Ecology and Sustainable Development
Hand-outs
Nurmukhanbetova G.A.
Cand.Sc.(Biology)
Topic 3: Demecology – the ecology of populations
The main idea: Note how mathematical models are used to examine variation in growth of a population.
Lecture outline:
1. Statistic and dynamic characteristics of population.
2. Growth curves patterns: J-shaped curve and S-shaped curve
3. Population regulation: Density-dependent and density-independent factors.
4. Human population patterns:
- Population numbers.
- Demographic transition and structure
- Population urbanization
1. Statistic and dynamic characteristics of population
Population – is a group of individuals of the same species that live in the same area.
Population is an elementary and evolutionary unit of biosphere.
Statistic characteristics:
 Population Number - number of individuals in a given population.
 Population Density – the number of individuals in a population in given area.
Population density = 100 fish / 1.000m3 of water = 1fish/ 10 m3 of water or 10 plants/ 1m2
Dynamic characteristics:
 Population growth – an increase in a population at a given time.
 Population growth rate – the change in the number of individuals in a population over time
2. Growth Curves Patterns:
Growth Curves – a graph showing the number of individuals in a population over time.
 The J-shaped curve – a growth curve that shows population growth which occurs indefinitely at a constant
reproductive rate.
The J-shaped curve presents two phases of population growth:
Lag-phase: little or no increase occurs in a population.
Exponential phase: an increase occurs in a population so rapidly that the number of individuals doubles in a specific
time interval and keeps doubling in increasingly shorter periods of time.
The J-shaped curve is also called as Exponential curve.
Biotic potential – the maximum rate at which a population could increase under ideal conditions – an environment
with unlimited resources. The biotic potential is rarely achieved by population in natural environment.
 The S-shaped curve – a growth curve that shows the period of relative stability in a population that occurs
because of environmental resistance (after its lag and exponential phases).
Carrying capacity – the largest population that a particular environment can support sustainably (long-term),
assuming there are no changes in that environment.
Environmental resistance – unfavorable environmental conditions that prevent organisms from reproducing
indefinitely at their biotic potential.
Factors that tend to reduce population growth rates: limits of food, light, space, oxygen etc.
Notice that the J-shaped curve is the lower part of the S-shaped curve!
J-shaped curve (Exponential curve)
Note that a population grows slowly during the
early lag phase and then very rapidly during the
exponential phase.
S-shaped curve (Sigmoidal or Logistic curve)
shows that population growth levels off after the
exponential phase. The top of this curve indicates
the carrying capacity of an ecosystem.
Ecology and Sustainable Development
Hand-outs
Nurmukhanbetova G.A.
Cand.Sc.(Biology)
Survivorship rate – a number of individuals survived over specific period of time.
Types of bell curves of survivorship rate: strait diagonal, convex, concave.
Population dynamics – periodical or not periodical changes in number, sex and age of a population affected by
abiotic and biotic factors (stable, changeable, explosive).
3. Population Regulation:
 Density-dependent factors – are factors that affect populations (reproduction and mortality rates) in different
ways depending on population density (availability of food, space, light, number of predators, parasitic
infections and disease etc.). Abiotic factors are sometimes density-dependent (space, light, oxygen).
 Density-independent factors –are factors that affect populations (reproduction and mortality rates) regardless
of population density (natural disasters as forest fire, earthquake, flood etc.).
4. Human population patterns:
 Population numbers
In 1650 ~ 500 million people.
In 2000 ~ 6 billion people.
In 2009 ~ 6.5 billion people.
This exponential growth rate can be explained by
increase:
 in food production

the rise of industry
 advances in medicine
Growth rate is about 70 million people per year. It
may be limited by the same density-dependent
factors that limit the growth of other populations –
unsustainable use of natural resources.
The growth curve of human population
Domestication – is the process in that plants and
animals are adapted or trained to live in a human
environment.
Domestication gave people a more reliable and
constant supply of food, which was one important
factor in determining human population size.
 Demographic transition – the process whereby a country moves from relatively high birth and death rates to
relatively low birth and death rates due to improved living conditions, that usually accompanies economic
development.
Stage 1 – Preindustrial: high birth and death rates, so population grows very slowly.
Stage 2 – Transitional – high birth rate and lowered death rate, so population grows very rapidly.
Stage 3 – Industrial – a decline in birth rate and low death rate, so slow population growth occurs. In developed
countries low fertility rate is due to high female literacy.
Stage 4 – Postindustrial – a population grows very slowly or not at all.
 Demographic structure
Demographics – the study of the factors that affect rates of birth, death, and growth in populations.
In making predictions about a population’s growth, it is necessary to know its age structure.
Population Age Structure – a distribution of population number by age accounting individual’s sex (Pyramid-like
diagram).
Population pyramids – the age structure diagram that is illustrated as a pyramid with the length of each tier
showing the number of males (left side) and number of females (right side) in a particular age group of individuals.
The overall shape of an age structure diagram indicates whether the population is increasing (Nigeria, Brazil), stable
(many European countries), or shrinking (Russia, Bulgaria, Germany).
Age groups of individuals:
- Pre-reproductive (0 – 14 years)
- Reproductive (15 – 44 years)
- Post-reproductive (45 years and older)
Age structure pyramid
Ecology and Sustainable Development
Hand-outs
Nurmukhanbetova G.A.
Cand.Sc.(Biology)
Replacement level fertility – the number of
children a couple must produce to “replace”
themselves (=2.1).
Total fertility rate (TFR) – the average
number of children born to each woman
(currently TFR=2.7).
A population that reduces its fertility rate to 2.1
children will continue to grow for another 30 to
50 years. This phenomenon, called population
growth momentum. It occurs because of the
large number of pre-reproductive and
reproductive individuals that exist in the
population.
Projection about world population (N) in 2050:
If TFR =2.5, N=10.9 (UNO high projection)
Stable
Growing
If TFR =1.5, N=7.9 (UN0 low projection).
Birth rate (Natality) – number of births (per thousand person) in any given year (or % per year) (B = dNn/dt)
Death rate (Mortality) – number of deaths (per thousand person) in any given year (or % per year) (M = dNm/dt)
Infant mortality rate – the number of infant deaths under age 1 per 1000 live births.
Natural increase of a population = Birth rate – Death rate
Total growth rate differs from the natural increase as it includes: immigration and emigration, births and deaths.
 Population urbanization
Urbanization – the process whereby people move from rural areas to densely populated cities.
In developing nations, most people live in rural settings, but their rates of urbanization are rapidly increasing. Rapid
urbanization makes it difficult to provide city dwellers with basic services such as housing, water, sewage, and
transportation systems.
Key terms:
Demecology
Population
Growth rate
Population Number
Population Density
Demography Structure
Population growth
Population growth rate
Biotic potential
Carrying capacity
J-shaped curve
S-shaped curve
Environmental resistance
Density-dependent factors
Density-independent factors
Demographic transition
Replacement level
(2.1 children)
Urbanization
Questions for review:
1. Name statistic and dynamic characteristics of population.
2. In what ways do J-shaped and S-shaped growth curves differ?
3. What is biotic potential and carrying capacity of an ecosystem?
4. How do biotic potential and/or carrying capacity produce the J-shaped and S-shaped population growth curves?
5. What is the difference between density-dependent and density-independent factors? How are they similar?
6. What is the population of the world?
7. Characterize four stages of the demographic transition.
Home task: Make a synopsis: Daniel D. Chiras, “Environmental Science: Creating Sustainable Future”
1. Draw the graph on Demographic transition (p.170).
2. Complete two tables and analyze the data on Population control strategies (p.176 and p.177).
Critical Thinking (Writing an essay):
1. Thomas Malthus in his essay “On population” (1798) argued that human population would be double every 25
years if natural resources were unlimited. How did this observation help Charles Darwin to form his theory of
evolution by natural selection?
2. Humans have tried to control insects with insecticides, such as DDT. Although DDT was effective for the first
ten years it was used, the insect population began to increase again. What caused DDT to lose its effectiveness?
3. Although population growth in some nations has slowed to nearly a standstill, the world population is still
increasing at an exponential rate. What might account for this rapid population growth?
4. Suppose that you want to predict the growth patterns for three small villages, all with the same number of
people (the hypothetical example). Draw the graphs and do the assumptions:
 One made up of families with children, parents, and grandparents – a population of mixed age and sex, the
second is made up solely of children, the third is made up of elderly people.
5. To predict accurately the growth pattern of a human population, why is it not enough to just consider human
numbers? What else must be considered?
6. How is human population growth related to natural resource depletion and environmental degradation?
7. Discuss some of the ethical issues associated with overpopulation. Is it ethical to have more than two children?
Is it ethical to consume so much in the way of material possessions? Is it ethical to try to influence a couple’s
decision about family size?