A population is a group of individuals of same species that live in a given geographical area, share or compete for similar resources and potentially reproduce.
Ex: All the cormorants in a wetland, rats in an abandoned dwelling, teak-wood trees in a forest tract, bacteria in a culture plate and lotus plants in a pond etc.
Population ecology is an important area of ecology as it links ecology to population genetics and evolution.
Population Attributes
Birth rates: Refer to per capita births.
E.g.: In a pond, there are 20 lotus plants last year and through reproduction 8 new plants are added. Hence, the current population = 28
The birth rate = 8/20 = 0.4 offspring per lotus per year.
Death rates: Refer to per capita deaths.
Ex: 4 individuals in a laboratory population of 40 fruit flies died during a week.
Hence, the death rate = 4/40 = 0.1 individuals per fruit fly per week.
Sex ratio: A population has a sex ratio.
Ex: 60% of the population is females and 40% males.
Age pyramid: It is the structure obtained when the age distribution (% individuals of a given age or age group) is plotted for the population.
For human population, age pyramids generally show age distribution of males and females in a combined diagram.
Population size or population density (N): It is the number of individuals of a species per unit area or volume.
Ex: population density of Siberian cranes at Bharatpur wetlands in any year is <10. It is millions for Chlamydomonas in a pond.
Population Growth
$\displaystyle \small \bullet$ The size of population is not static. It keeps changing with time, depending upon food availability, predation pressure and adverse weather.
$\displaystyle \small \bullet$ The main factors that determine the population growth are
Natality (B): It is the number of births in a population during a given period.
Mortality (D): It is the number of deaths in a population during a given period.
Immigration (I): It is the number of individuals of the same species that have come into the habitat from elsewhere during a given time period.
Emigration (E): It is the number of individuals of the population who left the habitat and gone elsewhere during a given time period.
$\displaystyle \small \bullet$ Natality and immigration increase the population density and mortality and emigration decrease the population density.
$\displaystyle \small \bullet$ If N is the population density at time t, then its density at time t +1 is
$\displaystyle \small N_{t+1}=N_{t}+[(B+I)-(D+E)]$
$\displaystyle \small \bullet$ Population density increases if B+I is more than D+E. Otherwise it will decrease.
$\displaystyle \small \bullet$ Under normal conditions, births and deaths are important factors influencing population density. Other 2 factors have importance only under special conditions.
$\displaystyle \small \bullet$ E.g. for a new colonizing habitat, immigration may be more significant to population growth than birth rates.
Growth model
Growth of population takes place according to availability of food, habit condition and presence of other biotic and abiotic factors.
Types of models
Exponential Growth
$\displaystyle \small \bullet$ In this kinds, growth occurs when food and space is available in sufficient amount.
$\displaystyle \small \bullet$ When resources in the habitat are unlimited, each species has the ability to release fully its innate potential to grow in number.
$\displaystyle \small \bullet$ The population grows in an exponential or geometric fashion.
$\displaystyle \small \bullet$ If in a population of size N, the birth rates as represented as ‘b’ and death rate as‘d’. Then increase and decrease in N during unit period time ‘t’ will be
dN / dt = (b – d) × N
Let (b – d) = r, then
dN / dt = rN
$\displaystyle \small \bullet$ The r in this equation is called ‘intrinsic rate of natural increase’.
$\displaystyle \small \bullet$ The r is an important parameter for assessing impacts of any biotic or abiotic factor on population growth.
r value for the Norway rat = 0.015
r value for the flour beetle = 0.12
r value for human population in India (1981) = 0.0205
The integral form of the exponential growth equation is $\displaystyle \small N_{t}=N_{0}e^{rt}$
where,
$\displaystyle \small N_{t}$ = Population density after time t
$\displaystyle \small N_{0}$ = Population density at time zero
r = intrinsic rate of natural increase
e = the base of natural logarithms (2.71828)
Logistic growth
$\displaystyle \small \bullet$ There is no population in nature having unlimited resources for exponential growth. This leads to competition among individuals for limited resources.
$\displaystyle \small \bullet$ Eventually, the ‘fittest’ individuals survive and reproduce.
$\displaystyle \small \bullet$ In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. It is called carrying capacity (K).
$\displaystyle \small \bullet$ A population with limited resources shows initially a lag phase, phases of acceleration and deceleration and finally an asymptote. This type of population growth is called Verhulst-Pearl Logistic Growth.
It is described by following equation:
$\displaystyle \small dN/dt=rN\left ( \frac{K-N}{K} \right )$
where,
N = Population density at time t
r = Intrinsic rate of natural increase
K = Carrying capacity
$\displaystyle \small \bullet$ Since resources for growth for most animal populations are finite, the logistic growth model is more realistic one.
Ex: All the cormorants in a wetland, rats in an abandoned dwelling, teak-wood trees in a forest tract, bacteria in a culture plate and lotus plants in a pond etc.
Population ecology is an important area of ecology as it links ecology to population genetics and evolution.
Population Attributes
Birth rates: Refer to per capita births.
E.g.: In a pond, there are 20 lotus plants last year and through reproduction 8 new plants are added. Hence, the current population = 28
The birth rate = 8/20 = 0.4 offspring per lotus per year.
Death rates: Refer to per capita deaths.
Ex: 4 individuals in a laboratory population of 40 fruit flies died during a week.
Hence, the death rate = 4/40 = 0.1 individuals per fruit fly per week.
Sex ratio: A population has a sex ratio.
Ex: 60% of the population is females and 40% males.
Age pyramid: It is the structure obtained when the age distribution (% individuals of a given age or age group) is plotted for the population.
For human population, age pyramids generally show age distribution of males and females in a combined diagram.
Population size or population density (N): It is the number of individuals of a species per unit area or volume.
Ex: population density of Siberian cranes at Bharatpur wetlands in any year is <10. It is millions for Chlamydomonas in a pond.
Population Growth
$\displaystyle \small \bullet$ The size of population is not static. It keeps changing with time, depending upon food availability, predation pressure and adverse weather.
$\displaystyle \small \bullet$ The main factors that determine the population growth are
Natality (B): It is the number of births in a population during a given period.
Mortality (D): It is the number of deaths in a population during a given period.
Immigration (I): It is the number of individuals of the same species that have come into the habitat from elsewhere during a given time period.
Emigration (E): It is the number of individuals of the population who left the habitat and gone elsewhere during a given time period.
$\displaystyle \small \bullet$ Natality and immigration increase the population density and mortality and emigration decrease the population density.
$\displaystyle \small \bullet$ If N is the population density at time t, then its density at time t +1 is
$\displaystyle \small N_{t+1}=N_{t}+[(B+I)-(D+E)]$
$\displaystyle \small \bullet$ Population density increases if B+I is more than D+E. Otherwise it will decrease.
$\displaystyle \small \bullet$ Under normal conditions, births and deaths are important factors influencing population density. Other 2 factors have importance only under special conditions.
$\displaystyle \small \bullet$ E.g. for a new colonizing habitat, immigration may be more significant to population growth than birth rates.
Growth model
Growth of population takes place according to availability of food, habit condition and presence of other biotic and abiotic factors.
Types of models
Exponential Growth
$\displaystyle \small \bullet$ In this kinds, growth occurs when food and space is available in sufficient amount.
$\displaystyle \small \bullet$ When resources in the habitat are unlimited, each species has the ability to release fully its innate potential to grow in number.
$\displaystyle \small \bullet$ The population grows in an exponential or geometric fashion.
$\displaystyle \small \bullet$ If in a population of size N, the birth rates as represented as ‘b’ and death rate as‘d’. Then increase and decrease in N during unit period time ‘t’ will be
dN / dt = (b – d) × N
Let (b – d) = r, then
dN / dt = rN
$\displaystyle \small \bullet$ The r in this equation is called ‘intrinsic rate of natural increase’.
$\displaystyle \small \bullet$ The r is an important parameter for assessing impacts of any biotic or abiotic factor on population growth.
r value for the Norway rat = 0.015
r value for the flour beetle = 0.12
r value for human population in India (1981) = 0.0205
The integral form of the exponential growth equation is $\displaystyle \small N_{t}=N_{0}e^{rt}$
where,
$\displaystyle \small N_{t}$ = Population density after time t
$\displaystyle \small N_{0}$ = Population density at time zero
r = intrinsic rate of natural increase
e = the base of natural logarithms (2.71828)
Logistic growth
$\displaystyle \small \bullet$ There is no population in nature having unlimited resources for exponential growth. This leads to competition among individuals for limited resources.
$\displaystyle \small \bullet$ Eventually, the ‘fittest’ individuals survive and reproduce.
$\displaystyle \small \bullet$ In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. It is called carrying capacity (K).
$\displaystyle \small \bullet$ A population with limited resources shows initially a lag phase, phases of acceleration and deceleration and finally an asymptote. This type of population growth is called Verhulst-Pearl Logistic Growth.
It is described by following equation:
$\displaystyle \small dN/dt=rN\left ( \frac{K-N}{K} \right )$
where,
N = Population density at time t
r = Intrinsic rate of natural increase
K = Carrying capacity
$\displaystyle \small \bullet$ Since resources for growth for most animal populations are finite, the logistic growth model is more realistic one.
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