TOPIC 10: AGE POPULATION STRUCTURE

Question 1

Age Structure

Answer 1

the number of different age classes + the number of individuals in each
age class
* indiv. in pop. in diff age ranges will contribute to po. in diff ways; young who cant reproduce won’t contribute to growth of pop. same with elderly take more resources from pop.

*bigger indiv. reproduce more

Question 2

Age class

Answer 2

= a discrete grouping for individuals born at ~ the same time

Question 3

Cohort

Answer 3

= Individuals that make up an age class

Answer 4

Important! –Because different age classes have different birth and death rates… age classes
contribute different numbers of offspring to the population

• higher death rates in humans as you get older in some species young have higher death rate

Question 5

Birth rate

Answer 5

the number of female offspring produced per female in a particular age class

• number of offspring produced will differ among age classes (e.g. age at sexual maturity, sexual senescence/menopause)

Question 6

Why do we only consider females in the birth rate?

Answer 6

• Concerned primarily with females because:
• birth rate is limited by the number of females
• difficult to quantify the contribution of each male

(only consider females b/c egg as a gamete are much more costly to produce sperm; males can fertilize many eggs but females are limiting factor can only carry one offspring

Question 7

Death rate

Answer 7

= the number of females that die per age class in the population
number of individuals dying will differ among age classes

Question 8

survivorship curves

Answer 8

number of individuals alive versus age
* 3 hypothetical curves - few populations fit one precisely

Question 9

Type 1 survivorship cure

Question 10

type 2

Question 11

type 3

Question 12

Population Growth

Answer 12

• Is a population growing (increasing in size from one generation to the next)?
• Birth (b) & death (d) differ among age classes…so, each age class contributes to
  population size differently

Question 13

• Population 1 –

Answer 13

– same number of individuals in all age classes

• Population size ↑ …. If b > d

should have some ind. that are or reproe age
as long as all indiv. combined have a higher avg. BR then avg DR
-> indiv. are being born at a higher rate then dying

Question 14

• Population 2

Answer 14

– only pre-reproductive adolescents and adults too old to breed

• Population size ↓…..If d > b

Question 15

Life Table

Answer 15

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

allows us to quantify whether the size of a population is increasing, stable or
decreasing

Question 16

Cohort (Dynamic) Life Tables

Answer 16

• follow one cohort (inds. within an age class) from birth to death
• determine d and b in each age class
  E.g. plants, sessile animals, mobile animals where dispersal is limited
• Difficult for highly mobile animals (cannot track them easily)
• Can take a long time to collect data (depends on lifespan)

Question 17

. Static (Time-specific) Life Tables

Answer 17

• snapshot of the population at one point in time
• estimate d by determining the age-specific survival independently for each age
  class at a specific time

E.g. highly mobile animals

• Difficult to determine the ages of all individuals
• can use stage or size classes if age cannot be determined
• Ignore variation in b & d among individuals in an age class (e.g. size, competitive ability,social status, genotype); within age class you have indiv. that are more competitive so will have more repro success
• Also, would need to be able to determine the age;

Question 18

• Underlying assumption

Answer 18

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

Question 19

Determining Age

Answer 19

• Biologists age organisms in several ways, depending on the species
• tree rings tell you have the exact age

Question 20

x =

Answer 20

age class
How many age classes will the cohort go through?

Question 21

nx

Answer 21

total number of individuals at each age x
Are there differences in the proportion of each age class?

Question 22

Fx =

Answer 22

= total number of offspring produced during each age class x
Is there an age class with the highest fecundity?

Question 23

lx =

Answer 23

= proportion of the original cohort surviving from birth to age x (survivorship)
* lx = nx/ n0

Question 24

dx =

Answer 24

proportion of the original cohort dying during each age class

dx = lx - lx+1

can sum the proportion of the cohort dying over a number of age classes

Question 25

qx

Answer 25

proportion of the original cohort dying versus surviving (age-specific mortality rate)
qx = dx / lx

• indicates the intensity of mortality at a particular age (qx cannot be
  summed)

Question 26

bx

Answer 26

= individual fecundity or birth rate (age-specific birth rate)
* the mean number of offspring produced per surviving individual:
bx = Fx / nx