10.2 Energy transfer through ecosystems

What is a trophic level & what do the arrows represent in a food chain/web?

• Trophic level = each stage in a food chain

• Arrows represent the flow of energy through the food chain

What do food chains always start with?

Photosynthetic (autotrophic) producers

Why do all living organisms need energy & matter from their environment?

• Matter is needed to make new cells (growth) & to create new organisms (reproduction)

• Energy is needed to drive all the chemical & physical processes of life (e.g. active transport & movement)

What is the difference between detritus and detritivore?

• Detritus = dead and waste matter that is not eaten by consumers

• Detritivore = an animal that eats detritus

(saprobiont = a microbe, e.g. bacterium/fungus, that lives on detritus)

What is matter?

• Measured in kilograms & comes in 3 different states: solid, liquid & gas

• It cannot be created, destroyed or used up

• The total amount of matter on Earth is constant (Earth is a closed system)

• The matter of a living organism is called its biomass

What is energy?

• Measured in joules & comes in many different forms (e.g. heat, light, kinetic, etc)

• These forms can be inter-converted but energy can never be created, destroyed or used up

• Energy can enter & leave the Earth (Earth is an open system)

• Total amount of energy on Earth remains roughly constant

What are pyramids of number?

• Show the numbers of organisms at each trophic level in a food chain

• Widths of bars = numbers

• In general, as you ascend the food chain, the size of the organisms increases & the number of organisms decreases

What are pyramids of biomass?

• Consider the total mass (biomass) of living organisms at teach trophic level

• The biomass should be dry mass → water stores no energy

• Biomass is found by drying & weighing organisms

• Usually pyramid shaped, since if a trophic level gains all its mass from the level below, then it cannot have more mass than that level

How much biomass is passed onto the next trophic level & why?

Only around 10% - most is lost at each trophic level due to:

• some of the biomass is used in respiration & converted to carbon dioxide & water, which are excreted

• some of the biomass is simply either not eaten or indigestible

What are pyramids of energy?

• Represent the flow of energy into each trophic level, so describe a period of time

• Always pyramidal & very shallow (energy transfer from one trophic level to the next is very inefficient)

• Less than 10% of energy is passed onto the next level → “missing” energy is lost as heat

What is matter lost as in food chains?

• Carbon dioxide due to respiration

• Uneaten parts (e.g. skin, bones, teeth, shells, wood, bark)

• Waste (e.g. faeces, urine)

What is energy lost as in food chains?

• Chemical energy in the uneaten parts

• Movement energy of consumers

• Heat energy, especially in the homeothermic animals