Lecture 9: Populations: Population growth & regulation Flashcards
per-capita phenomenon
Population growth via births and deaths
Total population growth
the individual reproductive rate multiplied
by the population size
the bigger the population size, what gets greater?
the numerical growth
what is an appropriate and convenient way to view growth?
in discrete time steps (generations, years)
when counting a population when should they be taken and why?
it needs to be taken at the same time each time step, to be sure that the same birth and death cycles are included
Geometric growth
- Growth via discrete time steps
- Population size is a function of starting population size, per capita growth factor, and number of time steps
- lines on the graph do not represent anything, data is only present on the dots
geometric growth formula
Nt = (N0)(λ^t)
explain the variables in the geometric growth formula
- N = Number of individuals
- λ (lambda) = Geometric growth factor
- t = Number of discrete time steps.
what is the geometric growth factor (λ)
- it is a multiplier
- ratio of population size/population size the previous year
what happens to organisms with a continuous growth?
they reproduce and die at a relatively steady rate at all times
what is exponential growth
-Growth (positive or negative) at a continuous rate that is a proportion of the total number of individuals at any given time.
-Population size is a function of starting population size, the growth rate, and the time that has elapsed
- lines on the graph represent the data
exponential growth formula
Nt = (N0)(e^rt)
explain the variables for the exponential growth formula
- r = Exponential growth rate.
- e = the exponential constant.
- N = Number of individuals
- t = Time elapsed.
what is e^r in the exponential growth formula replacing and why
- it replaces λ
- to describe that all individuals have a chance of reproducing at any time, not just at a discrete time step
geometric growth (λ) - decreasing population size
- 0 < λ < 1
- λ is in between 0 and 1, not inclusive
geometric growth (λ) - constant population size
λ = 1
geometric growth (λ) - increasing population size
λ > 1
exponential growth (r) - decrease in population size
r < 0
exponential growth (r) - constant population size
r = 0
exponential growth (r) - increase in population size
r > 0
if λ<1 or r<0, what does the graph look like?
it will be a negative exponential shaped graph
if λ=1 or r=0, what does the graph look like?
it will be a horizontal line
if λ>1 or r>0, what does the graph look like?
it will be a positive exponential shaped graph
what do we do when birth and death rates vary with age
we have to account for the growth rate/factor or each age class for accurate calculations
why is identifying age structures important
populations with the same birth and death rates but different age structures will grow at different rates
how is population growth shaped in a population with age structures
it will have different birth and death rates for different age classes
rapid and slow maturation in population growth
-Rapid maturation: limits age-class bias
-Slow maturation and bias in fecundity: maximizes age-class bias
what is a life table
it is used to organize the information for age structured populations and to calculate growth.
who does the life table typically track
-it tracks females and number of female offspring per reproductive female
-contribution of individual males are hard to track
Cohort tracking approach
follows a group of individuals from birth through to death
Cohort tracking approach - what it requires
requires that all individuals can to be marked and tracked for their whole life
Cohort tracking approach - pros and cons
- pros: provides rich data
- cons: no replication for strange years because age is confounded by time
Static age structure approach
Quantifies the survival and fecundity of all individuals of all ages in a population at a single time interval
Static age structure approach - what it requires
Requires a way to assess survival and fecundity within a single time interval.
Static age structure approach - pros and cons
- pros: All age classes face the same environment at the time of census, so age not confounded by time
- cons: it is unclear if the data is generalizable across years
general patterns of survivorship - type I
High initial survival, followed by age-specific mortality in later age classes (ex: big mammals)
general patterns of survivorship - type II
- Constant survival – organisms of all ages face the same likelihood of dying (small mammals)
- graph is negative and linear
general patterns of survivorship - type III
- High mortality early in life then escape major sources of mortality(ex: plants and invertebrates)
- graph is negative and exponential shaped
define fecundity
the amount of offspring an organism produces
consequences of age structured populations: example of fisheries
- fishing practices selectively target the most important age class of population stability
- results in the collapse of many fisheries worldwide
how were fishing practices selectively targeting the important age class
- edible fish have reproductive bursts later in life
- fisheries get paid more for catching bigger (older) fish
- these fisheries ended up catching the oldest class of large, reproductively mature fish that were responsible for the reproductive output of the population
what are cultural shifts in humans that slow population growth
- Fewer children: lower fecundity rate reflecting very high childhood survival
- Later reproduction: slower generation time reduces per capita growth rate per year, similar to later age of reproductive maturity
what factors constitute to human population growth
- Survival at all life stages is greatly improved globally.
- Fecundity rates in many parts of the world are unchanged from a historical time when many children died and adult life expectancy was much shorter.
