Lecture 8 Flashcards
One of the ecological maxims is:
“No
population can increase in size forever.”
Case Study: Human Population
In 1975, the population was growing at
an annual rate of nearly 2%.
At this rate, a population will double in
size every 35 years.
If this growth rate were sustained, we
would reach 32 billion by 2080.
But growth rate has slowed recently, to
about 1.21% per year.
If this rate is maintained, there would be
roughly 16 billion people on Earth in
2080.
Could Earth support 16 billion people?
show how survival and
reproductive rates vary with age, size, or life
cycle stage.
life tables
Information about births and deaths is
essential to __________
predict trends or future
population size.
Life Tables: Sx, x, Fx
Sx = age-specific survival rate—chance
that an individual of age x will survive to
age x + 1.
x, = survivorship—proportion of
individuals that survive from birth (age
0) to age x.
Fx, = fecundity—average number of
offspring produced by a female while she is of age x.
follows the fate of a
group of individuals all born at the same
time (a cohort).
cohort life table
survival and reproduction of
individuals of different ages during a single time period are recorded.
static life table
survivorship varies among human populations
US females VS Gambians
Gambians (born in different seasons –> hunger season with low crop yields from JUly to Oct. had lower survivorship)
is a plot of the
number of individuals from a
hypothetical cohort that will survive to
reach different ages.
survivorship curve
Survivorship curves can be classified into three general types.
Type I: lots of survivors to gradual decrease
Type II: linear decrease (constant decrease)
Type III: very high number of offspring, few age
Life table data can be used to
project the
future age structure, size, and growth rate of
a population.
A population can be characterized by its
“age structure” —
Age structure influences whether a
population will increase or decrease in
size.
the proportion of the
population in each age class.
Age Structure: (3 Growths)
Rapid Growth: more newborns and young people VS old (Guatemala, Nigeria, Saudi Arabia)
Zero Growth: population maintained over time (Spain, Austria, Greece)
Negative Growth: small youth population compared to older; “aging country,” population decline (Germany, Bulgaria, Italy)
Assume the population starts with 100
individuals:
Age class 0 (n0) = 20 individuals
Age class 1 (n1) = 30
Age class 2 (n2) = 50
Two things must be calculated
- Number of individuals that will survive to the
next time period. - Number of newborns those survivors will
produce in the next time period
The growth rate (lambda) can be calculated as the
ratio of the population size in year t + 1 (Nt+1)
to the population size in year t (Nt).
lambda = N at t +1 / N at t
When age-specific survival and fecundity
rates are constant over time, the
population ultimately grows at a _______
The age structure does not change from
one year to the next—it has a
fixed rate.
stable age
distribution.
Any factor that alters survival or fecundity of individuals can change the population
growth rate.
This can be used to develop management practices that decrease pest populations
or increase an endangered population.
Example: Loggerhead sea turtles are threatened by development on nesting sites and commercial fishing nets.
Even if hatchling survival were increased to
100%, loggerhead populations would
continue to decline.
Instead, population growth rate was most
responsive to decreasing mortality of older
juveniles and adults.
Population growth rate
- adult survivorship more important
- shrimping resulted in 5000 to 50 000 loggerhead deaths per year (turtles caught in nets and can’t escape)
Populations can grow exponentially when
conditions are favorable, but exponential growth cannot continue indefinitely.
In general, populations can grow rapidly
whenever individuals leave an average
of more than one offspring over
substantial periods of time.
If a population reproduces in synchrony
at regular time intervals (discrete time
periods), and growth rate remains the
same, ________ occurs
The population increases by a constant
proportion, so the number of individuals
added to the population becomes larger
with each time period.
geometric growth
In many species, individuals do not reproduce in synchrony at discrete time
periods, they reproduce continuously,
and generations can overlap.
When these populations increase by a
constant proportion, the growth is
exponential growth
