Topic 11 - Age Structure (BIOL 2300)

age structure

number of different age classes + the number of ind.s in each age class

age class

a discrete group of ind.s born at approx. the same time (e.g. ind.s whoa re born in 2006)

cohort

individuals that make up an age class

why is age structure important

different age classes have different birth and death rates… age classes contribute different number of offspring to the pop.

birth rate (b)

number of female offspring produced per female in an age class

• # of offspring will differ among age classes

*in reality, we often can’t just focus on females, e.g. species that males and females look the same

why is birth rate primarily concerned with females

• birth rate is limited by number of females

• difficult to quantify contribution of each male 

death rate (d)

number of females that die per age class in pop.

• # of ind.s dying will differ among age classes

survivorship curves

number of ind.s alive (log scale) versus age

• 3 hypothetical curves - few pop.s fit one precisely

• Type 1: low mortality through life, but high when older (e.g. humans)

• Type 2: equal probability of survival through life (e.g. birds, rodents, reptiles)

• Type 3: high mortality when young (e.g. fish, inverts, plants)

life table

a tabulation of b & d of each age class in a population

• use to quantify whether population size is increasing, stable or decreasing from one generation to the next

cohort life tables

follow one cohort (ind.s within an age class or born at the same time) from birth to death

• determine d and b in each age class

• e.g. plants, sessile animals, mobile animal where dispersal is limited

• difficult for highly mobile animals (cannot track them easily)

• can take a long time to collect data (depends on lifespan)

static (time-specific) life tables

snapshot of pop. at one point in time

• estimate d by determining he age-specific survival independently for each age class at a specific tim e

• e.g. highly mobile animals

• difficult to determine ages of all ind.s → can use stage or size classes if age cannot be determined →( e.g. stage: tadpole, adult frog) (e.g. size: tree diameter)

assumption of static (time-specific) life tables

stable age structure (=number of ind.s in each age class remains the same through time)

*often wrong in most cases, but we need to build an assumption

what do both cohort and static life tables ignore

both types ignore variation in b and d among ind.s in an age class (e.g. size, competitive ability, social status, genotype)

x - cohort life table

age class

“how many age classes will the cohort go through?”

nx -cohort life table

total number of ind.s at each age class x

• number of ind.s at each age class?

Fx - cohort life table

total number of offspring produced during each age class x

• number of offspring produced at each age class?