Transcript Population_ppt 1

Population
• A population consists of all the members of
a particular species that live within an
ecosystem and can potentially interbreed.
Factors that determine the size of
a population
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Births
Deaths
Migration
Organisms join a population through birth
or immigration
• Organisms leave a population through death
or emigration.
Population Equilibrium
Births
Immigration
Deaths
Emigration
Population growth occurs
• When the number of births plus immigrants
exceeds the number of deaths plus
emigrants.
Change in population
• (births – deaths) +(immigrants –emigrants)
=change in population size
In many natural populations, organisms
moving in and out contribute little to
population change, making birth and death
rates the primary factors tht influence
population growth.
Biotic potential
• The maximum rate at which the population
could increase, assuming ideal conditions
that allow a maximum birth rate and
minimum death rate.
Reproductive potential
• The maximum number of offspring that
each member of the population can produce,
is called reproductive potential.
• Reproductive potential increases when
individuals produce more offspring at a
time, reproduce more often, and reproduce
earlier in life.
Reproductive potential contd..
• Small organisms, such as bacteria and insects,
have short generation times. These organisms can
reproduce when they are only a few hours or a few
days old. As a result, their population can grow
quickly.
• In contrast, elephants and humans, become
sexually mature after a number of years. The
human generation time is about 20 years.
Environmental resistance
• Food and space
• Competition with other organisms, and
certain interactions among species, such as
predation and parasitism.
• Natural events such as storms, fires,
freezing weather, floods, and droughts.
How can environmental
resistance affect biotic potential?
• For example drought might kill plants
directly. Drought would also kill animal
populations that rely on these plants by
reducing reproduction and increasing the
number of deaths by starvation.
• The interaction between biotic potential and
environmental resistance usually results in a
balance between population size and
available reosources
“The elephant is reckoned the slowest breeder of all known
animals, and I have taken some pains to estimate its probable
minimum rate of natural increase; it will be safest to assume
that it begins breeding when thirty years old, and goes on
breeding till ninety years old, bringing forth six young in the
interval and surviving till one hundred years old; if this be so,
after a period of from 740-750 years there would be nearly
nineteen million elephants alive descended from the first
pair.”
Charles Darwin
But……. the planet isn’t covered with elephants. What
limits exponential growth?
population size
Set by environmental resistance
K
Set by biotic potential
Time
Biotic Potential Vs.
Environmental Resistance
Factors affecting biotic potential
• The age at which the organism first reproduces.
• The frequency at which reproduction occurs.
• The average number of organisms produced each
time.
• The length of organism’s reproductive life span.
• The death rate of individuals under ideal
conditions.
Growth rate of a population
• The growth rate (r) of a population is a
measure of the change in population size
per individual per unit of time.
• Birth rate – death rate = growth rate
• b-d = r
• Negative growth rate when death rate
exceeds birth rate.
Calculation of growth rate
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Example
Human population -10,000
Birth -1500
Deaths – 500
Calculate the growth rate.
Calculations
• Growth rate r = birth rate b – death rate d
• r=1500/10000 -500/10000 = 1000/10000
• =1/10 =0.10 = 10%
calculation – Population growth
• To determine the number of individuals added to a
population in a given time period, we need to
multiply the growth rate r by the original
population (N)
• Population growth = rN
• In the above problem growth rate =0.10 and the
original population is 10,000 and therefore the
population growth is 0.10x10,000 =1000
Population regulation
• Explain how population sizes in nature are
regulated?
• Discuss with your partner and if you need
help take the help of your text book and
then we can have discussion
Exponential growth
• Populations sometimes undergo exponential
growth, which means they grow faster and faster.
For example, if a pair of dogs give birth to 6
puppies, there will be 6 dogs in one generation. If
each pair in that generation has 6 puppies, there
will be 18 dogs in the next generation. The
following generation will have
54 dogs and so on. If the number of
dogs is plotted versus time on a graph,
the graph will have the shape as shown
Exponential growth: population not limited
by external factors
population size
N2
pop goes to infinity
slow increase
at first
N1
t1
t2
Time
Carrying capacity
Contd..
• The carrying capacity of an ecosystem for a
particular species is the maximum
population that the ecosystem can support
indefinitely. A population may increase
beyond this number, but it cannot stay at
this increased size.
Carrying capacity
• 1. why do you think the number of offspring
increased in the year 1850?
Doubling time
• Doubling time refers to the time it takes a
population to double in size at its current
rate of growth.
• Based on the calculation for exponential
growth, the doubling time can be
determined by dividing the constant 0.693
(about 0.7) by r.
Carrying capacity (K) = upper limit of
population size due to limits of
available resources (space, nutrients,
food, water….)
Density dependent regulation: factors connected to the
population density slow the rate of growth as
population size increases.
population size
resources become limiting
K
rapid (exponential) growth
Time
population size
Called “s-curve” in book
Time
idealized logistic growth
what a real population might do, with
oscillations and time lags
Density independent regulation: factors external to the
population can limit numbers. ex. If a pond dries up, all
fish will probably die, regardless of how many are there.
Another scenario, exponential growth and
crash (J curve)
population size
After a decline………
rebound
Critical number
extinction
Time
Human population estimates
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