Life Tables + Pop Growth Flashcards
What is survivorship?
Describes how many individuals in a population are expected to survive to any specific age (x)
What are type I survivorship curves?
Low mortality at the start of life, rapid increase at older ages
What are type II survivorship curves?
Constant mortality throughout life
What are type III mortality curves?
Relatively high mortality rate earlier in life, decreases later in life
How do we use survivorships in calculations?
Need to convert survivorships to probabilities - divide by the initial number
What is the purpose of life tables?
Summarise births and deaths for organisms at different ages of their lives
Show quantities like survivorship for different age classes
What are the types of life tables?
Cohort life table - represents age-specific rates over the lifetime of a cohort of organisms born during a short period of time e.g. one year
Period life table - represents age-specific rates during a specific time period of a certain population
What is lx?
Survivorship
Survival from age 0 to age x (0->1, 0->2, 0->3…)
e.g. at age zero 1000 individuals, at age one 500 individuals, survivorship (lx) at age 1 is 0.5
What is Sx?
Survival
Survival from age x to age x+1 (0->1, 1->2, 2->3…)
e.g. at age 1 = 500 individuals, at age 2 = 100 individuals
survival (Sx) = 100/500 = 0.2
What is fertility?
The per capita reproductive output of an individual
- Per capita = per individual
- Usually only female offspring are counted
What is mx?
Mean number of offspring (ox) produced by each surviving individual over the age x-1 to age x period
e.g. at age 2 = 100 individuals, 500 offspring
fertility = 500/100 = 5
What can be calculated from life tables?
- Measures of population growth
- Net reproductive rate
- Annual growth rate
- Generation time
What is R0?
Net Reproductive Rate
- The average number of female offspring produced by one individual female over her lifetime
What can R0 show?
- How much a population grows per generation e.g. R0 of 0.95 means a decline by a factor of 0.95 per generation
- A measure of fitness (lifetime reproductive success)
What does R0<1 mean?
Population is declining
- Females are not replacing themselves each generation
What does R0>1 mean?
Population is growing
What does R0=1 mean?
Stable population
- Every female adds one additional female per generation
How is R0 calculated?
∑lxmx (sum of products of lx and mx over all ages)
e.g. l0m0 + l1m1 + l2m2 …
What is T?
Generation time
- Average time between successive generations
- Average time between birth of individuals and birth of their offspring
How is T calculated?
As a weighted average of age(x) at reproduction of a cohort
T = ∑ (x lx mx) / ∑ (lx mx)
= ∑ (x lx mx) / R0
How are T and R0 combined?
R0 = 0.95
T = 2.5 years
This population declines by a factor of 0.95 every 2.5 years
What is λ?
Population growth rate
What does λ show?
How much a population will grow per unit time (e.g. years) in the long term
Useful as is a common scale when comparing pop growth
Works as a multiplicative factor
How is λ calculated?
λ ≈ R0^1/T
