Lecture 6: Age-structured populations and life histories
Age structure: the distribution of individuals among age classes within a population.
Geometric rate of population growth the ratio of the population size after one year to
that the beginning of the year:
o Will eventually reach a stable age distribution; each age class grows or declines
at the same rate
Fecundity: The number of offspring produced per reproductive episode.
o Reproductive period usually preceded by resource-accumulation phase
o Fecundity-survivorship trade-offs = cost of reproduction
x: age specific variables
nx: the number of individuals of age x in a population.
bx: female offspring produced per reproductive season (b
x : probability of being alive at age x
dx: death rate, proportion of individuals of age x dying by
m x mortality rate; proportion of individuals of age x
dying by age x+1
Shapes of survivorship curves
Type II survivorship curveconstant survival rate with age. Thus, survivorship declines
exponentially with age.
Type I survivorship curve high survival rate at the beginning, falls off abruptly with age
as the age-specific mortality rate increases (human; mortality is low early in life and
then increases rapidly later in life) .
Type III survivorship curvehigh mortality rate in early life, survival rate increase later
in life. (fishes; young individuals are extremely vulnerable to predation and other risk
factors, which they escape as they grow larger and mature). Net reproductive rate (R0): the expected total number of female offspring produced by
an average female over the course of her life span.
o ∑ would be the total # daughters produced by a mother who doesn’t die early;
multiplying by discounts expected production by the probability that some
mothers do die early
Generation time (T): the average period between the birth of an individual and the birth
of its offspring; average age at which a female gives birth.