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Objectives
This blog post provides readers with the following objectives. The reader will be able to:
o Explain the terms, population density, population growth, immigration, emigration,
birth rate and death rate.
o Outline the population sampling techniques
o Identify the factors which affect population size.
o Distinguish between density dependent and density independent factors that affect
population size.
o Explain the concept of biological pest control.
o Outline the advantages and disadvantages of biological pest control.
o Explain the concept of ecological succession.
o Explain how organisms in aquatic and terrestrial habitats are adapted to their habitat
Population Ecology Explained: Insights into Species
Interactions and Growth
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POPULATION DYNAMICS
Population dynamics is the study of short-term and long-term changes in the size and age
composition of populations, and the factors influencing these changes. It deals with the way
populations are affected by birth and death rates, and by immigration and emigration.
Population Size
Population Size the population size (denoted by N) is the number of individual organisms in a
population.
Population Density
Population Density is a measurement of population per unit area or unit volume i.e. the number of
individuals per unit area.
Population Density = Number of individuals
Area
Population Growth
Population Growth is the change in a population over time, or the change in the number of
individuals in a population per unit time. Population growth is determined by four factors, births
(B), deaths (D), immigrants (I), and emigrants (E).
The population growth of a period can be calculated in two parts, natural growth of
population (B-D) and mechanical growth of population (I-E).
Growth can be both positive and negative i.e. growth can be increasing or decreasing. The factors
which limit the growth of population are collectively called environmental resistance and can be
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grouped into abiotic and biotic factors.
Population Growth Rate
Population Growth Rate is the rate at which the number of individuals in a population increases in
a given time period.
Factors Affecting Population Size
o Natural disaster
o Emigration
o Food
o Oxygen
o Disease
o Immigration
o Birth rate
o Competition
o Water
o Death rate
o Predation
o Overcrowding
o Accumulation of toxic waste
o Light availability
Some of these factors are density dependent, others too are density independent
Density-dependent and -independent factors
Density Dependent Factors
They are density dependent because these factors relate to the size or density of the population.
These include predation, epidemic, disease, parasitism, competition, food availability,
accumulation of toxic waste. These factors lower the population and eventually stabilize it at a
particular point.
In some populations, the rate of increase in numbers, the population growth rate, decreases as the
density increases. For example, in studying the dynamics of bacteria or yeast, the population is
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