Chapter 9: Life Tables and Demography
9.1.
Age distributions, life tables, and survivorship curves
summarize survival patterns
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Cohort: a group of same-aged young that grow up and survive at similar rates
Age class: individuals of a particular age
Age structure: proportion of individuals in different age classes
○ Juveniles (pre-reproductive), reproductive (mature adults), post-reproductive
(senescent adults)
○ Plant and animal populations have age structures, but sometimes size or stage
or structure is more important
Life table: provides data on the number of individuals alive in different age classes and
the age-specific survival or mortality rates in these age classes
○ Number of individuals in different age classes can be calculated for any time
period
○ An imbalance in age classes can have a significant influence on a population’s
future
○ Help characterize whether or not a population is growing or declining, and help
identify which stage in the life cycle most heavily determines the rate of increase
or decline
Survivorship curve: graphical representation of the numbers of individuals alive in a
population at various ages
Cohort life table: life table following a cohort of individuals from birth to death
Static life table: life table on the age structure of a given population at one point in time
Factors that influence population growth:
○ Probability of survival
■ Survivorship curves, life tables, etc
○ Probability of reproduction
■ Fecundity: the potential rate of offspring production; influenced by body
size, temperature, latitude, habitat, food availability
Stable age distribution: for an exponentially growing population, when the proportion of
individuals in each age class becomes stable and remains constant; each age class
grows at the same exponential rate of increase
9.1.1. Age distributions reflect survival and mortality patterns
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Individuals in different age classes typically represent 1 year (though they can be
calculated for any time period)
Males often not included because they are typically not the limiting factor for population
growth
An increasing population should have a greater number of young, while a decreasing
population should have fewer
Imbalanced age classes can have a significant influence on a population’s future
○ Overfishing: older, larger fish of reproductive age are removed from the
population for food, limiting the number of new offspring
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Overgrazing: white-tailed deer overgraze vegetation and young trees, leaving
only older trees with foliage too far from the ground for them to reach; when the
older trees die, there are no young trees to replace them.
9.1.2. Static life tables provide a snapshot of a population’s age structure from a
sample at a given time
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Static life table: life table on the age structure of a given population at one point in time
Life table variables, symbols, and equations:
VARIABLE
SYMBOL
EQUATION
Age class
x
Interval at which age classes occur
Number alive at a
specified age
nx
Number of individuals alive at the start of each
age class interval
Age-specific
mortality
dx
dx = nx - nx + 1
Probability at birth of
surviving to a given
age
lx
lx = nx / n0
Age-specific
mortality rate
qx
qx = dx / nx
Average number
alive in age class
Lx
Lx = (nx + nx + 1) / 2
Total years lived
Tx
Tx = ∑ Lx
Life expectancy
ex
ex = (Tx / nx)
Age-specific fertility
mx
Average number of female offspring born to a
female parent of age x
Net reproductive
rate
R0
R0 = ∑ lx mx
Static life table limitations:
○ Assumes birth and death are constant through time
■ Example: If the rate of mortality is equal for 2-year-old sheep and
4-year-old sheep, but there are more 2-year-old sheep born in a particular
year due to favorable conditions, a greater number of 2-year-old sheep
would die of that age class leading to the assumption of a higher rate of
mortality in 2-year-olds
○ Assumes you sampled each age class in proportion to its numbers in the
population
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No independent method for estimating the birth rates of each age class
yet exists
Methodologically difficult to assess the age structure of a population; can be done
for species with growth rings, such as trees, fish scales, horns, sea urchins, and
some mollusks
9.1.3. Cohort life tables follow an entire cohort of individuals from birth to death
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Cohort life table: start with a group of individuals born in the same time frame/age
class, record their deaths until the cohort is all dea; can be age or stage based
Population censuses must be conducted frequently, but for a limited amount of time
(generally less than a year for insects or annual plants)
○ Age classes typically measured in weeks or months instead of years
More accurate than static life tables
9.1.4. Survivorship curves present survival data graphically
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Survivorship curves: graphically represent organismal survival at each age from static
or cohort life tables
Typically represented with age on the x-axis, and the logarithmic value of nx on the y-axis
○ Logarithmic scale allows the study of rates of change with time as opposed to
change in absolute numbers; also makes it easier to examine a wide range of
population sizes
Survivorship curves graphically represent organismal survival from both static or cohort
life tables
○ Type 1: most individuals die late in life (ex: humans)
○ Type 2: individuals die at a uniform rate (ex: squirrels)
○ Type 3: most individuals die at a young age (ex: octopuses)
Feature Investigation: Frederick Barkalow Jr. and colleagues constructed a
cohort life table for the Eastern Gray Squirrel
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Eastern gray squirrel is an important game animal in North America; Barkalow and
colleagues wanted to evaluate the effects of artificial nest boxes on populations
○ Team constructed a cohort life table for an area without nest boxes and another
for an area with nest boxes
○ Study found that nest boxes increased squirrel survival by reducing the mortality
associated with predation and bad weather for both young and adult squirrels
Global Insight: Hunting, over-collecting, and grazing can greatly affect
survivorship curves
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9.2.
Age-specific fertility data can tell us when to expect population
growth to occur
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Age-specific fertility and survivorship data help determine the overall growth rate per
generation called the net reproductive rate (R0)
○ Age-specific fertility (mx): the proportion of female offspring that are born to
females of reproductive age
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Net reproductive rate (R0): the average number of female offspring produced by
all the females in a population over the course of a generation, where a
generation constitutes the reproductive life of a female
Generally, generation time increases as organismal size increases