Ch.10- Population Growth and dynamics: Logistic Growth Flashcards
Density-independent factors:
When birth and death do not depend on population size(N): [floods, extreme temp.- usually abiotic factors]
Limits on population growth
Eventually, a critical resource limits reproduction ( Nesting sites, Food availability
Density-dependent factors:
If birth and death increase or decrease with population size: disease, predators- usually biotic factors
Logistic growth
- Sigmoidal (S-Shaped) pattern of growth on graph
- It is density-dependent
- assumes population can not grow indefinitely
Carrying capacity (K)
The Maximum population size that a habitat can support
Behavior of the logistic model
- Grows below K
- Decreases above K
- Remains constant at K
Inflection Point
- Separates the accelerating and decelerating phases of population growth
- When N=K/2 -> Maximum possible growth rate
Birth Rate with Limited resources
b=(b0)-aN
- This means that as population (N) increases, instantaneous birth rates (b) will decline.
- a is constant measuring the strength of density dependence
- b0 is birth rates w/ unlimited resources
Death rate with limiting resources
d=(d0)+cN
- This means that as population (N) increases, instantaneous death rates (d) will increase
- (d0) is death rate w/ unlimited resources
- c is constant measuring the strength of density dependence
Logistic Growth Equation
dN/dt= (b-d)N; if (b0-d0)=r and K=((b0)-(d0))/(a+c) then after some algebra the equation comes to
dN/dt=rN[1-(N/K)]
r- selected species (Maximize)
- evolutionarily adapted to exploit empty niches
- produce many offspring each with a low probability of surviving to adulthood
K- selected species (Live at)
- evolutionarily are strong competitors in crowded niches
- invest heavily in few offspring each with high probability of surviving to adulthood
Density- independent effects:
- Magnitude of the outcome is not related to population size
- Ex: hurricane, extreme weather, floods
- > Exponential growth
Density-dependent effects
- Magnitude of the outcome is related to population size
- Ex: starvation, competition
- > Logistic growth
Demography
The quantitative study of populations
Effective population size
number of individuals in a population that contribute offspring to the next generation
Other Influencing factors: Age Structure
- Rapid growth: more young than old
- zero growth: = pop. size between age groups
- negative growth: more old than young
Other influencing factors: Environmental Stochasticity
erratic or unpredictable changes in the environment
Other Influencing factors: Delayed density-dependence
growth does not immediately stop once r=K
->very common!
Other Influencing factors: Allee effect
- a decrease in the population growth rate as the population density decrease
- Ex: Passenger pigeon-were once the most abundant N. american bird but declined and couldn’t survive in low numbers
Other Influencing factors: initial population size
small populations more likely to go extinct by chance
Other Influencing factors: Sex ration
- usually 50:50 but not always
- Ex: turtle-sex is determined by temp. (warmer=female) so can have populations that are 90% one sex.
