2. Energy in Ecosystems

What you need to know...

    • Definitions of the terms species, population, producer, consumer, herbivore, carnivore and omnivore.

    • At each level in a food chain 90% of energy is lost as heat, movement or undigested materials.

    • Definitions and comparison of pyramids of biomass, energy and numbers.

    • Nitrogen in ecosystems

      • Animal and plant proteins are produced from nitrates.

      • The roles of nitrifying, denitrifying, root nodule and free-fixing soil bacteria.

      • Decomposers convert proteins and nitrogenous wastes to ammonium and nitrate.

    • Competition in ecosystems

      • Interspecific competition is when individuals of different species compete for the same resource in an ecosystem.

      • Intraspecific competition is when individuals of the same species compete for exactly the same resources.

Source: SQA


In this topic we look more closely at the transfer of resources between species in ecosystems. Organisms within a community depend on each other for energy, but also for other resources such as nitrogen. As well as studying the flow of energy and nitrogen through ecosystems, we'll also consider the affect competition for resources can have on species in ecosystems. Firstly however, you need to ensure that you are able to define the following terms:

Species: A species is a group of biologically similar organisms which can reproduce to produce fertile offspring (i.e. their offspring are healthy and can reproduce also).

Population: A population is a group of interbreeding organisms of a species in a particular area.

Producer: A producer is the organism in a food chain which fixes energy and produces organic compounds which are then available to other species in the food chain. Plants are producers as they use energy from sunlight to produce glucose from carbon dioxide through photosynthesis.

Consumer: Consumers are organisms which are not able to fix energy and must consume other organisms in order to obtain organic molecules.

Herbivore: Herbivores are animals which only consume plants.

Carnivore: Carnivores are animals which only consume other animals.

Omnivore: Omnivores are animals which consume both plants and animals.


As you already know, energy flows through food chains from the producer through different consumers. The producer, normally a plant, fixes energy into complex organic compounds which are used by the plant, and eaten by consumers. These consumers are in turn eaten by other consumers and so on. Have you ever wondered though why food chains are never longer than three or four species? The explanation is simple. At each stage in a food chain, most of the energy obtained by the organism is lost and is not available to the next organism in the chain. The reasons for this loss varies depending on the species, but can include the energy used by the organism to maintain its body temperature and move. In addition, not all of an eaten organism is digested and absorbed by the predator. As a result, at each stage in a food chain, only approximately 10% of the available energy is used for growth and passed on...meaning approximately 90% of the energy is lost (some of the lost energy in undigested materials will be released by decomposers). This means that the total available energy at each stage of the food chain is 90% less than the stage before, which is why there is a limit to the length of food chains. This is summarised in the diagram below.

Let's consider a simple food chain... Oak Tree -> Caterpillar -> Bluetit -> Sparrowhawk. There are various ways of visually summarising this food chain. Biologists often use pyramids to show the flow of resources through food chains, and these pyramids can be based on different measurements. The simplest form of pyramid is a pyramid of numbers. This is simply a count of a sample of the organisms of each species in the food chain. This normally produces a true pyramid shape as there quite often are a greater number of individuals of the species lower down the food chain. As you can see though from our example below, one oak tree can support a vast number of caterpillars.

We therefore normally do not use numbers to study energy flow in food chains. We can instead draw pyramids of biomass, or energy. A pyramid of biomass is a measure of the mass of a sample of each species in the food chain in grams in a certain area, whereas a pyramid of energy is measured in kJ per area per year. As you can see from the diagram above, because the biomass of the oak tree is larger than the combined mass of the caterpillars, we now get a true pyramid shape in this case. Pyramids of energy always have a true pyramid shape as energy is always lost at each stage of the food chain as explained above.


Another important resource which flows through ecosystems in Nitrogen (N). Nitrogen is a key component of amino acids, which are the repeating units of proteins, which we now know are crucial to all life. Although the air around us consists of about 78% nitrogen gas (N2), plants and animals are not able to use this nitrogen directly. We get our nitrogen from eating plants, or eating animals which in turn got their nitrogen from plants - but where do plants get their nitrogen from? This is where the nitrogen cycle comes in...

Perhaps the simplest place to start is with the legumes. The plant on the top right of the diagram above is a legume...legumes are plants such as beans and peas which have symbiotic relationship with certain bacteria. These bacteria live in 'nodules' on the roots of legume plants (shown in the image on the right from WikiMedia Commons) and are nitrogen-fixing. This means these bacteria can convert nitrogen from the air into a form which the plant can then absorb and use to produce amino acids and, ultimately, proteins.

Most species of plant do not have root nodules with nitrogen-fixing bacteria in them, so where do they get their nitrogen? Most plants need to absorb nitrogen in the form of nitrates (NO3). Nitrates are produced by nitrifying bacteria in the soil from ammonium (NH4). As shown in the diagram above, this ammonium can come from one of two sources. Some of the ammonium comes from nitrogen-fixing bacteria which live freely in the soil, but the rest comes from the decomposition of proteins from dead plants and animals and their nitrogenous wastes by decomposer bacteria and fungi. Not all of the bacteria in the soil are helpful however, some bacteria are denitrifying which means they convert nitrates back to nitrogen gas, making the nitrogen unavailable to most plants again.

You can explore the nitrogen cycle further with this animation [requires Flash].


Most ecosystems have limited resources for the communities they support. As a result, competition normally occurs between the organisms of a community. The resources organisms compete for depend on the species. For example, plants in a particular area will compete for light, whereas most animals would not. Resources which organisms will compete for include food, water and space. There are two types of competition which you need to know about: interspecific and intraspecific.

Interspecific is the type of competition that you probably most readily think of when considering competition. This is the competition between different species in an ecosystem for the same resources. We mentioned a good example of this in the previous topic. Red and grey squirrels are different species, but they compete for similar resources. This is interspecific competition.

Grey and Red Squirrels by Heidi Welch

We don't often consider the other type of competition, but if you think about it it's obvious. Members of the same species compete with each other for resources also. In fact, because members of the same species compete for exactly the same resources, this form of competition can be more intense than interspecific competition. Competition between members of the same species is called intraspecific competition. In addition to competing for resources such as food, water, etc., members of the same species will also compete with each other for a mate. As we'll learn later in this unit, the transfer of genes from parent to offspring is one of the primary drivers of evolution, and so members of a species will compete with each other for a suitable mate in order to reproduce and pass on their genes. For example, deer such as the ones in the picture below will physically compete for a mate. Much intraspecific competition does not involve direct fighting and simply involves the competition for resources as in interspecific competition.

Image by Peter Trimming

You can read more on competition on this website.