4.2 Energy flow

Energy FLOWS through an ecosystem Energy in ecosystems begins as sunlight. Some of this energy is converted into glucose by plants. Energy passes from plants to animals along a food chain At every step some of the energy becomes heat. A waste product of respiration, movement, or just keeping warm. As the heat dissipates in the environment it is lost to living things. Luckily more sunlight energy consistently arrives from the sun.

Food Chains vs. Food Webs

Food webs show all of the feeding relationships within an ecosystem

Following energy flow through an ecosystem An ecological pyramid is a diagram that shows the relative amounts of energy or matter contained within each trophic level in a food chain or web Pyramid of numbers Pyramid of biomass Pyramid of energy

Ecological Pyramids An ecological pyramid is a diagram that shows the relative amounts of energy or matter contained within each trophic level in a food chain or web A pyramid of energy shows the flow of energy from one trophic level to the next in a community. The units of pyramids of energy are, therefore, energy per unit area per unit time, for example, kJ m–2 yr–1  

Pyramids of Number These pyramids are the simplest to calculate and draw. They often show a reduction in numbers of organisms as you move along a food chain. But only if all the organisms are the same size. In a forest trees are usually much bigger than insects feeding on them

Pyramids of number (not very useful for understanding energetics of an ecosystem)

Pyramids of energy always ‘bottom heavy’

Pyramids of energy

A Completed Pyramid of energy Scale: 1cm = 10000 kJm-2y-1

The advantages of pyramids of energy The energy value is calculated over the whole year, which allows for seasonal variations in populations. The energy content of the different animals is calculated which takes account of size as well as the nutritional value of organisms. (1g of bone has less energy than 1g of muscle)

Drawing a Pyramid of Energy You need an energy value for each tropic level. units are kJm-2y-1 - that is “kilojoules of energy per metre squared of ecosystem per year” Make a scale using the largest energy value and the FULL size of the page. E.g.: for an A4 page (21 cm) then 1cm = 10000 kJm-2y-1 is a good scale.

Steps to draw a pyramid of Energy Choose your scale (1cm = 10000 kJm-2y-1) Decide the height of each horizontal bar (2cm is usually good) Calculate the length of each bar. Find the centre of the page Draw the producers bar at the bottom, centred on the page Add the primary consumers, and then secondary consumers, etc. For the tertiary consumes a thin vertical line will be OK Add the scale and a title to the graph

Be careful! : This pyramid has the wrong dimensions!

Biomass pyramids

Biomass Definition: the total mass of organisms in a given area or volume. organic matter used as a fuel, especially in a power station for the generation of electricity

Pyramids of biomass always ‘bottom heavy’ in terrestrial ecosystems

Pyramids of biomass May be inverted or spindle shaped in aquatic ecosystems: Affected by longevity, ‘turnover’ And reproductive potential of members of Trophic level

Even pyramids of biomass don't show the flow of energy very well. Biomass is a better way to measure the amount of living material in each trophic level if the organisms are different sizes. In many cases the pyramid of biomass shows the trend we expect – there is a smaller mass of organisms at higher trophic levels. They assume that the populations are the same all year round. We know that in aquatic ecosystems there is a ‘bloom’ of producers in the spring. Some algae reproduce very rapidly. Although there may be a small mass of algae in the water today, it may have doubled in mass by tomorrow. Even pyramids of biomass don't show the flow of energy very well.

How can we improve efficiency? Food production is more efficient if the food chain is short, because a higher percentage of energy is available to the top trophic level.

Biology Update! The “rule of 10” was based on aquatic ecosystems. It still generally applies… but… Recent studies have shown that energy efficiency can range from 0.05% to 20%.

Calculating energy efficiency Only 4 kJ of the original energy available to the bullock is available to the next stage, which might be humans. The efficiency of this energy transfer is: efficiency = 4⁄100 × 100 = 4% 33 kJ This bullock has eaten 100 kJ of stored energy in the form of grass, and excreted 63 kJ in the form of faeces, urine and gas. The energy stored in its body tissues is 4 kJ. So how much has been used up in respiration? The energy released by respiration = 100 - 63 - 4 = 33 kJ Only 4 kJ of the original energy available to the bullock is available to the next stage, which might be humans. The efficiency of this energy transfer is: efficiency = 4⁄100 × 100 = 4%

The energetics of food chains is a factor in the efficiency of food production for world hunger