Ecology

• Ecology is the study of the interactions between organisms and the interactions of these organisms with their environment.

• Terms related to ecology: (a) Habitat – The place where an organism lives (b) Population – A group of individuals of one species that live in a particular habitat (c) Community – All the organisms that live in a particular habitat. It consists of populations of organisms that live close enough to interact with one another. (d) Ecosystem – Consists of a community and its physical environment. Physical factors in the environment that the community interacts with include pH, temperature, light intensity, water and nutrient availability, oxygen / carbon dioxide availability and salinity.

Energy transfer in an ecosystem

• A food chain is a sequence of energy transfer in the form of food, between organisms in an ecosystem.

• Each level of the food chain is known as a trophic level.

• Primary producers are photosynthetic organisms (autotrophs) that are able to convert light energy from the Sun to chemical energy that can be transferred from one organism to another within the ecosystem. They can also convert inorganic nutrients in the soil to organic nutrients that can be transferred up the food chain.

• Consumers obtain their energy by consuming other organisms. They occupy a few trophic levels: (a) Primary consumers feed on primary producers directly. They are herbivores. (b) Secondary consumers are carnivores that eat herbivores. (c) Tertiary consumers are carnivores that eat other carnivores.

• Food chains can be combined to form food webs since some food chains are interconnected.

• In reality, energy flow in an ecosystem is not so direct. There are many different types of consumers that feed at different trophic levels. For example, parasites and scavengers feed on producers and consumers at every level. Decomposers (bacteria and fungi) obtain their energy from non-living organic material such as faeces, fallen leaves and dead organisms. During decomposition, nutrients from these dead organic matter are released into the soil for plants to use.

• Energy enters the ecosystem from the outside. Light energy from the Sun gets converted to chemical energy in producers during photosynthesis. Some of the energy is lost as heat during respiration and other metabolic processes. The rest gets converted into organic matter called biomass.

• The energy moves up the trophic levels as producers get consumed by primary consumers, primary consumers get consumed by secondary consumers etc.

• Energy is lost at every trophic level as heat in respiration, uneaten organism parts and through waste material.

• Organisms at each trophic level pass on much less energy to the next trophic level than they receive.

• Food chains seldom have more than 5 trophic levels as less energy is available at the higher trophic levels.

• Eventually, all energy supplied to the ecosystem is lost as heat. Energy has to be constantly supplied to the ecosystem from the Sun as heat cannot be recycled into useful forms of energy.

• Food chain:

Pyramid of Numbers and Pyramid of Biomass

• A pyramid of numbers shows the population of each trophic level in a food chain. The pyramid of numbers shown below means that at any one time in a given area, there are 1000 maize plants, 100 mice, 10 snakes and 1 fox. The size of each block in the pyramid is proportional to the number of organisms present in that level.

• The pyramid of numbers can sometimes be inverted.

• A pyramid of numbers is not an accurate estimate of the amount of energy at each trophic level because the population number does not always correspond to the amount of energy it can transfer to the next trophic level, e.g. a single tree can support a large population of aphids.

• A pyramid of biomass shows the dry mass of organisms at each trophic level in a food chain.