Download 12.2 - Demography

UNIT 5: Population Dynamics
12.2: Demography
pg. 593 – 599
Demography – is the study of the growth rate, age structure, and other
characteristics of populations.
Natality – is the birth rate in a population.
Mortality – is the death rate in a population.
Immigration – is the movement of individuals into the population.
Emigration – is the movement of individuals out of the population.
Populations are dynamic, forever changing, season to season, and year to
year. Demography is the study of these changes, both in size and density
through time.
Births and immigration (positive) and deaths and emigration (negative) are
used to determine growth rates of populations in a given time period, months,
years, 10 years, etc.
Life Tables
Life Table – is a chart that summarizes the demographic characteristics of a
population.
Cohort – is a group of individuals of similar ages.
Age-specific Mortality – is the proportion of individuals that were alive at
the start of an age interval but died during the age interval.
Age-specific Survivorship – is the proportion of individuals that were alive
at the start of an age interval and survived until the start of the next age
interval.
As a whole every species has a lifespan, yet individual’s lifespan varies.
Causes of death or factors could be; starvation, disease, predation, or the
inability to find suitable habitat.
A life table summarizes the demographic characteristics of a population of
the organisms being study. A group of organisms born around the same time
as the study group (or short lived organisms) is usually called the cohort and
are monitored until they die. Any life table has the life spans of individuals
divided up into intervals. (days, weeks, or months for short life spans or
years or ranges of years for longer living organisms)
Morality rate is the number of deaths in a population per unit of time. There
are two different views of this information, Age-specific mortality and Agespecific survivorship.
Age –specific mortality is the proportion of individuals that were alive at the
start of the age interval but are now dead before the next interval, while age
– specific survivorship is the proportion of individuals alive at the start of
the interval and are still alive as the next interval passes.
e.g.: data: For a 4 – 6 month period (age interval) initial population 722.
Number that died 195.
Age – specific mortality rate =
# that died during interval
# alive at the start of the interval
= 195
722
= 0.270
Age – specific Survivorship rate
= # still alive at the end of the interval
# alive at the start of the interval
= 722 – 195
722
= 0.730
The sum of the age-specific mortality rate and the age-specific survivorship
rate is equal to one.
0.270 + 0.730 = 1.000
A life table can summarize the proportion of the cohort that may survive to a
particular age. This statistic identifies the probability of a newborn will still
be alive at a given age. (interval 4 to 6 months)
Probability of being alive
at a specific age
= # alive at the start of the interval
# alive at the start of the initial age
interval
= 722
843
= 0.856
Conversely the probability that the newborn will not be alive at this age
Interval, is 0.144
1.0 00 – 0.856 = 0.144
Graph of age specific survivorship for the annual bluegrass plant by age interval.
Figure 2:
Survivorship Curves
Survivorship Curve – is a graphic display of the rate of survival of
individuals over the lifespan of a species.
Data about survival is used to create a survivorship curve graph, which
displays the survival of individuals over the lifespan of the species. There
are three general survivorship patterns: Type I, Type II, and Type III.
Type I: are relatively flat at the start, low death rate for the start and middle
years, then a sudden drop as death increase at the end. This is typical for
large animals, which have few offspring and must take care of them for a
period of time to reduce juvenile mortality. Specie that has this type of curve
also has long gestation periods and newborns are unable to care for
themselves.
Type II: this curve shows a constant rate of mortality in all age’s groups. It
shows a steady decline in survivorship. Factors that may affect their
mortality rate may be predation, diseases, and starvation. These type of
organisms have a short gestation period and often prey on type III organisms
but are preyed upon by type I organisms.
Type III: this type of curve drops down quickly at the start, expressing a
high death rate early in life, and then flattens out as the death rate declines
for a few individuals that survive. This type of curve is characteristic of a
species that produce a large number of offspring.
Figure 3: The survivorship curves (red) of many organisms, such as: a) the Dall mountain
sheep, b) the five-lined skink, and c) the perennial desert shrub, roughly match one of
three idealized patterns (blue).
The survivorship curves are used as a reference points for new data.
Fecundity
Fecundity – is the potential for a species to produce offspring in a lifetime.
Generation Time - is the average time between the birth of an organism and
the birth of the offspring.
Sex Ratio – is the relative proportion of males and females in a population.
Fecundity is the potential reproductive capacity of an individual or
population. Fecundity varies from species to species. Fecundity in a
population can increase or decrease depending on the environmental
conditions and other regulating factors.
Environmental conditions, such as plenty of food, can lead to an increase in
reproductive ability for a species. Climate may also have an impact on
fecundity of an organism.
Populations of sexually reproducing organism also vary in their sex ratio.
The proportion of males to females, the number of females is larger then
males, has a bigger impact because females produce the offspring. In this
situation one male can mate with a number of females. If males form a life
long bond to their mate (geese and swans) the numbers of males and females
can influence reproduction in the population.
The proportion of reproducing individuals in a population is relevant to the
conservation of species in which individuals are rare or widely disperse
across their habitat.
Changes in Population Size
Natality, immigration, mortality, and emigration are used to determine the
change in size of a population in a given time period. The population change
of any population can be calculated mathematically using the following
formula:
Population Change = (birth + immigration) - (death + emigration)
Population growth can occur if the birth and immigration is greater then the
mortality and emigration rate. If it were to be equal there would not be any
population growth, and population will decrease if the mortality and
emigration is greater then the birth and immigration rate.