BIOLOGY O LEVEL(FORM TWO) NOTES – BALANCE OF NATURE

BALANCE OF NATURE

THE NATURAL ENVIRONMENT

• The natural environment is made up of all living and non-living things that occur naturally on Earth. It includes air, water, animals, plants, microorganisms, stones, clouds, rocks, and soil.

IMPORTANCE OF THE NATURAL ENVIRONMENT

1. It is a source of food for organisms.
2. Provides shelter and security for organisms.
3. It provides an appropriate setting for organisms to reproduce and increase in number.
4. It allows living and non-living things to interact.

COMMON TERMS IN STUDYING THE NATURAL ENVIRONMENT

BIOTIC FACTORS: All the living components of the environment, such as plants, animals, and microorganisms.

ABIOTIC FACTORS: The non-living components of the environment, e.g., light, water, rocks, and soil.

ECOLOGY: The branch of biology that deals with the study of the relationship between living things and their natural environment.

POPULATION: The total number of a certain species of organisms in a community. Example: number of frogs in a pond.

COMMUNITY: The populations of different organisms living in a specific area called a habitat. Example: a grassland community could include grass, acacia trees, lions, antelope, giraffes, and cheetahs.

HABITAT: A specific area with a specific set of conditions that is appropriate for a certain community and where the community lives; it is the home of living organisms. Examples include tropical rainforests, deserts, swamps, ponds, grasslands, and oceans.

ECOSYSTEM: A natural unit made up of living and non-living things whose interactions lead to a self-sustaining system.

• An ecosystem is made up of communities.

BIOTIC FACTORS

Biotic factors are the living components in the environment. They can either be population factors or community factors.

1. POPULATION FACTORS

• Population density – The number of organisms in a given area.
• Dispersion – The geographical distribution of organisms in an area.
• Age structure – The distribution of population according to age.
• The ratio of males to females.
• The number of births.
• The number of deaths.
• Population growth – The rate at which the number of organisms is increasing.

2. COMMUNITY FACTORS

This includes food chains and food webs, which are feeding relationships that represent the flow of energy and recycling of nutrients in a community.

ABIOTIC FACTORS

These are non-living components of the environment. They can be climatic, soil, or geological.

CLIMATIC FACTORS

1. Temperature is the degree of heat in a place.
   • Organisms adapt to the temperature in their habitats in various ways.
   • Example: Some plants shed their leaves or roll them when it is very hot to avoid excessive loss of water.
   • Animals in very cold places have extra fat for insulation.
2. Light: It is obtained from the sun.
   • Affects the opening of stomata.
   • Important during photosynthesis.
   • Example: Some animals hunt in bright daylight (e.g., hawks), others hunt at dawn or dusk when there is dim light (e.g., lions), and others hunt at night (e.g., owls and bats).
3. Wind: It increases the rate of water evaporation from water bodies as well as from living organisms.
   • Wind is important in the formation of rain and the dispersal of some plant seeds.
4. Atmospheric pressure: When atmospheric pressure is high, the concentration of oxygen and carbon dioxide also becomes high, which affects gaseous exchange and respiration. The opposite is also true. This affects the distribution of both plants and animals.
5. Water serves as a habitat for a large variety of organisms. It serves as a solvent, a medium of transportation, and a temperature regulator.

AQUATIC FACTORS

These are factors that affect water bodies and life in water.

1. Salinity: The quantity of salts dissolved in water.
   • Aquatic plants have roots that absorb mineral salts and water depending on the concentration in the plant cells.
   • Some aquatic organisms are adapted to live in freshwater habitats while others live in saltwater habitats.
2. Wave action: Important for organisms living in the intertidal zone. These organisms are covered by water during high tide and exposed during low tide.
   • Such organisms include shrimps, different types of seaweeds, crabs, and prawns.

SOIL FACTORS

1. Soil texture – Refers to the size of soil particles.
   • Soil texture affects drainage, fertility, and distribution of plants.
2. Soil composition – The proportion of components of soil including mineral salts, air, microorganisms, water, and remains of living things.
   • These components affect soil fertility and hence plant growth.
3. Soil pH – The degree of acidity or alkalinity of soil.
   • Different types of plants grow in soils with different pH values. Example: tomato and pineapple grow in slightly acidic soil, while onions and cabbage prefer slightly alkaline soil.

GEOLOGICAL FACTORS

• These are factors concerned with the features of the land surface.

(i) Altitude – The height above sea level.
– At high altitudes, atmospheric pressure, temperature, and oxygen concentration are lower, while rainfall is higher compared to low altitude. This affects the distribution of both plants and animals.

(ii) Geological substratum – Refers to the various types of rocks that disintegrate to form the soil in an area. The chemical composition of the rocks is reflected in the chemical composition of the soil, determining soil pH and fertility.

(iii) Slope – Refers to the gradient of land. Slope can be steep, moderate, or gentle. Soil erosion is highest on steep slopes and lowest on gentle slopes. Soil erosion removes topsoil, making the soil less fertile, which interferes with plant growth.

Interaction of living and non-living things

The interaction of biotic and abiotic components of the environment is important for the completion of natural cycles such as the water cycle, the carbon cycle, and the nitrogen cycle.

The water cycle

The water cycle refers to how water circulates in the environment. Movement of water in the environment occurs as shown in the figure below:

Precipitation

In the water cycle:

1. Groundwater and run-off (water from rain) flow into streams and rivers.
2. The streams and rivers flow into lakes and oceans.
3. Water evaporates into the atmosphere from water bodies such as oceans and lakes and from plants through transpiration.
4. The evaporated water precipitates to form water vapor. Water vapor condenses to form clouds.
5. Wind causes clouds to move, for example, from above the ocean to above the land.
6. Rain falls and is absorbed by plants or forms groundwater and run-off. The cycle thus begins again.

Forests act as water catchment areas and prevent excess loss of water from the land. Wetlands, such as swamps and marshes, help to control flooding and are also important for water purification.

The carbon cycle

Carbon cycle refers to a biochemical cycle in the environment whereby carbon dioxide is taken up from the atmosphere and incorporated into plant tissues during photosynthesis.

Carbon cycle

Carbon dioxide from the atmosphere is absorbed by plants and used for photosynthesis. These plants serve as food for herbivores, which are in turn eaten by carnivores.

When plants and animals die, microorganisms cause decomposition and carbon dioxide is released into the atmosphere.

The remains of plants and animals after millions of years result in the formation of fossil fuels such as coal, natural gas, and oil. When these fuels are burnt, they release carbon dioxide into the atmosphere.

Respiration of living things also releases carbon dioxide into the atmosphere. The carbon dioxide is absorbed by plants and then the cycle starts again.

The nitrogen cycle

Nitrogen cycle refers to a biochemical cycle in the environment whereby nitrates in the soil are taken up by plant roots and may pass along food chains into animals that absorb it in this form. It must first be converted into either nitrates or ammonium compounds. The figure below shows the nitrogen cycle.

1. Nitrogen-fixing bacteria in the root nodules of legume plants carry out fixation by converting atmospheric nitrogen to nitrates.
2. Lightning converts atmospheric nitrogen into nitrates. The nitrates get into the soil and are later absorbed by plants. Plants use nitrogen compounds to produce plant proteins. Plants are eaten by animals. Animals use the nitrogen to produce animal proteins. When plants and animals die, decomposers such as bacteria and fungi feed on them. The decomposers release ammonia gas (NH₃) which contains nitrogen. The ammonia is converted into nitrites and then nitrates by bacteria. Denitrifying bacteria release nitrogen from nitrates back into the atmosphere.

INTERACTION AMONG LIVING ORGANISMS

The relationships among organisms in the environment can be explained in the form of predation, competition, and symbiosis.

PREDATION – This happens when one organism captures, kills, and feeds on another in order to get nutrients. Example: cats eat mice and sharks eat fishes.

COMPETITION – This is a relationship where organisms both need the same limited environmental resources for survival. Examples: lions and leopards both hunt antelope and zebra; hence lions and leopards are competitors. Organisms from the same species can also be competitors. Example: cows competing for grass.

SYMBIOSIS – This is a relationship whereby there is a close association between organisms. This association could take various forms, such as mutualism, commensalism, neutralism, synnecrosis, amensalism, and parasitism.

MUTUALISM – This is a relationship in which two organisms benefit each other. Example: The rhizobium bacteria in the root nodules of legumes convert nitrogen into nitrates for use by the plant. The bacteria get protection and nutrients from the plants.

COMMENSALISM – This is an interaction that is beneficial to one organism and neutral to the other organism. Example: when a bird builds a nest in a hole in a tree.

PARASITISM – In this association, one organism benefits while the other is harmed. Example: Plasmodium that causes malaria in human beings.

FOOD CHAINS AND FOOD WEBS

Food chains and food webs show the flow of nutrients and energy among organisms in the environment.

• Each organism in a food chain or food web represents a trophic level.
• Trophic level – The position that an organism occupies in a food chain or food web.
• Examples: producers like green plants, primary consumers like herbivores, and secondary consumers like carnivores.

PRODUCERS

These are organisms that can manufacture their own food. Example: green plants and photosynthetic bacteria. This is the first trophic level.

• Producers are eaten by primary consumers.
• Primary consumers are mostly herbivores such as rabbits, cows, buffaloes, wildebeests, goats, and sheep.
• Primary consumers form the second trophic level.

SECONDARY CONSUMERS

These form the third trophic level.

• They feed on primary consumers.
• They are mostly carnivores such as domestic cats, dogs, hyenas, lions, leopards, and cheetahs.
• This level can be followed by tertiary consumers which feed on secondary consumers, then quaternary consumers feed on tertiary consumers.

DECOMPOSERS

These are organisms that decompose dead organic matter.

• This is the final trophic level.
• These organisms feed on dead matter and break it down, thereby facilitating decomposition.
• The two main decomposers are the saprophytic fungi and saprophytic bacteria.
• The relationship between organisms at different trophic levels can be presented diagrammatically as follows:

FOOD CHAINS

A food chain is a linear relationship among the organisms of a community in which each organism feeds on the one preceding it.

• It presents energy flow from one trophic level to the next.
• Each organism feeds on, therefore derives energy from, the preceding one; in return, it is eaten by the next organism, providing energy for it.
• The arrows indicate the direction of energy flow.

Example of a food chain:

FOOD WEBS

A food web refers to several food chains interlinking together (a network of food chains).

• Most herbivores consume more than one kind of plant, omnivores consume more than one kind of plant and animal, and decomposers consume more than one kind of herbivore.

Example of a food web:

SIGNIFICANCE OF FOOD CHAINS AND FOOD WEBS

1. Food chains and food webs facilitate the flow of energy in the environment.
2. They help to maintain the balance of the total numbers of organisms in the environment.