Levels of ecological organisation
A habitat is a type of environment in which an organism lives. For example, the habitat of a giraffe is grassland (savannah) with groups of trees such as Acacia. The habitat of a woodlouse (Oniscus) is a humid, dark place such as beneath the bark of a rotting log. The habitat of a mangrove tree is a muddy sea shore that is regularly flooded by the tide.
A population is a group of organisms of the same species that lives in the same place at the same time. If the species is a sexually-reproducing one, the organisms in the population are able to interbreed with one another. For example, all the giraffes in a particular area of savannah make up the giraffe population.
A community is all the organisms, of all the different species, that live in the same place at the same time. For example, all the giraffes and other animals, all the plants, all the fungi and all the bacteria make up a community in the savannah. Each type of habitat tends to have its own typical community.
An ecosystem is the interactions that take place between all the organisms in a community and their non-living environment. For example, an ecosystem in an area of African savannah would include the predator-prey relationships between giraffes and lions, the feeding relationships between grass and giraffes, the exchanges of oxygen and carbon dioxide between the air and the living organisms, the availability of mineral ions in the soil that can be taken up by plant roots, and so on. Strictly speaking, an ecosystem is not simply a place but a dynamic series of interactions between organisms and their environment.
A niche is the role of an organism in an ecosystem. Different species have different niches, although these may overlap. For example, both giraffes and zebras are herbivores that require open grassland and a water supply. However, giraffes are able to browse on vegetation from high tree branches, whereas zebras graze on grass and other low-growing plants.
Note that you are expected to have studied an ecosystem in an area familiar to you.
Energy flow through ecosystems
Living organisms require energy to maintain metabolic processes that keep their cells alive. Most of this energy is released from organic molecules such as glucose by respiration. The energy released Is used to make ATP.The energy can then be released in smallquantities, exactly when and where it is required, by hydrolysing the ATP to ADP and inorganic phosphate.
Each organism therefore needs a supply of energy-containing organic molecules in order to be able to make ATP. Organisms that can use energy from other sources, such as sunlight, to make these organic molecules are calld producers. In most ecosystems, the producers are plants, which make carbohydrates by photosynthesis. They absorb energy from sunlight and incorporate it into carbohydrates, where it is stored as chemical potential energy.
Animals and fungi depend on taking in organic molecules that were originally synthesised by plants. They are consumers.
A food chain shows the pathway by which energy is passed from one organism to another. The energy is transferred in the form of chemical potential energy in food. The arrows in the food chain indicate the direction of energy transfer. A food web is a network of interconnecting food chains.
The position at which an organism feeds in a food chain is called a trophic level. Producers are at the first trophic level, primary consumers (herbivores) at the second trophic level, secondary consumers (carnivores that feed on herbivores) at the third trophic level, and so on.
Large quantities of energy are lost in the transfer between one trophic level and the next. For example, only about 10% of the energy in the grass in an area of savannah is passed on to herbivores. This is because:
• Not all the grass is eaten. Some is trampled or covered by animal droppings, or may grow too low to the ground for animals to be able to graze it. Pollen from grass flowers may be blown away by the wind before it is eaten. Leaves may die and fall to the ground before they are eaten.
• Not all the grass is available to be eaten. The roots, for example, are underground where few animals will find and eat them.
• Of the grass that is eaten, much is indigestible inside the alimentary canals of the herbivores. Cellulose and lignin are difficult to digest and may simply pass out in the faeces rather than being absorbed into the herbivores' bodies.
• The grass plants require energy themselves, which they obtain by respiration. This breaks down organic molecules to carbon dioxide and water, and the energy is eventually lost as heat, so is no longer available to herbivores.
The diagram shows the quantities of energy transferred between organisms in a food chain in a salt marsh. The figures are in kJ m -2y -1. Only three trophic levels are shown.
efficiency = (kJ of energy transferred to secondary consumers : kJ of energy transferred to primary consumers) x 100
= (30:300) x 100
(a) define the terms habitat, niche, population, community and ecosystem and be able to recognise
examples of each ;
(b) explain the terms autotroph, heterotroph, producer, consumer and trophic level in the context of food chains and food webs ;
(c) explain how energy losses occur along food chains and discuss the efficiency of energy transfer
between trophic levels;