TOPIC 10: AGE POPULATION STRUCTURE Flashcards
Age Structure
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
Age class
= a discrete grouping for individuals born at ~ the same time
Cohort
= Individuals that make up an age class
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
Birth rate
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)
Why do we only consider females in the birth rate?
- 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
Death rate
= the number of females that die per age class in the population
number of individuals dying will differ among age classes
survivorship curves
number of individuals alive versus age
* 3 hypothetical curves - few populations fit one precisely
Type 1 survivorship cure
type 2
type 3
Population Growth
- 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
- Population 1 –
– 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
- Population 2
– only pre-reproductive adolescents and adults too old to breed
- Population size ↓…..If d > b
Life Table
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
Cohort (Dynamic) Life Tables
- 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)
. Static (Time-specific) Life Tables
- 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;
- Underlying assumption
stable age structure (=number of individuals in
each age class remains the same through time)
Determining Age
- Biologists age organisms in several ways, depending on the species
- tree rings tell you have the exact age
x =
age class
How many age classes will the cohort go through?
nx
total number of individuals at each age x
Are there differences in the proportion of each age class?
Fx =
= total number of offspring produced during each age class x
Is there an age class with the highest fecundity?
lx =
= proportion of the original cohort surviving from birth to age x (survivorship)
* lx = nx/ n0
dx =
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
qx
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)
bx
= individual fecundity or birth rate (age-specific birth rate)
* the mean number of offspring produced per surviving individual:
bx = Fx / nx
