PRODUCTIVITY AND ENERGY FLOWSource: http//web3.ibo.org/

ENERGY ENTERS THE ECOSYSTEM AS SUNLIGHT

• Only 2% of the light energy falling on a tree is captured and turned into chemical energy by PHOTOSYNTHESIS

• The rest is reflected, or just warms up the tree as it is absorbed

Gross productivity is a measure of total energy captured

• It is measured in joules (J)

• It is the energy captured or assimilated by an organism

• for a plant, it is the sunlight actually fixed by the leaves during photosynthesis

Energy is stored in leaf as sugars and starches, which later are used to form flowers, fruits, seeds,etc.

Net productivity • Plants have to use some of the energy they capture

to keep themselves growing:– they move water and stored chemicals around– they make hormones and enzymes to control their

metabolism– they make flowers, fruits, new leaves and stems

• the energy left over after they have used what they need is called the net productivity

• the energy used by the plant for essential tasks is called respiration energy, and eventually it is released to the environment as heat

So how are gross and net productivity related ?

Net productivity = Gross productivity – Respiration Energy

or using symbols NP = GP - R This equation applies to animals too,

but more on that later….. The productivity of a plant is called:

PRIMARY PRODUCTIVITY

So, what about animal productivity?• Animals must eat other organisms to obtain energy, unlike

plants which photosynthesize• Animals may eat plants or animals or both• Not all the energy in food is absorbed (assimilated) into an

animal’s body• Unassimilated food is ejected as faeces or droppings

So gross productivity = food assimilated or gross productivity = food eaten – energy in

faeces

What about net productivity for an animal (secondary consumer)?

• Gross secondary productivity = Energy eaten - energy in faeces

• As well as keeping themselves alive, animals must use energy to move and keep warm - plants need rather less energy- but in the end it, as in plants, it all turns to heat

• Net secondary productivity (NSP ) = food eaten - faeces - respiration energy

so NSP = GSP- R (just like plants)

?

Net and gross productivity in the food web

• In a food web diagram, you can usually assume that:– the energy input into an organism represents the

GP– the energy output from that organism to the

next trophic level represents the NP– the difference between GP and NP = R

(respiration energy ) and/or loss to decomposers

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Now check what you have understood!

– Complete this energy flow diagram: • label GPP, NPP and R for the primary producer• add arrows to show missing energy pathways (3 in total)• fill in the blank box to explain why some sunlight is not fixed

by plant

SUN PLANT

DECOMPOSERS

RESPIRATION

……………………..……….

HERBIVORES

The following data were collected in a study of secondary productivity in a population of woodlice. Some of the woodlice produced offspring during the experiment.

Source: IB Examination Paper 1 May01

Estimated Dry Mass at start of experiment (g)

Estimated Dry Mass at end of experiment (g)

Adult woodlice 1.53 1.59Young woodlice - 0.63Food (dead leaves) 22 19.45Fecal matter - 0.84

ANSWER1. What is the Gross Productivity (g) of this

population over the period of the experiment?Answer: GP = Food Eaten – Fecal Loss GP =

(22 - 19.45) – 0.84GP = 1.71

2. What is the Net Productivity (g) of this population over the period of the experiment?

Answer: NP = Biomass (dry mass)NP = 1.59 – 1.53 + 0.63NP = 0.69

ANSWER

1. What is the Gross Productivity (g) of this population over the period of the experiment?

2. What is the Net Productivity (g) of this population over the period of the experiment?