Unit Six

Population and natural resources

6.1 Population ecology
 Ecology is the study of the interaction between those diverse groups of organisms and the physical environment.
 The levels of ecological organization include individual, population,community, ecosystem, biome and biosphere
 A population is a group of interacting individuals of the same species with common characteristics living and
interbreeding within a given area.
 Population ecology is the study of the processes of interaction and changes that affect the distribution and abundance
of populations in the environment.
6.1.1 Population size, density and dispersal
Population size
 It is the total number of individuals present in a particular habitat where it is designated by the capital letter "N"
 In studying populations, locating and counting each individual is very difficult.
 Scientists use sampling technique and count individuals in the sample area and infer the total population to estimate
the larger population in that particular habitat.
 Different sampling methods can be used to determine the size and density of a population of organisms.
1) Quadrat method:commonly used for plant
2) Mark-recapture method: commonly used for animal
 In studying the human population, a population census is conducted once every 10 years at least.
1) Studying populations using the Quadrat method
 It is a method used to study immobile organisms such as plants and small, slow-moving organisms.
 A quadrat is a square made from sticks and string or by usinga wood, plastic, or metal square and placed on the
ground to count organisms in each square.
 Many quadrats are located randomly throughout the habitat at several locations, to make sure that the recorded
numbers are representative for the overall habitat. Then, counted numbers from each quadrat can be used to
estimate the population size and population dnsity within the entire habitat
Example: A student wanted to know how many plants there were in a 50 ㎡ field and threw many quadrats to
cover 10 ㎡ and counted 50 plants. The student used the equation above to calculate the estimated number of
plants in the whole field (50㎡).
2) Populations Estimation by Mark-recapture method
 It is a method used to determine the population size of animals that move from one place to another.
 The procedure for this method is:
1) Capture a sample of animals, mark them with tags, bands, paint, or other body markings,and release them
back into the environment, allowing them to join with the rest of the population.
2) Capture a sample of animals that includes both marked and unmarked individuals.
3) Estimate how many individuals there are in the total population using the ratio of marked to unmarked
individuals.
Example- Let us assume that researchers captured 60 birds, marked and released them back into the forest. After
some time, they came back and captured another 100 birds. Of the birds that were captured for the second time,
they found that 20 were already marked and 80 were unmarked. Using this data, they can estimate population
size as follows:

Studying the human population

 The human population can be studied using a census.
 A census is a systematic collection of information from all units or individuals in the population, or a complete
enumeration of the members of a given population.
 It involves the official counting of a population in the territory of a country and the collection of information on
selected demographic and socioeconomic characteristics of the population in terms of age, sex, ethnic group, religion,
marital status, household size and structure, occupation, economic activity status, etc.
 Data obtained from the population census can be used to calculate measures of population size,density, age, sex, birth
rate, death rate,fertility rate, life expectancy, population growth rate

Biology handout for grade 11. Unit 6 Set by Ambaw F. 2016E.C. 1

  There are two methods of conducting a census. These are:
A. De Facto Method of Census:
 In which the government fixes one date for conducting the census throughout the country
 It is usually done on a full-moon night because it is presumed that all households are present at their
residences during that time.
B. De Jure Method of Census
 The government fixes the enumeration period of two or three weeks. hThe enumerators collect information
from households by visiting them frequently to fill in the required information. Persons residing temporarily
at a place are not counted, whereas only persons residing permanently at aplace are counted in it.
Population density
 It is the number of individuals within a specific area or volume,i.e., population size divided by the total land area.
 It is the measurement of the size of a population as a percentage of the total land area occupied by the people. For
example, the number of people per 10 square kilometers is a population density. Similar to all properties of a
population,
 Density is a dynamic characteristic that changes over time as individuals are added to or removed from the
population. For instance, birth and immigration can increase a population's density, whereas death and emigration can
decrease it.
Population dispersal
 Species dispersal patterns refer to how the individuals in a population are distributed in space at a given time.
 There are three dispersal patterns of species. These are:
1) Uniform dispersion: individuals of a population are spaced more or less evenly.
2) Random dispersion: individuals are distributed randomly, without a predictable pattern.
3) Clumped dispersion- individuals are clustered in groups.
 The dispersal pattern of individuals in a population provides more information about how they interact with each
other and with their environment than a simple density measurement.

6.1.2 Exponential and logistic growth in populations

 Population growth is the increase in the number of humans on Earth.
 The global human population as well as population of Ethiopia has been increasing rapidly,and has continued to
struggle for energy, food, water,and medical care availability.
 Population growth rate
 It is the percentage rate of change within a specified number of individuals in a population.
 It is the average annual rate of change of population size during a specified period.
 It measures how fast the size of the population is changing.
 In determining population growth, it is important to consider the death and birth of the organism. Thus, growth rate is
determined by subtracting the death rate (D)from the birth rate (B). Considering death and birth rate as single factor
(r), growth rate can be expressed in a simple equation as:
 Scientists use different methods and models to describe changes that occur in a population thereby predict future
changes. The two models are:
1) Exponential growth model describes populations increase in numbers without any limits to their growth.
2) Logistic growth model describes populations' increases in number in the presence of limits, and due to an
increase in number.
Exponential growth model
 It represents the growth of a population without environmental constraints in which the population size (N)increases
exponentially.
 It shows the growth of a population in an ideal environment with unlimited resources to use.
 The population growth graph of this model shows a J-shaped curve.
dN/dT = rN Where dN/dT = growth rate of the population in a given instance, d = change,
N =population size, t = time, and r = the per capita (per individual) rate of increase,
 The value "r" can be positive, negative, or zero, i.e., the population growth rate (dN/dt)is proportional to r.
 r>0: increase
 r= 0:no change
 r<0:decrease
Biology handout for grade 11. Unit 6 Set by Ambaw F. 2016E.C. 2
  Exponential growth model is used when the per capita rate of increase (r) takes the same positive value regardless of
the population size.
Logistic growth model
 It is used when the per capita rate of increase (r)decreases as the population increases towards a maximum limit.
 The population graph of this model is resulting in S-shaped curve
 The maximum population size that the available resources in an environment can sustain/support is called carrying
capacity (K).
Examples
1. The following data shows the exponential population growth rate for ten generations with r =1. Draw the graph.
Calculate the exponential growth rate for each generations and draw the figure when r = 0.6 (you can use excel to
draw).dN/dt=ΔN=r N1,N
Generation Ni (Population Size) rN N
1 100 100 200
2 200 200 400
3 400 400 800
4 800 800 1600

2. The following data shows a sample of the logistic population growth rate for ten generations with r =1 and K =
250. Draw the graph. Calculate the logistic growth rate for each generations and draw the figure when r= 0.6. 0.9, 1.4
and 2 (K-N- and K = 100, 150, 200
and Generation Ni (Population Size) rN
N)/K
AN Nf 300(you can use excel
to draw). dN/dt =ΔN=r
1 100 100 0.6 60 160
Ni(K-N/K),
2 160 160 0.36 58 218
3 218 218 0.13 28 246
4 246 246 0.02 4 250
5 250 250 0.00 0 250

6.1.3 Demographic structure

 Population size and density describe a population at one particular point in time, whereas demography describes the
dynamics of a population.
 Demographic parameters that scientists use include population size, crude death rate, population growth rate,
population density, infant mortality rate, fertility rate, crude birth rate, migration, and life expectancy.
 Crude birth rate is the number of live births per 1000 people in a year.
 Crude death rate is the yearly annual number of deaths per 1,000 people.
 Infant mortality rates refer to the annual number of deaths of children aged less than a year old per 1000 of those
born alive.
 Life expectancy is a measure that interests many people. It refers to the number of years that a human being can
live in relation to the current mortality rates.
 Fertility rate is a measure of the number of children born.
 Mortality rate is a measure of the number of people who die.
 Migration means the movement of people from one area to another area
 Immigration-movement of people into an area to take up permanent residence
 Emigration refers to the movement of people out of an area to another place of permanent residence.
 In demography, age and sex are the most commonly studied elements of population composition.It is the number of
individuals in terms of sex and age in the population.

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  The age-sex structure is an important factor that influences population growth. This is because younger people are
more likely to reproduce, whereas older people have higherrates of dying.

Population pyramid
 It is a graphical representation of the age and sex composition of a specific population.
 The population pyramid represents the age-sex structure of a population and states the complex social narrative of a
population through its shape.
 Demographers use these simple graphs to evaluate the extent of development for a given population in a country and
to make predictions about the types of services that population will need, such as schools, hospitals, homes, etc.
 A population pyramid is popularly known as an "age pyramid" or "age and sex pyramid."
 There are three types of population pyramids as shown in Figure.
a) Expansive population pyramids
 They are used to describe populations that are young and growing.
 The pyramid' shape has a broad base and a narrow top. Expansive population pyramids show the presence of
a larger percentage of the population in the younger age group.
 Populations with this shape usually have high fertility rates with lower life expectancies.
Eg Many third world countries including Ehiopia, kenya and other has expansive
b) Constrictive population pyramids
 They are used to describe populations that are elderly and shrinking.
 They have smaller percentages of people in the younger age group.
 Countries with higher levels of economic development have constrictive population pyramids because access
to quality education and health care. Eg. USA and other
c) Stationary population pyramids
 They are used to describe populations that are not growing.
 They show an almost equal proportion of the population in each age group in that it tapers off at the top.
 There is not a decrease or increase in population; it is stable.
 Developed countries, where birth rates are low and overall quality of life is high. Eg. Italy
Population pyramids in Ethiopia
 The population of Ethiopia was 42.6 million in 1984, 53.5 million in 1994, 73.5 million in 2007 and 83.7 million in
2012 and is currently estimated to be more than 110 million. (EPHS, 2014)
 Ethiopia has a younger population, with 45 percent of its population under the age of 15 years and only 4.8 percent of
its population aged 60 years or older. (NPC, 2007)
 Women between the ages of 15 and 49 account for 23% of the total population.
Survivorship Curves
 A survivorship curve is a graph of the number of individuals surviving at each age interval versus time.
 There are three types of survivorship curves.
a) Type I
 Organisms produce relatively few offspring and provide a lot of care to the offspring, increasing the
likelihood of their survival.
 As a result, most of the offspring survive to adulthood, so they can reproduce.While
 Mortality is low in the early and middle years, it occurs mostly in older individuals.
Eg. Humans and most mammals.
b) Type II
 Organisms produce moderate numbers of offspring and provide some of these organisms survive their
younger years. Eg. some birds
 The death /mortality rate is relatively constant throughout the entire life span,
c) Type III
 Organisms produce many offspring but provide them with little or no care. As a result,
 Relatively few offspring survive to adulthood.
 Mortality is highest at early ages, but it is lower at later ages.
Eg. plants, invertebrates, and many species of fish
6.1.4 Population regulation
 There are two resistances to population growth.
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 1) Density-dependent factors
 They refer to the density of the population at a given time that affects its growth rate.
 They are living (biotic) factors that affect a population.
 They include predation, competition (interspecific and intraspecific), accumulation of waste, and diseases
2) Density-independent factors
 They are factors that influence the growth rate of the population regardless of its population density
 They are non-living (abiotic) factors that regulate the population growth.
 They include weather, natural disasters, and pollution

6.2 Natural resources

 Natural resources refer to any natural substances or materials that are available naturally in the envionment and that
are used by human beings.
 They include oil, coal, natural gas, metals, stone, air,sunlight, soil, water, animals, birds, fish, and plants.
 Natural resources are used to make food, fuel, our clothes, cars, televisions, computers, and refrigerators,which
provide us with heat, light, and power.
 Natural resources can be classified in various ways as renewable and non-renewable, or inexhaustible and
exhaustible, respectively.
a) Renewable natural resources
 They are resources that can be replaced after utilization. Resources that cannot be exhausted even after
continuous utilization are termed renewable resources..
 They are available continuously and their quantity is not noticeably affected by human consumption.
However, renewable resources
 They do not have a rapid recovery rate and are susceptible to depletion if they are overused.
Eg. sunlight, air, trees, water, wind, hydropower, solar and wind energy, biomass energy,
b) Non-renewable natural resources
 They are is a natural substance that is not replenished with the speed at which it is consumed.
 These resources may take millions of years to form and replenish.
Eg. minerals, fossil fuels, coal, and natural gas.
6.3 Conservation of natural resources in Ethiopia
 Most of the natural resources are limited.
 Conservation is the care, protection, and wise use of natural resources so that the resources can be used for future
generations.
 Conservation is also the preservation, management, or restoration of natural environments and the ecological
communities habituated to human beings.
 Conservation aims to manage human use of natural resources for the current public benefit and sustainable social and
economic utilization.
 Ethiopia has a wide range of natural resources such as plants, animals, water, and soil.
VWildlife in Ethiopia
 Ethiopia has huge wildlife resource potential that has national and global importance.
 Wildlife refers to living things that live in the natural environment and are not yet domesticated.
 Importance of wildlife resources: they are used to maintain natural ecological processes, store genetic material for the
future, secure wildlife tourism, and contribute to the national economy, and manufacture goods. Examples of wildlife
in Ethiopia are shown in the figure below
Eg. Walia Ibex, and the Ethiopian Mountain Chameleon.
 Loss and decline of wildlife is caused by
o Natural disasters such as earthquakes, floods, droughts, tornadoes and wild fire are the major causes of loss
and decline of wild life.
o Human activities such as expansion of agricultural activities, development activities, deforestation, poaching,
illegal wildlife trade, habitat change, climate change, overexploitation, and pollution.
 Conservation mechanisms
 Wildlife conservation is the practice of protecting wildlife and their habitats.
 Wildlife conservation in Ethiopia has been practiced based on wildlife conservation areas.

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  Ethiopia has established 24 national parks, 2 wild sanctuaries, 6 wild life reserves, 5 community conservation
areas and more than 10 controlled hunting areas,
Plants in Ethiopia
 Ethiopia have enabled various plant species to survve.
 Plant species in Ethiopia are diverse and have a rich endemic element.
 Ethiopia is considered a centre for crop genetic diversity.
 Plant species are being lost and declned due to natural and manmade activities.
Eg. deforestation, climate change, invasive species, overexploitation, pollution and land degradation
 Conservation mechanisms: conservation mechanisms for plant species in Ethiopia include
i. Indigenous practices or cultural practices: Taboos or forbidden practices for using or consuming some plant
species; domestication; rserves; selective harvesting; sacred groves, etc.
ii. Modern practices: practices used to establish botanical gardens, control invasive species, recover, restore, and
preserve genes in a bank.
Soil and water in Ethiopia
 The major causes of soil erosion and the decline in quantity and quality of water are human activities such as
deforestation, poor land husbandry practices, and wastes emitted from industries and homes.
 Conservation of soil and water is mandatory.
 Conservation mechanisms: soil and water conservation measures involve both mechanical and biological measures.
a) Mechanical measures: include constructing bunds, terraces, diversion ditches, check dams, micro-basins, and
hillside terraces.
b) Biological measures: include enclosing degraded land from human and animal interferences; tree seedling
production; planting of tree seedlings on farmlands (agro-forestry); afforestation.
6.4 Impact of traffic accident on wild and domestic animals
 Car accidents are the major cause of reductions in wildlife populations and other.
 Mortality would be high in other national parks and other wider areas where there are a number of crossing roads.
 Some of the methods to reduce traffic accidents on both wildlife and domestic animals include designing structures
that allow safe passage for animals and wildlife, constructing underpasses to promote habitat connectivity and
encourage natural movements, and considering natural wildlife habitats when roads are constructed and deriving
slowly.
6.5 Impact of human activities on the environment
 The following is a list of the major activities that affect the environment.
 Burning of coal, natural gas
 Using refrigerants and coolants
 Applying pesticides
 Waste incineration and waste disposal
 Using various volatile chemicals
 Agricultural activities
 Industrial and other sewage discharges
 Construction of infrastructure and transportation
 Urban development  Mining activities
6.5.1 Environmental pollution
 It is the contamination of the components of the environment that adversely affect the normal environmental
processes.
 Environmental pollution can be caused by natural events such as forest fires and active volcanoes, but mainly by
human activities.
 Types of environmental pollution: The major types of pollution are air pollution, water pollution, land pollution,
noise pollution, and light pollution.
 Pollution of all kinds can have negative effects on the environment and wildlife that in turn have impacts on the
health and wellbeing of human beings.
Air Pollution and its Effects
 It is the contamination of air in atmosphere by harmful substance.
 Air pollution is caused by air pollutants being added into the environment at rates that exceed the natural capacity
of the environment to dissipate and dilute or absorb them.
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  The major air pollutants are carbon monoxide, nitrogen dioxide, ozone, lead, sulfur dioxide, smog and particles.
 The human activity that released pollutants are transportation, industrial activities, the burning of coal and natural
gas, the use of refrigerants and the use of various volatile chemicals.
 Air pollution is the leading environmental health risk and a major cause of environmental degradation in the
world.
 Water Pollution and its effect
 It is the contamination of water bodies. It interferes with the beneficial use of water. For instance, the presence of
an excessive
 The major causes of water pollution are usually human activities such as factories, industrial
agricultural wastes, sewage, transportation of oil, use of chemical fertilizers and insecticides,
herbicides, pathogenic microorganisms, and Water hyacinth (Emboch)
 Water pollution is the major cause of the depletion of fish populations, the destruction of biodiversity,
and waterborne diseases that lead to death.
6.5.2 Climate change
 Climate is the usual weather of a place that can vary for different seasons.
 Climate change is a change in the usual patterns of weather.
 Climate change can affect living organisms in the environment.
 Human activities can change the climate by changing the natural components of the environment.
6.5.3 Global warming
 It is an average rapid rise in the earth's temperature.
 The major causes of global warming are greenhouse gases released from human activities such as burning fossil fuels
and industries.
 It has an effect on living things. Some of the effects include flooding, melting of ice caps,rising oceans, loss of
species, health problems.
6.5.4 Ozone layer depletion
 One of the major problems caused by air pollution is the depletion of the ozone layer.
 Ozone (O3) is made of three oxygen atoms and it is a toxic gas.
 The atmosphere has four layers. These are the troposphere, stratosphere,mesosphere, and thermosphere.
 Ozone is found in the stratosphere of the atmosphere.
 The ozone layer protects the earth from these harmful radiations (UV rays). It absorbs the radiation from the sun,
thereby preventing it from entering the earth's atmosphere.
 The major cause of ozone layer depletion is the release of CFCs (chlorofluorocarbons), which are used in
refrigerators and fire extinguishers, hydrofluorocarbons (HCFCs), and halons.
 The process is as follows:
1) chlorofluorocarbons (CFCs) released
2) CFCs rise into ozone layer in the stratosphere, which contains ozone (O3)
3) UV radiation releases chlorine (Cl) from CFCs
4) Cl destroys ozone (Cl + O3 ---> ClO + O2 )
5) Depleted ozone allows more UV radiation to pass through the atmosphere, and (6) more UV radiation causes
more skin cancer
 Effects ozone layer depletion: it exposes to UV rays that can lead to:
 Skin cancer and cataracts and weaken the immune system response.
 Ageing of the skin, making one look much older than he/she actually is.
 Cause respiratory diseases such as chest pain, difficulty in breathing, or even throat irritation.
 Affects other living beings, such as terrestrial and aquatic animals and plants.
 It also Increases the formation of ground-level ozone (smog)
6.5.5 Acid rain
 It is rain with more acidic content (lower pH value) than natural rain.
 It is caused by high concentrations of acid-forming pollutants that dissolve in freshly condensed water vapor in the
atmosphere.

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  The major causes of acid rain are the burning of fossil fuels, electric power-generating facilities, industrial processes,
exhausts emitted from the internal combustion engines that serve as sources of sulfur dioxide and nitrogen oxides,
etc.
 The burning of fossil fuels (coal or oil) for energy contains sulfur and nitrogen, which combine with oxygen to form
sulfur dioxide (SO2), and nitrogen dioxide, which contribute to the acidity of rain. Moreover, carbon dioxide (CO2)
in the atmosphere makes rain slightly acidic. This is because carbon dioxide and water combine to form carbonic
acid, commonly known as carbonated water.
 Effects of acid rain:
 Acid rain causes great damage to plants, soil, and water that in turn affects many living organisms that
depend on these resources for their survival.
 Acid rain can harm plants by damaging the outer leaf surfaces by stripping away the waxy protective coating
from the plant leaves, resulting in the leaching of nutrients such as calcium, magnesium, and potassium out
of leaf tissue and drying
 Acid rain can keep seeds from germinating and degrade the available nutrients in the soil.
 It affects soil microorganisms, aquatic animals such as fish, amphibians and human beings by reducing the
quality of the water people use for drinking and cleaning and can cause acute toxicity or chronic health
problems due to dissolved trace metals.
6.5.6 Loss of Biodiversity
 Biodiversity refers to the variety of life on earth in terms of genes, species, individual organisms within a given
species, and biological communities from an ecosystem to the global biosphere.
 The loss of biodiversity is a decrease in biodiversity with regard to the number genes, species, and biological
communities in the world.
 The rapid growth of the human population has caused rapid ecosystem change and massive loss of biodiversity across
the world.
 The loss of biodiversity is mainly related to the permanent ecological changes in the ecosystems, landscapes, and the
global biosphere because of the exponential growth and demands of the human population.
 The major causes of the loss of biodiversity are:
 Loss of habitat and degradation.
 Overexploitation
 Invasive species
 Pollution
 Climate change
 Effects of biodiversity loss:
 The loss of biodiversity is increasingly threatening the earth's ability to provide humans with things such as food,
water, fertile soils, and protection from pests and disease.
 Loss of biodiversity affects the economic systems and human society.
6.5.7 Toxic bioaccumulation
 Bioaccumulation is an increase in the concentration or accumulation of chemicals in living organisms' body tissues.
 Persistent bioaccumulative toxic substances (PBTs) are chemicals that do not degrade easily in the environment.
PBTs are typically accumulated in fatty tissues and are slowly metabolized, often increasing in concentration within
the food chain.
 Sources of toxic bio-accumulates: organic compounds and metals include synthetic chemicals that contain halogen
atoms (particularly fluorine, chlorine,or bromine), DDT, and metals, such as lead and mercury.
 The process of bioaccumulation occurs as follows (Figure 6.22):
a) Plants absorb small amounts of toxic substances, often pesticides or pollutants.
b) These plants are eaten by primary consumers in low concentrations.
c) The toxin cannot be excreted, so when the primary consumers are eaten by secondary consumers, the toxin is
absorbed by secondary consumers.
d) This repeats as secondary consumers are eaten by higher-level consumers.
e) At each trophic level of the food chain, the toxins remain in the tissues of the animals, so the concentration of
toxins becomes the most concentrated in the body tissues of the animals at the top of the food chain.

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  Effects of toxic bio-accumulates: They have a variety of toxic properties, resulting in a diverse array of adverse
health effects such as mutagenic damage to DNA, cancer, neurological toxicity, reproductive toxicity, developmental
toxicity, and immune system damage.
 Lead contamination of air, soil, or drinking water can ultimately result in significant exposures in fetuses, infants, and
children, resulting in impaired brain development. When mercury is consumed by fish with plankton, it passes
through the food chain and damages the nervous systems and the reproductive systems of mammals, including
humans.
6.5.8 Resource depletion
 Resource depletion occurs when the consumption of natural resources becomes faster than they can recover and
become scarce.
 Natural resources have been depleted primarily because of human activities
 The major causes of the depletion of natural resources are:are
1) Overpopulation.
2) Poor farming practices.
3) Overconsumption of natural resources
4) Industrial and Technological Development
6.6 Indigenous conservation practices in Ethiopia
 Indigenous knowledge is a body of knowledge built over generations by a group of local people living in a particular
environment.
 Ethiopia has a known indigenous culture, tradition, religion, and knowledge.
 The people of Ethiopia have indigenous knowledge about natural phenomena, natural resources, and their
environment.
 Peoples in Ethiopia have developed indigenous knowledge in a wide range of fields of conservation of natural
resources, such as the conservation of soil, water, crop seeds (selection and preservation), forest, medicinal plants,
animals, etc.
 The most commonly used soil and water indigenous conservation practices among the Konso people are
 making terraces
 contour ploughing
 crop rotation
 mixed cropping
 Surface mulching
 Agro forestry.

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