Biology

Key Unit Competence

Describe the factors affecting population size and the importance of natural resources

Learning Objectives

Learning Objectives By the end of this unit, I should be able to:
– State and define population characteristics.

– Explain factors that affect population density.

– Explain population growth patterns.

– Explain the terms renewable and non-renewable resources.

– Explain how environmental resistance affects the balance of nature.

– Explain the importance of natural resources in growth of the Rwandan economy and methods of conservation.

– Demonstrate methods used in estimating populations by using quadrats and line transects.

– Research how the human population has grown over the past 250 years.

– Compare statistics on the population age-sex structure of developing and developed countries.

– Analyse the costs and benefits of managing renewable and non-renewable resources.

– Support that human population explosion impacts negatively on the environment.

– Recognize that some resources are renewable and others are non-renewable and that effective use of these resources is of great value.

– Justify the practice of family planning as a tool for reducing population explosion.

Pictures A and B represent ecological populations. In biology, an ecological population is a group of organisms of the same species that live in the same area at a certain period of time. The population is the unit of natural selection and evolution. How large population is and how fast it is growing are often used as measures of its health.

1.1 Population characteristics

Activity 1.1

Discuss the following terms in relation to population:

1. Density

2. Age structure

3. Growth pattern

4. Birth rate

5. Death rate

A given population is characterized by its density, age structure, growth patterns, birth and death rate.

1.1.1 Population density

Population density is the number of individuals of the same species per unit area or volume. For example, the number of Acacia tree species per square kilometer in the Akagera National park in Rwanda or the number of Escherichia coli per millilitre in a test tube express the density of these individuals per square kilometre in a natural forest and per millilitre in a test tube.

1.1.2  Population age structure

Age structure is the number or proportion of individuals in each age group within a population. The figure 1.1 below provides the distribution of the population according to age.

Information is included by sex and age group as follows: 0-14 years (children), 1524 years (early working age), 25-54 years (prime working age), 55-64 years (mature working age), 65 years and over (elder age). The age structure of a population affects a nation’s key socioeconomic issues. For example, countries with young populations (high percentage under age 15) need to invest more in schools while countries with older populations (high percentage ages 65 and over) need to invest more in the health sector.

The shapes of the age-sex structure pyramids shown above show the age sexstructure of a developing and developed country. The main characteristics of developing countries including some of the African countries in terms of population growth include high death rate; high birth rate and low life expectancy, while the main characteristics of developed countries such as most European countries in terms of population growth are low death rate, low birth rate and longer life expectancy

1.1.3  Population explosion

Population explosion is the rapid increase in number of individuals of a particular species. For example, the world’s human population increase since the end of World War II is attributed to; an accelerating birth-rate, a decrease in infant mortality and an increase in life expectancy. Such human population increase impacts negatively the environment. For instance, human population explosion contributes to pollution leading to; ozone depletion, eutrophication, acid rain, global deforestation, soil erosion and desertification.

As the figure 1.3 above indicates, the World population is exponentially growing. This is the reason why most countries, including Rwanda, are practicing the family planning. Family planning is the practice of controlling the number of children in a family and the intervals between their births. If a married couple is sexually active, they have to adopt at least one family planning technique such as contraception and timing of reproduction. Other techniques commonly used include; sexuality education, prevention and management of sexually transmitted infections, preconception counselling and management, and infertility management.

1.1.4  Population growth patterns

Population growth patterns are graphs (population growth curves) in which increases in size are plotted per unit time. When a population size increases, the growth rate also increases. The larger the population becomes, the faster it grows. The factors that contribute to the population growth are immigration of new species as well as the birth rate. Population growth is also influenced negatively by emigration and the death rate. 

1.1.5  Birth and death rates

Birth rate is the ratio of live births in a specified area to the adults in population of that area. It is usually expressed per one thousand individuals per year. It is estimated from this calculation:

Death rate is the ratio of deaths to the adults in population of a particular area during a particular period of time. It is usually calculated as the number of deaths per one thousand individuals per year and it is estimated from this calculation:

Self –assessment 1.1

1.Distinguish between population density and age structure.

2. There are 100 adult elephants in a population of an area. Each year, 10 elephants are produced while 2 elephants die.

a.  Calculate the birth rate of this population.

b.  Calculate the death rate of that population.

3. Explain the impact of population explosion on the environment.

4. Describe the family planning techniques.

1.2 Population density: Dependent and independent factors affecting population density 

Populations are differently distributed. The distribution and the density are controlled by environmental factors, which can either increase or decrease the population size by affecting birth rate, death rate, immigration and emigration (Table 1.1 below). These factors are grouped into two major categories: Density -dependent factors and Density- independent factors.

1.2.1 Density-dependent factors

Density dependent factors are factors whose effects on the size or growth of the population vary with the population density. The types of density dependent factors include: availability of food, predation, disease and migration. However, food availability is considered as the main factor.

1.2.2 Density-independent factors

Density independent factors can affect the population without being necessary based on the density. They include; natural disasters (droughts, floods, hurricanes and fires), temperature, sunlight, seasonal cycle, human activities, and levels of acidity, cited among many others.

Self-assessment 1.2

Discuss the ways by which natural disasters (droughts, floods, hurricanes and fires) affect the population growth.

1.3  Methods or techniques of measuring population density

1.3.1 Quadrat method

A quadrat is a square frame that marks off an area of ground, or water, where you can identify different species present and/or take a measurement of their abundance. Before any experiment, the decision on a suitable size for the quadrat and the number of samples to use is taken. Samples must be selected randomly to avoid any bias, such as choosing to take all of samples from the place with fewest species simply because it is the easiest to do. This would not represent the whole area you are surveying.

A quadrat method enables the calculations of 3 aspects of species distribution including; species frequency, species density and species percentage cover. The results can be used to calculate species frequency and species density.

1.3.2 Species frequency

Species frequency is a measure of the chance (probability) of a particular species being found within any one of the quadrat, and it is found simply by recording whether the species was present in each analysed quadrat. For example, if a quadrat is placed 50 times, and a given plant was identified in 22 samples, then the species frequency for this plants equals:

1.3.3 Species density

Species density is a quantity of how many individuals there are per unit area, for example, per square meter. To achieve this, one takes the total number of counted individuals and then divide it by the number of quadrats done. An example is: Total number of individuals = 200

Total area of quadrats = 480m2

Species density  200 480 = 0.417v  individuals/m2.

1.3.4 Species cover

Species cover is a measure of the proportion of ground occupied by the species and gives an estimate of the area covered by the species as the percentage of the total area. For example, if there are 100 small squares in one quadrat, then the squares in which the plant species is present are counted. If plants are found in 25 squares within that quadrat, the conclusion is that the plant covers 25% of the area.

1.3.5 Line transect method

Line transect is a tape or string laid along the ground in a straight line between two poles as a guide to a sampling method used to measure the distribution of organisms. For example, the investigation on change at the edge of a field where it becomes very marshy is done by randomly selecting a starting point in the field and lay out a measuring tape in a straight line to the marshy area, and then sample the organisms that are present along the line, which is called a transect. The simplest way to do this is to record the identity of the organisms that touch the line at set distances – for example, every two meters.

1.3.6 Capture-recapture method

Activity 1.3.2

Mukamana is a fish farmer in Bugesera district. She wanted to know the total population in her fish pond. She netted 240 fishes and tagged (marked) their opercula with aluminium discs. She released those fishes into the pond. After one week, she netted again 250 fishes among which 15 had the aluminium discs. Calculate the estimated population from marked individuals.

Capture-recapture method involves capturing the organism, marking it without any harm, and release it in the same area so that it can resume a normal role in the population. For example, fish can be netted and their opercula is netted with aluminium discs, birds can be netted and rings can be attached to their legs, small animals may be tagged by dyes, or by clipping the fur in distinctive pattern, while arthropods can be marked with paint. In all cases, some form of coding may be adopted so that individual organisms are identified. Having trapped, counted and marked a representative sample of the population. 

At a later stage, the population is trapped again and counted, and the population size is estimated using the Lincoln index as follows:

Estimated total population =

Where: N1: the number of organisms in initial sample,

N2: the number of organism in a second sample,

N: the number of marked organisms recaptured.

1.4 Population growth patterns and environmental resistance

1.4.1. Population growth patterns

1.4.1. Population growth patterns  

Population growth patterns are graphs also called population growth curves in which the increases in size are plotted per unit time. Two types of population growth patterns may occur depending on specific environmental conditions:

a. Exponential growth pattern/J-shaped curve/J-shaped curve

Exponential growth is a pattern of population growth in which a population starts out growing slowly but grows faster as population size increases. An exponential growth pattern also called J- shapes curve occurs in an ideal, and unlimited environmental resources. In such an environment there will be no competition. Initially population growth is slow as there is a shortage of reproducing individuals that may be widely dispersed. As population numbers increase, the rate of growth similarly increases, resulting in an exponential J-shaped curve. Exponential population growth can be seen in populations that are very small or in regions that are newly colonized by a species.

b. Logistic growth pattern / sigmoid growth curve

Logistic growth is a pattern of population growth in which growth slows and population size levels off as the population approaches the carrying capacity. A logistic growth pattern also called S-shaped curve occurs when environmental factors slow the rate of growth.

The sigmoid or S- shaped curve represented by the figure 1.8 shows three main stages in population growth: The lag phase where there is a slow growth, the log phase or exponential growth phase, also called logarithmic phase, in which the number of individuals increases at a faster rate and the plateau phase or stationary phase, in which the number of individuals are stabilized.

Causes of the exponential phase are various and include the plentiful of resources such as; food, space or light, little or no competition from other organisms, and favourable abiotic factors such as; temperature or oxygen and reduced of lack of predation or diseases. The stationary phase, however is caused by a balanced number of; births plus the number of immigrations and the number of deaths plus the number of emigration. Other causes may include; the increase of mortality caused by predators and diseases, excess of wastes and competition for available resources such as food, space, shelter and minerals. Some of these causes may include the carrying capacity explained as is the maximum number of individuals that a particular habitat can support.

1.4.2 Environmental resistance

Environmental resistance is the total sum of limiting factors, both biotic and abiotic, which act together to prevent the maximum reproductive potential also called biotic potential from being realized. It includes external factors such as predation, food supply, heat, light and space, and internal regulatory mechanisms such as intraspecific competition and behavioural adaptations.

1.4.3 Environmental balance

A balance of nature is the stable state in which natural communities of animals and plants exist, and are maintained by competition, adaptation and other interactions between members of the communities and their non-living environment. Every biotic factor affects or causes a change in the natural environment. For example, when a living organism interacts with the environment, this causes a change in the environment. The following are some of the examples of biotic factors and their effects on balance of nature:

– Respiration:  when animals are respiring, they take in oxygen and give out carbon dioxide (CO2) from respiration. The CO2 can be taken in by plant leaves and be used in the process of photosynthesis to make food and give out oxygen. 

– Predation: when animals, for example, predate on other animals, this reduces the numbers of prey, which in turn affects the ecosystem.

– Parasitism:  cause diseases that may slow down the growth rate of a population and/or reduces the number of organisms.

– Competitors: when organisms compete over nutritional resources, this could reduce the growth of a population.
 

Self-assessment 1.4

1. Explain any 3 biotic factors that affect the balance of nature.

2. Distinguish between carrying capacity and biotic potential.

3. Explain how environmental resistance affects the population growth.

1.5.1. Natural resources

Natural resources refer to materials or substances occurring in environment and which can be exploited for economic gain. Natural resources such as; solar energy, wind, air, water, soil and plants are renewable natural resources while others including fossil fuels, oil, coal natural gas cited among many others are non-renewable natural resources. A renewable resource can or will be replenished naturally in the course of time, while a non-renewable resource is a resource of economic value that cannot be readily replaced by natural means on a level equal to its consumption.

1.5.2. Importance of natural resources in economic growth of Rwanda 

– Water is used for; irrigation, domestic activities, industrial use, and mining.

– Lakes and rivers are source of food (fish) for humans and contribute for recreation (tourism).

– Land serves as the storehouse of water, minerals, livestock, and home for wild animals which generate an income in different ways. 

– Minerals including gravel, coal, metals, oil, clay, sand, stones…are used for construction and for income generation. 

– Soil contributes to agricultural crop production, and supports forest and food crops.

– Trees are the major sources of timber, construction materials and firewood and contribute to fight against erosion, water and air purification and wind protection.

– Some plants are source of food and money for humans and other animals 

– Some animals including; mountain gorillas in Volconoes National Park, lions in Akagera National Park and many other wild animals contribute to economic development of the country through tourism.

1.6 Methods of conserving natural resources

They are various and different methods used for conservation of natural resources and they include:

– Use of alternative sources of power such as; solar and wind energy: These alternative sources of energy are bio friendly particulars because they do not produce harmful gases that damage the ozone layer, compared to the burning of fossils fuels such as; coal and charcoal. They are also; cheap to use, not easily depleted, and are renewable.

– Tree planting to prevent soil erosion: This entails planting trees and other vegetation to  control soil erosion  caused by wind and water. Trees and vegetation are essential in the maintenance of the ecosystem. They also act as home for most insects, birds and some symbiotic plants. This creates a habitat for wildlife therefore conserving wildlife altogether.

– Practicing of judicious ways to conserve water in our homes: This entails simple practices like ensuring that taps are closed when they are not in use. Taking less time in the shower aids to conserve lots of water per month.

– Use pipelines to transport oil: During oil transportation on ships, spills can happen which will negatively affect both plant and animal life. Therefore, use of pipelines is more recommended.

  – Growing vegetation in catchment areas: Catchment areas act as a source of water that flows in; streams, rivers and oceans. Vegetation in the catchment areas allows sufficient infiltration of water into deeper soil layers thus leading to formation of ground water.

– Prior treatment of human sewage and Industrial wastes: Water flowing from industries comes with many toxic wastes that must be treated before getting to the natural water bodies. This reduces harm inform of pollutants e.g. chemical and thermal forms.

  – Harvesting rain water: This is done through usage of water tanks that collect water during the rainy season and maintain use during dry periods.  This reduces tension on water reservoirs (e.g. lakes).

– Practice of in-situ or on-site conservation of wildlife: This involves conservation of fauna and flora in their natural habitats. This entails setting up measures that protect areas such as national parks and game reserves.

– Practice Ex-situ or offsite conservation of wildlife: It involves the conservation of animals and plants outside the natural habitats. These include areas such as; pollen banks, DNA banks, zoos, seed banks, botanical gardens, tissue culture banks among others.

– Formulation of policies and regulations to curb poaching: Poachers continue to kill many animals such as; elephants, rhinos, leopards for their tusks and skins which are sold off in the black market. Poachers are a major threat to our biodiversity as they are slowly making some species extinct. These regulations will ensure that poaching is done away with.

– Practice judicious ways of conservation energy: Such practices include  switching off the lights  when not in use, unplugging electrical appliances when not in use. Plugged-in appliances continue to use electricity even when not in use. Other practices include spending less time when taking hot showers

. – Use of biogas in our homes: Around the World, Liquefied Petroleum Gas (LPG) is the most rampant source of fuel in our homes today. Continued LPG use results into the depletion of oil reserves, biogas is therefore an alternative. Biogas is mainly produced from cattle dung, biogas plants are a source of both biogas and manure.

– Use of bio-fuels: For more than a century, fossil fuels have been a major source of energy. However, they are depleting rapidly, this calls for alternative sources of fuel such as bio-fuels which are mainly from plant species. Bio-fuels are known to be bio friendly and they reduce the occurrence of air pollution.

– Ensure the recycling of wastes: These wastes include; plastics, paper bags that have resulted to tones of garbage. Recycling entails re-manufacturing of already used materials. This reduces the amount of waste available reducing soil and water pollution.

– Make use of electronic mails: Electronic mails are paperless and present a good way to minimize the usage of paper. Technology has made this possible reducing the usage of paper and envelops. This has reduced the production of paper and also minimized cutting down of trees.

– Purchase hybrid cars instead of the conventional cars: Hybrid cars use a combination of electricity and minimal amounts of gas to run them. This is a break from the use of petroleum consuming cars that are now in large numbers.

– Water the lawns and farms in the evening: Watering the farm when it is dry and hot results to increased water evaporation and a lot of water is used for the same. During the evening, the weather is much cooler reducing evaporation thus conserving water.

– Reuse old furniture: It is common to dispose of old furniture and opt for new furniture. The old furniture should be sold off for use or donated to charity where they can be reused. The old furniture can also be re-sculptured and redecorated to save wood. This will reduce deforestation.

– Practice crop rotation: Planting the same crops for a long period of time reduces soil fertility. The practice of crop rotation will restore and maintain soil fertility thus conserving the soil.

– Translocation of wild animals: The growing population has led to human encroaching on the wildlife habitat. This has resulted to human-animal conflict where the wildlife are killed by humans as a way of protecting themselves from them. Translocation involves moving wild animals to adjacent areas and fencing to curb the conflict.

– Establish special schemes to preserve endangered plant and animal species: This includes; botanical gardens, sanctuaries that may be established to protect the endangered species so that they can be available for future generations.

– Constructions of reservoirs: This will regulate the amount of water that is used daily. The dams also act as a source of hydro-electric power which is another alternative source of energy.

– Formulate regulations to stop over fishing: Over fishing interrupts aquatic life and depletes the fish available in our water bodies. In some cases, it poses a threat to the endangered aquatic species. Regulations to avoid over fishing should be put in place.

– Construction of terraces in sloping land: This will prevent soil erosion as water tends to run downhill.