Topic outline

  • General

  • Unit 1 : Interdependence among organisms in an ecosystem

      After studying this unit, I should be able to classify examples of species interactions.

    By the end of this unit, I should be able to:
    • Define and differentiate between intraspecific and interspecific relationships in ecosystems using relevant examples.
    • Discuss positive and negative effects of competition among individuals of the same species and between those from different species.
    • Identify features that allow a predator to kill and feed on its prey.
    • Interpret graphs and data for predator-prey relationships in an environment.
    • Appreciate the interdependence of living organisms in an environment.• Illustrate commensalism and amensalism.

    1.1 Meaning of intraspecific and interspecific interactions in organisms

    We already know that the universe is inhabited by millions of species of organisms. However, none of these organisms is self-reliant. They all depend on each other. For example, they depend on each other for food, shelter or even mates. This is called interdependence. Sometimes the interdependence does not seem obvious with some organisms. It may be direct or indirect depending on the relationship between the organisms in question. Also, some relationships have mutual benefits while others do not.

    Activity 1.1: Investigating interactions between living organisms

    1. Take a walk and observe an anthill.
    •   Observe what happens when termites are building the anthill.
    •   Note down the various activities carefully.
    2. Come up with a list of roles of different types of termites during the construction work. Fill the following table

    Table 1.1: Interactions in the environment

    3. Watch the video on worker bees. Note the roles of the various bees in the video. Write them down in a table like the one above.
    4. Now, look at the photograph of a pride of lions scrambling for meat of a zebra. Watch a video on the same.
    5. Compare what you have seen happening in procedures 1, 2 and 3. What can you conclude about animal interaction in the scenarios given?

    We have already seen that organisms depend on one another in an ecosystem.
    There are two types of dependencies:
    • One in which organisms belonging to the same species depend on each other. This is known as intraspecific interaction.
    • The other in which dependence is between organisms of different species. This is known as interspecific interaction.

    1.2 Intraspecific relationships

    This type of relationship involves interaction between organisms of the same species. In lions, for example, the females are the hunters while the males are required for protecting the family. In bee colonies, we have the worker bees and those in charge of protecting the queen bee, they are known as soldier bees. Can you think of similar relationships in other organisms?

    Activity 1.2

    1. Watch a video on relationships of organisms with one another:
    2. From the video, come up with the definition of intraspecific interactions and give the examples of intraspecific interactions. Come up with a table like the one below.

    Table 1.2: Interspecific interaction

    3. Compare your work with that of your classmates.

    Intraspecific relationships are of two types: competition and cooperation interactions.

    (a) Competition
    Competition is a relationship that occurs between two organisms either of the same species or different species that require or use the same resources.They depend on the same resources found in the ecosystem, for example, food, space, water and shelter. When this happens between organisms in the same species, it is intraspecific competition. If it happens between organisms of different species, it is interspecific competition. Wildebeests and zebras in the savannah both eat grass. This is an example of interspecific competition.

    Competition is only found in organisms in the same ecological niche or closer niches. For instance, there may not be any competition between an elephant and a rabbit. Though they are found in the same habitat, they do not depend on the same food. Also, there will be no competition between spiders and lions, though both are carnivores. However, grazers eat the same food i.e. grass. They therefore compete for food. This has brought conflict between livestock and wildlife near human settlements. In some areas, livestock must compete for the limited grass with wild grazers such as zebras and gazelles.

    The more similar two kinds of organisms are, the more intense the competition between them. If one of the two competing organisms is better adapted to live in the area than the other, the less fit organism must migrate, die or shift to a slightly different niche in order for it to survive.

    •  Competition for the same resource by two organisms of the same species is referred to as intraspecific competition. Normally, male animals of the same species compete for the female animals.

    • Competition for same resource by organisms from different species is called interspecific competition. Cheetah and lion compete for flesh.
    Competition is an instinct in all organisms for their survival. Organisms tend to compete for resources when the supply is not enough.


    (b) Cooperation

    This is a kind of interaction in which organisms of the same species live together and share work between themselves. Every member of the species has a diverse role and they closely work together for their own group benefit. This kind of relationship can be observed in social insects such as ants, termites, bees among other groups of organisms.

    Self –evaluation Test 1.1
    1. What is intraspecific competition?
    2. Why is competition among organisms of the same species healthy?
    3. Give the importance of cooperation among organisms living together in a community.

    1.3 Interspecific relationships
    This type of relationship involves the interaction between organisms belonging to different species.

    Activity 1.3: Investigating interspecific relationships

    1. Observe the pictures of these animals:
    • Ticks
    • Lice
    • Fleas
    • Tsetse flies
    • Mites
    2. Where do you think these animals are usually found? How do they get their food?
    3. Bees, butterflies and birds are often seen visiting flowers. What do you think they do on the flowers? Is this important or not?
    4. What do the above observations tell you about interspecific relationships between organisms?

    There are many types of interactions between different species of organisms. Examples include: parasitism, predationgrazing, competition, mutualismneutralism, commensalism,amensalism and allelopathy.Have you ever heard of these terms? In the next section, you will study about these types of interactions in detail.

    (a) Parasitism
    This is a non-mutual relationship between different species of organisms. In this type of relationship, one species benefits while the other suffers. The organism that benefits is the parasite. It obtains nutrients from another organism called the host. Can you give examples of parasites and their hosts in real life?

    Adaptations of the parasites include:
    (a) Being smaller than their hosts.
    (b) Reproducing relatively faster.
    (c) Having penetrating and attachment organs.
    (d) Surviving in areas with low oxygen concentration.

    These adaptations enable them to survive in or on the host. They usually have no intention of killing their hosts. However, they end up harming or even killing the host.Further, parasites can either live inside or outside the host. When the parasite lives inside the body of the host, it is called an endoparasite. Examples of endoparasites are tapeworms, trypanosomes, liver flukes and roundworms.

    When the parasite lives outside the body of the host, it is called an ectoparasite. Examples of ectoparasites are ticks and fleas.

    Effects of parasites

    Many diseases which affect humans, domestic animals and crops are caused by parasites. Also, all fungi which are not saprophytic are parasitic. Diseases such as potato blight in plants and ringworms in human beings are caused by fungi which are parasitic. We have already seen that parasites can either kill or interfere with the health of their hosts. Parasites that do not kill their hosts may also reduce their rate of growth.A reduced growth rate will result in a slow increase in the population of the host.

    (b) Predation

    Predation is the hunting, killing and eating of one organism by another for food. A predator is an organism that kills and eats the other. A prey is an organism that is killed and eaten.

    Can you give examples of other animals that are predators or preys?Predators have many feeding adaptations. For example:

    (a) They have acute senses that locate and identify a prey. Many have structures such as claws, teeth, fangs and stings that they use to catch and subdue or poison the prey. Examples are rattlesnakes, which locate their prey with special heat-sensitive organs located near their eyes, they kill small birds and mammals by injecting them with toxins through their fangs.

    (b) Predators that pursue their prey are fast and agile. Those that lie in ambush, for example lions, are often camouflaged in their environment. Preys that do not move usually protect themselves chemically (bypossessing toxic chemicals) or physically (by having hard shells, spines or thorns).
    (c) Mobile prey depend mainly on their ability to escape from their predators. Preys also camouflage from predators. This makes it difficult for predators to locate them, for example chameleons.
    (d)Some predators such as frogs and chameleons have long tongues to help them catch insects.
    (e) Predators such as eagles have sharp eyes facing forward thereby giving them greater sense of sight which aids in locating their prey.

    Some examples of predator-prey relationships are:
    • A spider preying on an insect.
    • A lion preying on a gazelle.
    • A lizard preying on an insect.
    Effects of predation

    Predation helps in controlling the population size of both the predator and the prey. As the population size of the prey decreases, the predators are left with less and less food to eat and their population size decreases due to starvation. This gives the prey time to increase in population. In a stable predator-prey relationship, the populations of the predator and the prey are regulated by each other. This is well illustrated in fig 1.8
    showing the relationship between the impala which is the prey and the lion which is the predator. Analysis of the graph shows that when impala increase in number, food become abundant to lions and they start to increase in numbers. When the number of impala start to decline, competition for food among lions increases something which makes lions to starve and die. The cycle of interdependence starts again.

    Therefore, predation brings about population balance in an ecosystem. In order to avoid predation, preys exhibit different kinds of behavioural adaptations. Examples include avoiding predators, fleeing from enemies or defending themselves.

    Activity 1.4: Interpreting data on predator – prey relationships

    The following table shows hypothetical data on the populations of lions, zebras, antelopes and leopards in three national parks in Rwanda, namely Akagera, Nyungwe and Virunga.

    Table 1.3: Population of wild animals in national parks

    1. Identify predators and preys from the table.
    2. Come up with bar graphs on the population distribution of the animals in the three national parks.
    3. Assuming that lions eat zebras only and leopards eat antelopes only, comment about the population of the various animals in the parks.
    4. If you were a conservationist, what would you do in the three parks?

    (c) Grazing

    This is a method of feeding in which herbivores like cows, sheep and goats feed on grasses and plants. Look at the following picture. What is going on in the picture?


    Some domestic animals eat grass and plant parts. They are known as herbivores. Herbivores occupy the first level of feeding as primary consumers. They transfer energy from producers to the secondary consumers. Grazing is the eating of grass while browsing is eating parts of woody plants and leaves.

    Did you know: Animals grazing is not seen as predation because plants are not killed by the grazers. It is also not parasitism because a parasite is always with the host either inside or outside. Grazers are not always with the plants they feed on.

    (d) Mutualism

    Mutualism refers to an interaction between organisms of two different species where both benefit. Through this relationship both species enhance their survival, growth or fitness. It is the opposite of parasitism where only one species benefits and the other is harmed. Examples of mutualism include:

    • The relationship between ungulates like cows, goats, sheep and some bacteria found within their intestines. The ungulates manage to use cellulose which is broken down by cellulase enzyme produced by the bacteria. The bacteria benefits by getting nutrients and shelter whereas the ungulate is able to obtain energy from the cellulose broken down.

    • The relationship between a buffalo or a cow with the white egret which eats ticks from the buffalo or cow. The buffalo or cow benefits when ticks (parasites) are removed from its body while the birds benefit when they get food (ticks). • The relationship between bees and flowering plants. The bees get nectar as food while the flowers are pollinated by the bee.
    (e) Neutralism

    Neutralism is a term that describes the relationship between two species that interact but do not affect each other. It describes interactions where the health of one species has absolutely no effect whatsoever on that of the other. However, it is not easy to prove that the health of both species is not affected. This term is usually applied to relationships which are insignificant. An example is in a grassland ecosystem where hares, deers, frogs, live together without affecting one another.

    (f) Commensalism

    Commensalism is a relationship between two organisms where only one benefits while the other is not affected. In other words, one organism benefits while the other neither benefits nor gets harmed. Examples include:

    •  A fish known as Remora follows a shark wherever it goes so that when a shark kills a prey and eats it, there are some leftovers which are eaten by a Remora fish. Since only leftovers are eaten, the shark is not deprived of its food, neither does it get harmed in any way.
    • Mites will attach to wasps, flies or beetles for transportation. Can you think of other examples of commensalism? List them down in your notebook.
    (g) Amensalism

    Amensalism is any relationship between organisms of different species, whereby one organism is inhibited in terms of growth or is destroyed completely while the other organism remains unaffected. Examples include:

    • Hooves of cattle trampling grass and other plants. The grass and the plants are crushed and may die while the animal remains unaffected.
    • Plants with thick canopy block sunlight and prevent other plants beneath from growing.

    • Think of other examples of ammensalism. Write them down in your notebooks and share with other members of your class.
    heart Allelopathy

    Allelopathy is a biological phenomenon where one plant chemically inhibits the growth of another. In essence, plant allelopathy is used as a means of survival in nature, reducing competition from plants nearby.

    Activity 1.5: Observing how some plants affect others

    1. Look at the plants below. Do you know them?

    2. Visit a place where the plants above are growing.
    3. Observe the trend in the growth of other plants around the area. What can you conclude?

    Some plants produce chemicals known as allelochemicals. These chemicals influence germination, growth, survival, and reproduction processes of other plants. These chemicals can work in two ways. They can affect another organism positively or negatively. This means an organism that receives the biochemical can benefit or be harmed. Those plants that produce chemicals that harm others are said to be allelopathic.

    Trees are good examples of allelopathic plants. For instance, eucalyptus use allelopathy to protect their space by using their roots to pull more water from the soil so that other plants cannot thrive. Some like the maple plant use their allelochemicals to inhibit germination or impede development of nearby plant life. Most allelopathic trees release these chemicals through their leaves, which are toxic once absorbed by other plants. Trees that are known to exhibit allelopathy include maple, pine and eucalyptus.

    Self-evaluation Test 1.2

    1. Suggest biological methods of controlling:
    (a) Weeds in the farm.
    (b) Invasive species in a field.
    2. What kind of relationship is portrayed in the following interactions?
    (a) Worms in human intestines
    (b) A cat catching a mouse
    (c) Lions fighting over buffalo carcass
    (d) Cows grazing in a field
    (e) Insect in a flower
    3. The data below shows imagined numbers of how a predator and a prey affect each other's populations in an ecosystem over a period of several years.

    (a) What conclusion can you make about the effect of the predator on the population of the prey?
    (b) In what way does the population of prey affect the population of the predator?
    (c) What would happen if all the predators died of a disease?
    (d) How can a farmer apply the knowledge about the relationship between predators and preys?
    (e) Draw a graph to represent the above data.

    Unit summary

    •  Interdependence is mutual dependence between organisms.
    •  Interspecific relationships are the relationships that show the interactions between the organisms belonging to different species.
    • Intraspecific relationships are the relationships that show the interaction between organisms that belong to the same kind of species.
    • Parasitism is a relationship in which one organism benefits from the relationship while the other is harmed.
    • Predation is a kind of relationship in which one form of species serves as food to the other species. This involves a predator and prey relationship where one specie is hunted by the other as food to eat.

    • Competition is a relationship in which the two or more species are competing with each other to utilise the same limited resources that are necessary in order for them to survive.
    • Amensalism is a kind of relationshipin which the population of one species is inhibited while the population of another species is not affected.
    • Neutralism is a kind of relationship in which population of does not affect the other.
    • Commensalism is a relationshipbetween species in which one of the organisms benefits from the relationship while the organism is neither benefited nor harmed.
    • Allelopathy refers to the chemicalinhibition of one species by another.
    • Grazing is a method of feeding in which a herbivore feeds on plants such as grasses.

    1. Symbiosis is a mutually beneficial relationship between different organisms. Which among the following interactions is it closely related to?
    A. Neutralism
    C. Commensalism

    2. What is interdependence?
    3. Indicate whether the relationships below are commensalism or amensalism.
    (a) Barnacles attached to the sides of a whale. _______________
    (b) Cattle egrets: birds that follow grazing cattle. ________________
    (c) A flatworm attached to a horseshoe crab_______________
    (d) A donkey grazing in a field. ________________

    4. Differentiate between intraspecific and interspecific relationships using examples.
    5. Explain why a predator is a factor in determining the population of the species it eats, but a scavenger is not.
    6. Look at the photograph of a lion below. Identify features that makes the lion a good predator.


    7. What are the economic benefits of weeds and pests?
    8. What is the main disadvantage of allelopathy in plants?
    9. How are predators and their preys adapted to their environments?
    10. In your opinion is parasitism beneficial? Why?
    11. With reference to interdependence in plants and animals, explain why environmental conservation is important to the economy of Rwanda.
    12. The data in the following table shows the relationship between predator and prey in an area of savanna grassland. Use it to answer the following questions.

    (a) Draw a graph to represent the data in the above table.
    (b) Explain the population dynamics in the table.
    (c) Suggest animals that could depict such a relationship.
    (d) What is the impact of predator –prey dynamics in the ecosystem?

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