Pyramids of numbers, biomass and productivity

STARTER: Plot these values on a bar graph Position in food chainNumber of organisms Producers100 Primary consumers50 Secondary consumers25 Tertiary consumers10 Colour the bars using the key below Producer – Green Primary consumer – Yellow Secondary consumer – Blue Tertiary consumer - Red

Ecological Pyramids These are graphical models of the quantitative differences between the amounts of living material stored at each trophic level of a food chain. They allow us to easily examine energy transfers and losses There are three types which measure: Number Biomass Productivity (Energy)

Pyramid of Numbers A pyramid of numbers shows the number of organisms at each trophic level of the food chain

Now… Cut out each of the bars and place them onto a separate piece of paper… Which bar will go at the bottom? Which bar will go at the top? Stick them down…

Label! Label each side of the pyramid with the following terms: Left SideRight Side ProducersPlants Primary consumersHerbivores Secondary consumersCarnivores Tertiary consumersTop carnivores

Pyramid of numbers The length of each bar gives a measure of the relative numbers Pyramids have producers at the bottom, usually in the greatest number

Pyramid of numbers But some may have a large single plant, a tree, as the producer so the base is one individual

Pyramids of numbers AdvantagesDisadvantages Simple easy method of giving an overview Good for comparing changes in population numbers over times or season All organisms are included regardless of their size – therefore some pyramids are inverted They do not allow for juveniles or immature forms Numbers can be too great to represent accurately

GROUP ACTIVITY

Pyramids of number

lettuce

snail

hedgehog

flea

If we start to think about how many of each organism there are, we can start to build a pyramid of numbers...

Is it really fair that 1 lettuce counts for the same size box as 1 flea?

Pyramids of biomass

STARTER: Is it really fair that 1 lettuce counts for the same size box as 1 flea in a pyramid of number? Explain your reasoning? A better way is to look at the quantity of living material (biomass) at each feeding (trophic) level...

lettuces snails hedgehogs fleas Pyramids of biomass are always “pyramid- shaped” as at each stage of a food chain energy (biomass) is lost through respiration and body waste.

Pyramids of biomass A pyramid of biomass contains the biomass (mass of each individual x number of individuals) at each trophic level. The units of a pyramid of biomass are units of mass per unit area, often grams per square metre (g m -2 ) or volume of water (g m -3 ) or sometimes as energy content (joules, J) A pyramid of biomass is more likely to be a pyramid shape

Pyramids of biomass

AdvantagesDisadvantages Overcomes the problems of pyramids of numbers Only uses samples from populations so it’s impossible to measure biomass exactly Organisms must be killed to measure dry mass The time of year affects the results Two organisms with the same mass do not have the same energy content

GROUP THOUGHT: What is wrong with this pyramid of biomass?

STARTER Why are biomass and numbers SMALL at HIGHER trophic levels?

Now try building a pyramid of biomass

STARTER: Energy loss in a food chain

Pyramids of productivity show That the amount of available energy decreases down the food chain It takes a large number of producers to support a small number of primary consumers It takes a large number of primary consumers to support a small number of secondary consumers

PYRAMIDS OF PRODUCTIVITY show the flow of energy between tropic levels

Pyramids of productivity AdvantagesDisadvantages Shows the actual energy transfers and allows for rate of production Allows for comparison of ecosystems based on relative energy flow Pyramids are not inverted Energy from solar radiation can be added It is very difficult to collect energy data as the rate of biomass production over time is required It is difficult to assign some species to a particular trophic level when they may be omnivorous

Group Activity Make a poster with the information given in the handout Things to include: 1. Units for the different pyramids 2. Explanations for the shape of each pyramid of number 3. Explanation for why a pyramid of biomass is always ‘pyramid shaped’ 4. Description of how a pyramid of energy shows energy flow through a food chain