Population A population consists of all the members of a particular species that live within an ecosystem and can potentially interbreed.

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

• Births

• Deaths

• Migration

• Organisms join a population through birth or immigration

• Organisms leave a population through death or emigration.

Population Equilibrium

BirthsImmigration

DeathsEmigration

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?

Time

po

pu

lati

on

siz

e

Set by biotic potential

K

Set by environmental resistance

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

• 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

here.

Exponential growth: population not limited by external factors

Time

po

pu

lati

on

siz

e

N1

t1

slow increase at first

t2

N2 pop goes to infinity

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.

Time

po

pu

lati

on

siz

e

rapid (exponential) growth

resources become limiting

K

Time

po

pu

lati

on

siz

e

idealized logistic growth

what a real population might do, with oscillations and time lags

Called “s-curve” in book

tend to be random, unpredictable events, & don’t lead to constant population levels

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)

Time

po

pu

lati

on

siz

e

Critical number

After a decline………

rebound

extinction

Human population estimates

http://www.prb.org/Content/NavigationMenu/PRB/Educators/Human_Population/Population_Growth/Population_Growth.htm