Population growth

life tables

summarize proababilities that an age class, or cohort will survive and reproduce in any given year over the individuals lifetime

Cohort

a group of individuals that are all born at the same time

life tables: survivorship (ix)

the proportion of offspring that are still alive at each age

life table variable (x)

year or age class

Life tables variable (N)

the number of cohorts remaining in the population

life tables variable (Nx)

number remaining in particular age class

life table variable (mx)

fecundity

life table variable (dx)

death rate of original cohort that dies each year

Fecundity (mx) def

the number of offspring an individual can have in its lifetime

typically refers to the number of FEMALE offspring a female can have in her lifetime

Age specific fecundity

average number of female offspring produced by a female in each age class 

life table for Zootoca vivipara

as survivorship declines fecundity increases

Net reproductive rate

indicates whether the population is increasing or decreasing

if the sum is <1.0 the population is getting smaller 

=1 its staying the same 

>the population is growing 

survivorship curves

LOG of the number of survivors vs age

type I survivorship curve

A type I survivorship curve depicts a population with low mortality early in life and high mortality late in life.

Examples include humans, elephants, and whales

type 2 survivorship curve 

type 3 survivorship curve 

A type III survivorship curve depicts a population with high mortality early in life and high survival later in life.

This is a common pattern in many species of insects and in plants such as dandelions and oak trees that produce hundreds or thousands of seeds

why cant we have high survival and high fecundity

restricted amount of time and energy

limites resources 

life history tradeoffs

shaped by natural selection: to maximize an organisms

general pattern with high fecundity

high fecundity, fast growth rate, sexual maturity at young age, produce many offspring, lower survivorship

general pattern for high survivorship 

slow growth rate, invest in resources in traits that allow them to compete with other individuals, lower fecundity 

To calculate the number of individuals that survive to the next age class

we multiply the number of individuals in an age class by the annual survival rate of that age class (Nx) x (Sx)

s the size of the population, N, approaches the carrying capacity of the environment, K, the ratio N/K approaches 1 and the term in parentheses approaches zero.

At this point, the population’s growth rate is zero, which means that the population has achieved a stable equilibrium.