Biology and Health Sciences

Key unit competency:  To be able to explain the concepts applied in environmental Biology including interaction and interdependence of organisms.

Learning objectives

After studying this unit, I should be able to:
• Define terms used in ecology.
• Construct and interpret simple food chains, food webs, pyramids of biomass
and numbers.
• Appreciate the interdependence of living organisms.
• Appreciate the role of green plants and interdependence of living organisms.

Introduction

Look at the picture below. Can you explain what is happening? What is the importance
of the action in the picture? Supposing one of the animals in the picture becomes
extinct, what would happen?

Talk to your friend about how such relationships in your area are regulated. What does this tell you about what you will learn in this topic?

2.1 Concepts of ecology

An old saying goes ‘no man is an island’. This seems to apply to all other organisms on earth. Organisms do not exist in isolation. They depend on each other for survival. Plants obtain their energy from the sun. Herbivores like cows eat the plants for survival. On the other hand, carnivores like lions feed on herbivores if they are to survive. Through this interdependence, organisms have developed feeding relationships. The scientific study of these relationships between organisms
and their environment is called ecology. Therefore, ecology seeks to explain the connection between plants and animals and their surroundings. It also provides information about the benefits of the
ecosystem and how we can use the Earth’s resources efficiently. The study of ecology enables us to appreciate the environment and leave it healthy for future generations.
Research Activity
1. Using text books and the internet research on the meaning of the following words:
• Biodiversity
• Ecosystem
• Population
• Community
• Habitat
• Niche
• Biotic and abiotic factors
• Food chains and food webs
2. Discuss the meaning of these words with a classmate.
3. Note the meaning of the words in your notebooks.

During the study of ecology, the following terms are widely used.
• Ecosystem: This is a stable unit of nature consisting of all communities interacting with each other and their surrounding physical environment. Examples of ecosystem include a pond, a grassland and desert.
• Biosphere: This is the part of the Earth and its atmosphere capable of supporting life. It is an area where organisms live, including the ground and the air.
• Producer: This refers to all green plants (for example, beans, mango tree and pine) which manufacture their own food by the process of photosynthesis.
• Habitat: An area in which an organism lives. The habitat for a leopard should have the right amount of food (gazelle, rabbits and impala) water (a lake, river or spring), and shelter (trees or dens
on the forest floor).

                     Fig. 2.2: Flamingoes in their habitat

• Community: A combination of different species of organisms living together in an area. An example is a forest of trees and undergrowth plants, inhabited by animals and rooted in soil containing bacteria and fungi.
• Population: This is the total number of organisms of the same species living together in a specific area at a certain time.
• Biotic factors: These are the living components of an ecosystem. These may be plants, animals, fungi, and any other living things.
• Abiotic factors: These are the nonliving components of an ecosystem. They include pH, sunlight intensity and temperature.
• Niche: This is the position that an organism occupies in a habitat. It includes where it lives, its role and feeding habits, for example, a garden spider is a predator that hunts for prey among plants, while an oak tree grows to dominate a forest canopy, turning sunlight into food.
• Carrying capacity: The maximum size of a population that a given areacan support without straining.
• Edaphic factors: These are all nonliving
components of an ecosystemboth chemical and physical. They are related to abiotic factors.
• Environment: This is a combination of all factors that affect the life of an organism.
• Biodiversity: This is the variety of life in the world or in a particular habitat or ecosystem.

• Biome: This is a large naturally occurring community of animals (fauna) and plants (flora) occupying
a major habitat. Examples include: forest, grassland, freshwater, marine and desert.
• Biomass: This is the total mass of organisms in a given area or volume.
Activity 2.1: To investigate various concepts of ecology
Procedure
1. Take a walk in the school compound or the surrounding area.
2. Look for the various organisms e.g: termites, cockroaches, birds, grasshoppers, earthworms and
grasses. Alternatively, look at these animals:

3. Find out the following for each of the organisms.
• Where it lives.
• Its role in the environment
• The factors in its surroundings (habitat) that attract it to this specific area.

4. Give a rough estimate of the number of some of the organisms in your environment.
• Are the numbers of these organisms influenced by the availability of what they eat?
That is the amount of grass and number of grasshoppers noticed.
5. Record your observations.
6. Share your findings with the rest of the class. Study questions
(a) What role does each of the or g a n isms pl ay i n the environment?
(b) State at least two physical aspects that attract the organisms to this particular habitat.
(c) Which organisms are abundant in the area studied?
(d) Where are the grasshoppers more concentrated in relation to the amount of grass in the area?
Explain.

Ecosystems

Ecosystems are also known as biomes. They are made up of both the living component (biotic factors) and the non-living component (abiotic factors).
There is a close interaction between the two components whereby each affects the other. Ecosystems are broadly subdivided into two: terrestrial and aquatic ecosystems.

Activity 2.2 Class Activity
1. You will be provide with pictures or photographs of different ecosystems by your teacher.
• Study the pi c tures and photographs carefully.
• Identify the different ecosystems shown in the photographs.
• Identify both biotic and abiotic factors in the photographs and pictures.
2. Can you identify in your locality where the ecosystems seen in (1.) above are found?
3. Suggest how important wetlands are to the community.
• What problems will the community face when wetlands are interfered with? Aquatic ecosystem
Aquatic biomes or ecosystems form the largest part of the biosphere. Since water bodies contain different amounts of salt in them, they are classified into two: marine and fresh water ecosystem.
(a) Marine ecosystems
These are aquatic ecosystems made up of water with high salt concentration. Oceans, seas and all organisms in them form part of such ecosystems. This covers the largest part of the earth’s
surface.

                 Fig 2.4: A mangrove forest forms part of the marine ecosystem

(b) Freshwater ecosystems These are made up of water with a less salt concentration. Lakes, wetlands and rivers with all organisms in them fall under this category of ecosystems.

                       Fig 2.5: Fresh water ecosystem

Terrestrial ecosystems This is the land ecosystem. Terrestrial ecosystems are always named according to the climate and physical features that define them. They include savanna
grasslands, deserts, temperate grasslands and forests. Terrestrial biomes usually cross cut into each other without clear boundaries.

     Fig 2.6: Terrestrial ecosystem

Biotic and abiotic factors

Discussion corner
1. Talk to a classmate about the following:
• Non–living things that you interact with in the environment.
• Ways in which non-living things are important to living organisms.
2. Share your findings in class. Communities and their physical surroundings constitute an ecosystem.
The environmental conditions that affect a community are temperature, light, pH, wind, salinity and atmospheric pressure. These physical factors that affect the type of organisms and their distribution
are called abiotic factors. Communities include living organisms that is plants and animals. These are known as biotic factors.

Abiotic factors

These are the environmental factors in an ecosystem. They describe the physical conditions in the ecosystem.

These factors influence the type of organisms living in an ecosystem. They also influence the distribution of the organisms in that ecosystem. The abiotic factors in an ecosystem include
the following:
(a) Light - The sun is the main source of light energy in all ecosystems on earth. Light is important in any ecosystem because plants use it to make their own food.
(b) Temperature – Temperature variation in an ecosystem affects the type and distribution of organisms found in it. Some organisms prefer high temperatures while others prefer low temperatures. Therefore they will inhabit different parts of the ecosystem.
(c) Atmospheric pressure – This is the pressure exerted by air in the atmosphere. Atmospheric pressure affects the amount of oxygen in the air. At the sea level, air pressure is high. This reduces at higher altitudes. Therefore different organisms will occupy different altitudes based on their endurance.
(d) Salinity – This is the degree of salt concentration in water. This is a factor that is mainly found in aquatic habitats. Some organisms live in salty environments while others live in fresh water environments.
(e) Humidity – This is the amount of water vapour in the atmosphere. It influences evaporation and
transpiration rates.

(f) pH– This is the degree of acidity or alkalinity. The pH of an ecosystem affects the type of distribution of organisms found in it.
(g) Wind – This is moving air. Air moves in different directions and speed. Wind influences environmental factors such as temperature and humidity.

Biotic factors

There are many different people who form the school community. There are teachers, parents, students, secretaries, cooks and watchmen. The school cannot be able to run efficiently if all these
people do not perform the jobs or tasks they are supposed to perform. If the night guard slept while on duty, then robbers might get into the school and steal all the students' books. This would affect the learning process. With the interelationship among different people, the school runs smoothly. A
similar form of interelationship exist among organisms in any ecosystem.
Biotic factors are the entire living components in an environment. They affect each other either positively or negatively.
Autotrophic organisms such as plants use energy from the sun and simple elements to make food. Without them other species of organisms in the area cannot survive. Herbivores eat food from plants; they affect the population of the plants and at the same time support other organisms that depend on them. Organisms in an ecosystem interact in many ways.

Activity 2.3: Class Activity

1. Your teacher will show you a film or video on wildlife interdependence.
2. Watch the film or video carefully.
3. From the film or video you have watched.
• Can you identify the biotic and abiotic factors in the film?
4. Share your findings with the class.

Self-evaluation Test 2.1 

1.  Match the following terms and their definition. 

2. Non-living things affect the life of living things. Explain.

2.2 Energy flow in ecosystems 

Its common knowledge that a bicycle cannot move unless the person ridding it peddles! Human beings cannot perform their daily activities like talking and moving if they do not eat good food. Where do living things obtain energy from? How is energy transfered from one organism to another?     

The primary source of energy is the sun. Energy enters ecosystems in form of sunlight. Part of it is then converted into chemical energy by the green plants (producers) through the process of photosynthesis. For this reason they are known as Autotrophs. 

 Plants pass on this energy to the grazers who feed on plants for food. These organisms that feed on plants cannot manufacture their own food. They are therefore known as heterotrophs. Those organisms that feed on plants directly are known as herbivores. Herbivores fix part of the energy into their bodies; the rest is lost into the surroundings. They are also in turn eaten by the carnivores who take in part of this energy losing the largest part in form of heat and waste excreta.

The carnivores are grouped into first level, second level up to the topmost level. The top most carnivores receive the least portion of energy as most of it has been lost. The last trophic level in an ecosystem is that of the decomposers. They decompose organic matter thus allowing the recycling of nutrients. Decomposers are saprophytic organisms. They act on the dead remains of organisms in all the other levels.

It should be noted that sometimes in an ecosystem, organisms can feed on both plants and animals. In this case, they are known as omnivorous organisms. Examples of such organisms are pigs and human beings. 

In ecology, a trophic level refers to the position an organism occupies in a food chain. Green plants form the first trophic level known as the producers. Some organisms such as herbivores feed directly on the plants to obtain energy. Other organisms obtain this energy indirectly by feeding on the herbivores. Collectively, these organisms are called consumers because they cannot make their own food. Herbivores like grasshoppers form the second trophic level. First level carnivores like the lizards form the third trophic level, followed by snakes on the fourth level and then eagles come on the fifth level as top level carnivores. Some consumers called the detrivores derive their energy from non-living organic material such as feaces and fallen leaves.

The table below gives a summary of organism we expect in each trophic level 

Table 2:1: Organisms in different trophic levels

Food chain 

A food chain is a linear representation of how organisms eat each other before they are eaten in return. It is a sequence describing a feeding relationship between producers and consumers. When the feeding relationship is put down schematically, what is formed is known as a food chain. 

Energy in an ecosystem can be passed on from the sun through several organisms. Insects like grasshoppers feed on grass. The grass is the producer because it makes its own food. The grasshoppers are the primary consumers because they obtain chemical energy directly from the plant. The insects are eaten by birds which obtain chemical energy from them. The birds are secondary consumers. If a bird is eaten by a wild dog, then the wild dog is quarternary consumer. This is an example of food chain as shown below. 

Food web 

It is rare in an ecosystem to find that organisms eat only one type of food. They always have other options in case one is not in place to be consumed. A food web is therefore a complex series of interconnected food chains. It shows feeding relationships between various species of organisms in a given community. A food web is composed of all possible food chains in any given ecosystem. When a schematic representation is made to show other options an organism has for food, a food web is obtained.

In the food web identify as many food chains as possible. Note that the higher organisms such as the snakes and owls depend on more than one particular food. Also note that decomposers such as the bacteria and fungi are involved in all parts of the food web. The food web comprises the relationship within a community. It is easily made by arranging organisms in their trophic levels. The arrows in the food chain indicate the flow of energy from the food web. One organism can provide energy to more than one trophic level.

Activity 2.4: Investigating trophic levels occupied by organisms 

1. Take a walk in the school compound or to an area around the school. 

2. List all the organisms found in the school compound. 

3.  Group the organisms according to their trophic levels. 

4.  Record your work in a table. 

5. Construct several food chains using the organisms you have listed. 

6. Model a simple food web from the organisms above. 

Study questions 

(a) How are the organisms in one trophic level related to those in the next trophic level? 

(b) From the food chains, identify organisms that can be eaten by more than one organism. 

(c) Discuss in groups the importance of each tropic level in a food chain. 

(d) Discuss the effect of removing one tropic level from the food chain.

How energy flows in an Ecosystem 

Study the diagram below. What does it show?

From the diagram above, we can see that energy from the sun flows through producers to the consumers. It also shows that not all energy from the producers is transferred to the consumer. 

Energy flows into a food chain from the sun to plants through the process of photosynthesis. It then moves up the food chain to higher trophic levels. Since the transfer of energy from one trophic level to the next is inefficient, less energy enters higher trophic levels. 

• What brings about the loss of energy at each trophic level? 

Several factors account for the loss of energy as one moves from one trophic level to the next. They include the following: 

• Respiration 

• Part of the energy is lost as undigested food matter. 

• The rest is lost as excretory products.

Self-evaluation Test 2.2 

1.  Consider this food chain 

     Grass ->Grasshopper->Birds.

    (a) The birds in this food chain are: 

          A. Producers  

          B. Primary consumers 

         C.  Secondary consumers 

         D.  Tertiary consumers 

    (b) If a disease killed all the birds, what effect will it have in this food chain? 

2. A vulture feeds on dead bodies of animals. The vulture is best described as:

          A. An omnivore  

          B. A carnivore 

          C. A decomposer 

          D. A scavenger 

3.  An ecologist carried out a survey to estimate the number of organisms in a certain dam. The following table shows the record of the survey. 

    (a) Which of the above organisms are consumers of the last order? 

    (b)  Which organism is likely to get finished first in the dam? Give a reason to support your answer. 

    (c)  Draw a possible food chain that shows the energy flow in the dam.

4.  Explain why food chains are not endless; they are limited to four or five and rarely six organisms.

2.3 Ecological pyramids 

What does the word pyramid reminds you of? The Egyptian pyramids probably? Look at the picture below, what shape does it have?

Compare the shape of the Egyptian pyramid above and the ecological pyramids discussed below.

Ecological pyramids are diagrams that show how important factors in an ecosystem such as energy, biomass and population size change at each trophic level. Traditionally, these diagrams place the primary producers (green plants) at the bottom. The highest trophic levels are placed at the top. The size of the portion of the diagram associated with each trophic level shows the amount of factor in consideration. A food chain can be expressed in a measurable way by using pyramid of numbers or pyramid of biomass.    

Pyramid of biomass 

This is a diagrammatic representation of mass or weight of organisms in each trophic level in a food chain. If the dry mass of all organisms at each trophic level of a food chain can be weighed, the mass can be used to draw a type of diagram called pyramid of biomass. 

Biomass means the dry mass of any living material at any trophic level in a food chain. Biomass reduces as one moves from the producers to the various levels of consumers. This is the same trend observed with the amount of energy.

A pyramid of biomass is a chart drawn to scale showing the amount of biomass at each stage in a trophic level. The bars become narrower from the base to the top of the pyramid.

It is always hard to construct a pyramid of biomass because of the following reasons 

• Measuring biomass often means death of the organism. 

• Organisms may belong to more than one trophic level in an ecosystem. It is therefore not accurate to represent it with only one bar. 

Note: When making a pyramid of biomass, you must use a scale. 

Pyramid of numbers 

The number  of all organisms at each trophic level of a food chain can be counted. These numbers can be used to draw a diagram called  pyramid of numbers. The number of organisms in each level can be obtained by totalling the population of all the species making up that level for instance: 

    • Total number of plants.  (Producers) 

    • Total number of herbivores. (Primary consumers) 

    • Total number of carnivores. (Secondary consumer) 

The pyramid is therefore a diagramatic representation of numbers of organisms in each trophic level in a food chain. In this way, it is possible to know the number of organisms that are capable of transferring energy from one trophic level to the next. The Figure below shows pyramid of numbers. Its shape as we can see is like that of a pyramid. This is called an upright pyramid.

The pyramid indicates that organisms transferring energy to the next energy levels decrease as we rise up.  Sometimes the pyramid is not upright, for instance, if you were to construct pyramid of numbers using the tree as a habitat. It would be the only producer and the consumers such as caterpillars and birds would be many. If we were to construct pyramid of numbers, the smallest box would represent the tree. It would be at the bottom and not at the top. The shape of this pyramid would be inverted as can be seen in the figure below.

Self-evaluation Test 2.3

1.  Plot the following data by placing the producers at the bottom. Use the scale 1cm = 50g/m² for length and 1 cm height for each level.

(a) What does this pyramid indicate?

(b) Explain why the dry weight of the carnivores must be low.

2.  If the total weight of producers in an ecosystem is 1000 kg, what would you expect the total weight of secondary consumers to be?

3.  Study the food chain given below Oak tree -> insect larvae  -> insectivorous birds 

     The food chain can be represented in _______ diagram.

(a) Which of the pyramids above (A, B, C and D) best represents this food chain?

(b) Identify the producer in the food chain.

(c) Name the source of energy entering this food chain.

(d) Name two main groups of organisms referred to as decomposers.

4.  Do you agree with the statement below? ‘All food chains begin with green plants’ Explain your answer.

Unit summary

• Ecology is a branch of Biology that deals with the relationship between organisms and how they relate with their physical surroundings.

• Biotic factor is any living component that affects an organism.

• Abiotic factors are non-living conditions which influence where plants or animals live.

• A food chain is a linear sequence that shows how organisms dependent on one another for food.

• A food web is a system of interlocking and interdependent food chains.

• Trophic levels are the several hierarchical units in an ecosystem, comprising of organisms that share the same function and nutritional relationship in the food chain.

• The sun is the main source of energy in the ecosystem.

• Green plants are the producers in the ecosystem. They are able to manufacture their own food through the process of photosynthesis.