, know when to run …..(Kenny Rodgers, The
Gambler)
• Patterns of survival and
reproduction
• What patterns in the 3
types of survival?
r-selected species
• Reproduce early and
have many small
offspring
• High r
• Short generation time
• many are semelparous
• poor competitors
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K-selected species
• Reproduce late have few
large offspring
• Low r
• big investment in few
offspring
• long lived with most young
maturing to reproduce
• many iteroparous reproductive effort spread
out over time
Metapopulations
• Definition
• Migration and extinction drive population
dynamics
• Variations
• Examples
Metapopulations
• r -- % occupancy
• b – colonizations
• d – extinctions
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Metapopulations
P = 1 – e/c
e = extinctions
c = colonizations
Variations
• Simple
(true metapopulation)
• Source-sink:
Patch occupancy
increases with
decreasing e/c
Variations
• Patchy population
Examples
• California mountain
sheep
• corridors
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Examples
• California
checkerspot
butterfly
Demographic and environmental
stochasticity
• Random variation in pop size resulting
from random variation in:
– birth and death rates (demographic)
– biological and physical environment
(environmental)
• Cause extinction in small pops
Small Populations
• Demographic stochasticity
• Environmental stochasticity
• Reduced genetic variation
Dusky seaside sparrow
(Ammodranus maritimus nigrescens)
• Lived in Florida marshes
• Population decline:
– Habitat destruction/conversion
– Mosquito control
• Last 5 individuals were male
• Died June 18, 1987, at Disney
World…really!
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Reduced Genetic Diversity
• Small populations and
Genetic drift
• Inbreeding
depression
• H = 1 – 1/2Ne
• Ne is size of
population
measured as the
number of
breeding
individuals
Consequences of lowered genetic
variability
• Inbreeding
depression
• Population ability to
adapt to changing
conditions
• Examples
Loss of genetic diversity
Inbreeding Depression
Ipomopsis aggregata, scarlet gilia
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