Energy, Ecosystems & the effects of human activity

Anything in red is directly from the spec and anything else in bold is of high importance but not on the spec. Italics represent side notes to consolidate understanding but are not necessary.

Ecosystem – Living (biotic) and non-living (abiotic) components in an area surrounding a species

Habitat – The natural home of an population of organisms

Community – all the organisms living in an ecosystem

The food chain

The sun is ultimately the source of all ecosystems as the producers (plants) at the bottom of the food chain use photosynthesis to convert sunlight into energy that the plants use to fill itself up with sugars as starch. Herbivores like rabbits then eat the plants, converting the plant’s stored energy (in sugars) into it’s own energy stores like fat. They are thus a primary consumer (the first to consume another organism). Animals higher up the food chain, such as carnivores then eat the animals and the chain continues. Each level of this chain is called a trophic level. This chain eventually has to end as more and more energy is lost to the surroundings. Detritivores (eg. earthworms) and decomposers (eg. bacteria and fungi) in the soil feed off of the dead organisms at each trophic level and contribute to the recycling of nutrients and thus the chain continues.

This is summarised below, where three trophic levels are shown:


Energy loss

Plants usually use only 1-2% of light energy to perform photosynthesis. Animals respire and excrete waste that also results in a loss of energy. The further you go up a food chain, the less energy there will be due to an accumulation of all these factors from the lower trophic levels.

Photosynthetic efficacy (PE) is a plant’s efficiency to perform photosynthesis (this is where we get the 1-2% from). You need to know how to calculate it via the following equation:


Gross production refers to the amount of product formed. However, some of this is used up in respiration. Net production considers this loss and is useful in calculating crop yield. It is calculated by the following equation:

net production


Primary and secondary productivity represent the energy available to primary and secondary consumers in the food chain as calculated above.

Relating to this is also energy transfer between trophic levels, calculated as followed:

Screen Shot 2016-06-02 at 10.12.56.png

For herbivores, only ~10% of all ingested plant material is actually converted into body mass (ie. used). The rest of the energy lost through respiration, due to avoiding parts of the plant (eg. roots) or an inability to digest cellulose (plant fibre). As cellulose is undigested, it is excreted directly into the soil, where saprobionts (decomposing bacteria) digest it.

Carnivores are much more efficient as their diet is rich in digestible protein from other animals.

Pyramids of Energy

We use three types of pyramids when we’re trying to represent energy conversion between organisms in food chains:

  • Number
  • Biomass 
  • Energy


This is the easiest measure to make (counting isn’t hard!). However, it doesn’t take into account the size of each species and thus is pretty hard to draw to scale and thus be representable data.

Screen Shot 2016-06-02 at 09.38.39.pngBiomass

Biomass is difficult to obtain accurate measurements and takes a long time to get (compared with counting at least).

Neither methods give us any help in identifying productivity and both can give inverted pyramids that don’t appear very representable.

Screen Shot 2016-06-02 at 09.46.01.png


This is by far the most accurate and representable way of quantifying energy transfer between trophic levels. It tells us the required energy to sustain a trophic level. Because energy is constantly being lost at each trophic level (respiration & excretion), the pyramid is always a pyramid, with the smallest energy at the top and the largest at the bottom.

Screen Shot 2016-06-02 at 09.48.38.png

Communities and succession

As nice as it is to say “that’s an ecosystem, that is“, ecosystems are constantly changing. Species-species interactions and environmental changes can drastically affect an ecosystem. For example…

Primary succession = the introduction of new species into a previously baron area (eg. after a volcano that wipes out all life).

Secondary succession = the reintroduction of species to an area that was previously occupied by other species (eg. forest fire that destroys habitats but opens up a world of bacterial heaven in the soil)

When an ecosystem is starting, there is great diversity in the species inhabiting the area. Soon though, natural selection and competition starts to take place that creates a constant balance between the numbers of each species to reach an almost equilibrium of sustained life for each species (eg. a well established woodland). This point is called a climax community (it’s reached it’s optimum conditions).

A good example that the A level course likes you to know about is the story of a plant based ecosystem transforming from a baron bare rock land to a thriving forest. It’s magical.

  1. Algae and Lichen take their places as the pioneer species (the first ones) that start to eat away at the bare rocks because no one else can (wahey, no competition!)
  2. This – along with rain contributing and the increase in decomposition – leads to the formation of a very rubbish soil
  3. Nearby spores from mosses blow over and improve the quality of the previously very rubbish soil so that simple plants like grass can grow
  4. As this decomposition continues, the soil get’s EVEN BETTER to the point where shrubs appear
  5. Eventually, you’ll get your long-living oak trees appearing, which is the climax community (as there’s a balance between grass/shrub/trees etc.)

Some stages of succession show a dominant organism (algae -> moss -> grass -> shrubs -> trees), these are referred to as sere where all the seres eventually reach climax with each other.

Human activity can alter the succession series including:

  • Grazing of sheep (or other farmlife)
  • Farming of the land (crops)
  • Deforestation and soil erosion

It’s not all doom and gloom though. Humans can also have positive impacts on maintaining optimum conditions of an environment. For example, on the moors red grouse birds feed off of the pioneer stage of heather (plant) and seek shelter in the ‘building’ phase. Humans routinely burn back heather in the autumn to encourage the reformation of the plant in the pioneer and building phases instead of moving onto the mature phase that the red grouse doesn’t benefit from.