8.5 Ecosystems

Biotic components (ESGB5)

Biotic components are living things that shape the ecosystem. Each biotic factor needs energy to do work and for proper growth. To get this energy, organisms either need to produce their own energy using abiotic factors, or interact with other organisms by consuming them. Biotic components typically include:

• Producers: also known as autotrophs include all green plants. Producers make their own food using chemicals and energy sources from their environment. The producers include land and aquatic plants, algae and microscopic phytoplankton in the ocean. Plants use photosynthesis to manufacture sugar (glucose) from carbon dioxide and water. Using this sugar and other nutrients (e.g. nitrogen, phosphorus) taken up by their roots, plants produce a variety of organic materials. These materials include starches,lipids, proteins and nucleic acids.
• Consumers: are also known as heterotrophs. They eat other organisms, living or dead, and cannot produce their own food. Consumers are classed into different groups depending on the source of their food. Herbivores (e.g. buck) feed on plants and are known as primary consumers. Carnivores (e.g. lions, hawks, killer whales) feed on other consumers and can be classified as secondary consumers. They feed on primary consumers. Tertiary consumers feed on other carnivores. Some organisms known as omnivores (e.g.crocodiles, rats and humans) feed on both plants and animals. Organisms that feed on dead animals are called scavengers (e.g., vultures, ants and flies). Detritivores (detritus feeders, e.g. earthworms, termites, crabs) feed on organic wastes or fragments of dead organisms.
• Decomposers: (e.g. bacteria, fungi) also feed on organic waste and dead organisms, but they can digest the materials outside their bodies. The decomposers play a crucial role in recycling nutrients, as they reduce complex organic matter into inorganic nutrients that can be used by producers. If an organic substance can be broken down by decomposers, it is called biodegradable.

Abiotic components (ESGB6)

Abiotic components are the non-living chemical and physical factors in the environment that affect ecosystems. Abiotic components play a crucial role in all of biology. Abiotic factors are broadly grouped into physiographic, edaphic and climactic factors and atmospheric gases.

1. Physiographic factors

Physiographic factors are those associated with the physical nature of the area. The main physiographic factors we will look at are slopes, aspect and altitude.

• Slope: is the gradient or steepness of a particular surface of the Earth. The slope affects the rate of water run-off. A steep slope encourages fast run-off of water and can cause soil erosion. The soil tends to be shallow and infertile with reduced plant growth. Plants are small and few animals are present. A gentle slope favours slower flow of surface water, reduces erosion, and increases availability of water to plants. The direction and steepness of a slope also influences the surface temperature of the soil.
• Altitude: is the height of the land above sea level. At high altitudes the temperature is lower, the wind speed is greater, and the rainfall less. Environments at higher altitudes are also more likely to experience snow conditions. Altitude plays a role in vegetation zones. At high altitudes, less plant and animal species are found. Plants growing at mid-altitudes experience more stunted growth. Plants at sea-level are abundant.
• Aspect: refers to the position of an area in relation to the sun or wind or wave action. It is the direction that the slope faces i.e. North, East, West. In South Africa rain fall is more common on the south-eastern slopes, therefore these tend to be forested or rich in vegetation. The slopes facing the other way (north west) tend to be drier.

2. Edaphic factors

Edaphic factors are those factors related to the soil. The qualities that may characterise the soil include drainage, texture, or chemical properties such as pH. Edaphic factors affect the organisms (bacteria, plant life etc.) that define certain types of ecosystems. There are certain plant and animal types that are specific to areas of a particular soil type. The particular factors we will consider include the pH of the soil and soil structure.

• pH of soil is a measure of how acid or alkaline soil is and can be measured by using the pH scale. The pH scale ranges from 0 to 14. Neutral solutions have a pH value of 7. Acid solutions have a pH value of less than 7 and alkaline solutions have a pH value greater than 7. Litmus paper or universal indicator can be used to determine whether a solution is acid or alkaline.

• Soil Structure: the decomposed organic matter, called humus gives topsoil its dark colour. It supplies plants with nutrients and helps the soil retain water. Soils rich in humus are fertile soils. The specific soil type is determined by the size of particles e.g sand has very large sized particles, clay has very small sized particles and loam has a mixture of particle sizes. If you roll moist soil between your fingers, clay soil feels sticky, sandy soil feels gritty and loam soil feels soapy. The water retention capacity of soils is the ability of soil to retain different amounts of water. Clay soil retains a large amount of water. Sandy soil retains very little water. Loam soil retains a moderate amount of water.

3. Climatic factors

• Sunlight: is essential for the process of photosynthesis. Producers, such as plants, rely directly on the sun. Heterotrophs, such as animals, use light from the sun indirectly by consuming plants or other heterotrophs. All organisms receive the energy required for survival through the break down of sugars and other molecular components that are produced by the autotrophs. These sugars are then broken to release the energy stored in them, by the process of cellular respiration.

• Temperature: varies greatly across different parts of the Earth and throughout the year. Temperature affects the rate of evaporation and transpiration and causes seasonal changes in weather. Seasonal variation in vegetation also occurs as the germination of seeds requires warm temperatures. Plants and animals have special adaptations that make them suited to the temperature of their specific environment. Temperature affects the rate at which photosynthesis, cellular respiration and decomposition take place. As you learnt in the earlier section on enzymes, this is linked to the optimal temperature profile for enzymes. The rate of reaction increases with increasing temperature and decreases at lower temperatures.

• Water: is one of the most important factors in the ecosystem. It is the main component of living cells and is essential for all living organisms. About \(\text{80}\%\) of the human body and \(\text{90}\%\) of the plant body consists of water. Water is not evenly distributed over the Earth. It is abundant in aquatic ecosystems and least abundant in deserts. Plants are adapted to the available amount of water in the following ways:

  • Xerophytes are plants that are able to live in dry habitats, or in regions with low annual rainfall. These plants are resistant to drought, have to cope with a shortage of water, high temperatures and light intensities and dry warm winds. We discussed in detail the adaptations developed by xerophytes in order to avoid water loss in the earlier chapter on plant structure.

  • Mesophytes are plants that need an average, regular supply of water.

  • Hydrophytes are plants that are able to live entirely or partially submerged in water or in very wet soil. These plants have to cope with a water surplus.

4. Air/gases

• Wind: speeds up evaporation and assists in pollination of plants and the dispersal of their seeds.

• Air: is composed of \(\text{78}\%\) nitrogen, \(\text{21}\%\) oxygen, \(\text{4}\%\) carbon dioxide and water vapour. Look ahead to the section on nutrient cycles to read more detail. Oxygen is used in cellular respiration and combustion and is returned to the environment by the process of photosynthesis. Carbon dioxide is a product of cellular respiration and decayed organic matter. It is removed from the atmosphere by plants during the process of photosynthesis. Nitrogen is needed by all living organisms for the synthesis of proteins. The amount of water vapour found in the air remains constant on average, however, it can vary greatly from one place to another. Some parts of the earth are prone to high humidity levels, while other locations have very dry air. Much of what we consider weather is caused by water vapour. The clouds in the sky are largely made up of it, and it is the condensation of this vapour into droplets that creates rain and snow.