Energy flow, productivity and biological magnification – Concept of an Ecosystem (Senior 6 Biology) | Ganzaa

Concept of an Ecosystem

Lessons

Senior 6 · Biology

Concept of an Ecosystem

5 lessons

Lesson 3 of 5

Energy flow, productivity and biological magnification

25 min

Why are lions rare?

In Akagera, a visitor may see hundreds of impala and zebra in a day, but lions only with luck. Lions are not rare because they are weak. They are rare because of energy: the grass of a savanna captures only a small part of the sunlight that falls on it, the grazers turn only a small part of the grass into new body tissue, and the lions turn only a small part of the grazers into new lion. At each step most of the energy is lost as heat. So an ecosystem can feed many grazers but only a few top predators.

In Senior 2 you drew food chains, food webs and pyramids. Here you will put numbers on the flow: productivity, the efficiency of each transfer, and why some poisons become more concentrated up a food chain.

Definition

Trophic level: the feeding position of an organism in a food chain: producers (level 1), primary consumers (2), secondary consumers (3), tertiary consumers (4).

Gross primary productivity (GPP): the rate at which producers convert light energy into chemical energy in organic compounds by photosynthesis, per unit area per unit time. Unit: $\mathrm{kJ\,m^{-2}\,yr^{-1}}$ .

Net primary productivity (NPP): the rate at which energy is stored in new plant biomass, after the plants' own respiration: $\text{NPP} = \text{GPP} - R$ . NPP is the energy available to consumers and decomposers.

Ecological efficiency (efficiency of energy transfer): the percentage of the energy in one trophic level that becomes energy in the next, $\frac{\text{energy in level } n+1}{\text{energy in level } n} \times 100$ . It is often about 10 %, but ranges from under 1 % to about 20 %.

Biomass: the mass of living material in an area, usually measured as dry mass, $\mathrm{g\,m^{-2}}$ .

Following the energy

Figure 1 follows the energy through one square metre of savanna for one year (illustrative values, in $\mathrm{kJ\,m^{-2}\,yr^{-1}}$ ).

Figure 1: Energy flow through a savanna grassland (illustrative values, kJ m⁻² yr⁻¹; arrows not to scale). Orange arrows: heat lost in respiration. Brown arrows: dead matter and faeces to decomposers. Dashed arrows: the losses of the secondary consumers.

Why so little sunlight is fixed. Most of the light that reaches a plant is never used: some is reflected or passes through the leaves, much has a wavelength that chlorophyll does not absorb, and photosynthesis is often limited by carbon dioxide, water or temperature. Of the light that is absorbed, much is lost as heat in the reactions themselves. Typically only 1 to 2 % of the sunlight falling on a crop, and less in most natural ecosystems, ends up as GPP.

Why so little passes from one trophic level to the next.

Not all of a level is eaten: roots, wood and dead leaves go to decomposers instead.
Not all that is eaten is digested: cellulose, bone and hair leave in the faeces (egestion).
Of what is absorbed (assimilated), much is used in respiration for movement, active transport and, in birds and mammals, keeping the body warm. That energy leaves as heat.
Some is lost in excretion, as urea.

Energy therefore flows through an ecosystem in one direction and leaves as heat; it is not recycled. Nutrients, as you will see in Lesson 4, cycle.

Worked example 1: GPP, NPP and efficiency of photosynthesis

Use the values in Figure 1. Calculate (a) the NPP, (b) the percentage of the sunlight converted into GPP, and (c) the percentage of GPP that is available to consumers.

(a) $\text{NPP} = \text{GPP} - R = 36\,000 - 20\,000 = 16\,000\,\mathrm{kJ\,m^{-2}\,yr^{-1}}$ .
(b) $\frac{36\,000}{6\,000\,000} \times 100 = 0.60\,\%$ .
(c) $\frac{16\,000}{36\,000} \times 100 = 44\,\%$ (2 s.f.). The plants use the other 56 % of their GPP in their own respiration.

Check the units: every value is per square metre per year, so the percentages are valid. If one value were per hectare, you would first convert: $1\,\mathrm{ha} = 10\,000\,\mathrm{m^2}$ .

The energy budget of a consumer

For any consumer, the energy it eats (ingestion, $I$ ) is shared between faeces ( $F$ ), respiration ( $R$ ) and new biomass (production, $P$ ); energy lost in urine is often included in $R$ for simplicity:

I = F + R + P \qquad A = I - F \qquad P = A - R

where $A$ is the energy assimilated (absorbed from the gut). Two ratios compare animals:

\text{assimilation efficiency} = \frac{A}{I} \times 100 \qquad \text{net production efficiency} = \frac{P}{A} \times 100

Endotherms (birds and mammals) spend much of their assimilated energy on keeping warm, so their net production efficiency is low, often only 1 to 5 %. Ectotherms (insects, fish, reptiles) do not heat their bodies from respiration in the same way, so they convert a larger share, often 10 to 40 %, into biomass.

Worked example 2: a young bull and a grasshopper

Illustrative data. A young bull eats grass containing $144\,000\,\mathrm{kJ}$ a day, loses $50\,000\,\mathrm{kJ}$ in faeces and $88\,000\,\mathrm{kJ}$ in respiration (including urine and methane). A grasshopper eats leaves containing $100\,\mathrm{J}$ , loses $63\,\mathrm{J}$ in faeces and $22\,\mathrm{J}$ in respiration. Compare their net production efficiencies and explain the difference.

Bull: $A = 144\,000 - 50\,000 = 94\,000\,\mathrm{kJ}$ ; $P = 94\,000 - 88\,000 = 6\,000\,\mathrm{kJ}$ .
Net production efficiency $= \frac{6\,000}{94\,000} \times 100 = 6.4\,\%$ .
Grasshopper: $A = 100 - 63 = 37\,\mathrm{J}$ ; $P = 37 - 22 = 15\,\mathrm{J}$ .
Net production efficiency $= \frac{15}{37} \times 100 = 41\,\%$ .
The bull is an endotherm: it respires a great deal to keep its body at about $38.5\,^\circ\mathrm{C}$ , so only 6.4 % of what it absorbs becomes new tissue. The grasshopper is an ectotherm, and turns 41 % into biomass.
The bull digests its food better, though: its assimilation efficiency is $\frac{94\,000}{144\,000} \times 100 = 65\,\%$ , against $37 %$ for the grasshopper, thanks to the microorganisms in its rumen that digest cellulose.

Ecological pyramids

A pyramid draws each trophic level as a bar, producers at the base. Figure 2 shows the three kinds.

Figure 2: Pyramids of numbers, biomass and energy. The biomass values are from a study of the English Channel quoted in OpenStax Biology 2e.

	Pyramid of numbers	Pyramid of biomass	Pyramid of energy
Each bar shows	Number of individuals at each level	Dry mass per unit area at one moment	Energy flowing through each level per unit area per year
Can it be inverted?	Yes: one tree can feed thousands of insects	Yes: in lakes and seas, phytoplankton reproduce so fast that a small standing crop feeds a larger mass of zooplankton	Never: energy is lost at every transfer
Weakness	Ignores size: a tree and a blade of grass each count as one	A snapshot: ignores how fast each level is replaced; drying kills the organisms	Hard to measure; needs data over a whole year

Worked example 3: transfer efficiencies in a lake food chain

In a lake (illustrative data), the energy in new biomass each year is: phytoplankton $12\,000$ , zooplankton $1\,300$ , small sardine-like fish $140$ and predatory fish $12$ , all in $\mathrm{kJ\,m^{-2}\,yr^{-1}}$ . Calculate the efficiency of each transfer, and use your answers to explain why this food chain has only four levels.

Phytoplankton to zooplankton: $\frac{1\,300}{12\,000} \times 100 = 10.8\,\%$ .
Zooplankton to small fish: $\frac{140}{1\,300} \times 100 = 10.8\,\%$ .
Small fish to predatory fish: $\frac{12}{140} \times 100 = 8.6\,\%$ .
Overall, the predatory fish receive $\frac{12}{12\,000} \times 100 = 0.1\,\%$ of the energy fixed by the phytoplankton. A fifth level at about 10 % would receive only about $1\,\mathrm{kJ\,m^{-2}\,yr^{-1}}$ : too little to support a population of large animals that must hunt over a wide area. So food chains rarely have more than four or five levels.

Bioaccumulation and biological magnification

Energy thins out up a food chain, but some poisons do the opposite. A substance that is persistent (not broken down by organisms or the environment) and fat-soluble (stored in fatty tissue, not excreted in urine) builds up in each organism over its lifetime: this is bioaccumulation. Because each predator eats many prey over its life, and keeps the poison from all of them, the concentration rises at each trophic level: this is biological magnification (biomagnification). Examples are the insecticide DDT, polychlorinated biphenyls (PCBs) and methylmercury.

Table: a persistent insecticide in a lake food chain (illustrative data)

Level	Concentration / parts per million (ppm)	Magnification from the level below
Water	0.00005
Phytoplankton	0.04	$\times 800$
Zooplankton	0.2	$\times 5$
Small fish	1.0	$\times 5$
Fish-eating bird	25	$\times 25$

The bird's tissues hold $\frac{25}{0.00005} = 500\,000$ times the concentration in the water. Top predators, including people who eat large predatory fish, carry the highest doses. DDT in birds of prey caused thin eggshells that broke during incubation, and populations crashed until DDT was banned for farm use in many countries from the 1970s. The Stockholm Convention (2001) now restricts DDT worldwide to controlling disease vectors such as malaria mosquitoes.

Warning

Three common mistakes

"Energy is recycled in an ecosystem." Energy flows one way and is lost as heat; only matter (carbon, nitrogen, phosphorus) is recycled.
"10 % is a law." It is a rough average. Real transfer efficiencies range from under 1 % to about 20 %; always calculate from the data you are given.
"Biomagnification means the poison gets stronger." The substance does not change: its concentration in tissue rises, because it is stored rather than broken down or excreted.

CrashCourse: "Ecosystem Ecology: Links in the Chain" (Crash Course Ecology #7). Watch for primary productivity, the loss of energy at each trophic level, and biological magnification.

Tip

Exam technique. In calculations, write the formula, substitute with units, and give the answer to the number of significant figures in the data. When you explain an energy loss, name where the energy goes (heat from respiration, faeces, uneaten parts, urine), not just "energy is lost".

✦

Practice: energy flow

Define net primary productivity and state the equation that links it to GPP.
A tea field has a GPP of $42\,000\,\mathrm{kJ\,m^{-2}\,yr^{-1}}$ and loses $23\,500\,\mathrm{kJ\,m^{-2}\,yr^{-1}}$ in respiration. Calculate its NPP.
Give three reasons why most of the light falling on a leaf does not become GPP.
A goat eats food containing $20\,000\,\mathrm{kJ}$ , egests $8\,000\,\mathrm{kJ}$ and respires $11\,400\,\mathrm{kJ}$ . Calculate its assimilation efficiency and net production efficiency.
Explain why a pyramid of energy can never be inverted, but a pyramid of biomass can.
Primary consumers in a field store $1\,600\,\mathrm{kJ\,m^{-2}\,yr^{-1}}$ and secondary consumers $96\,\mathrm{kJ\,m^{-2}\,yr^{-1}}$ . Calculate the efficiency of transfer and suggest one reason it is below 10 %.
Using the idea of energy transfer, explain why a hectare of maize eaten by people feeds more people than the same maize fed to pigs.
A pesticide is at 0.002 ppm in a pond and 18 ppm in a heron. Calculate the magnification factor, and give two properties of the pesticide that explain it.

Show answersHide answers

NPP is the rate at which energy is stored in new plant biomass after the plants' respiration; $\text{NPP} = \text{GPP} - R$ .
$42\,000 - 23\,500 = 18\,500\,\mathrm{kJ\,m^{-2}\,yr^{-1}}$ .
Any three: light is reflected; light passes through the leaf; light of wavelengths not absorbed by chlorophyll (green); light misses the chloroplasts; photosynthesis is limited by another factor (CO₂, temperature, water); energy is lost as heat in the reactions.
$A = 20\,000 - 8\,000 = 12\,000\,\mathrm{kJ}$ : assimilation efficiency $= \frac{12\,000}{20\,000} \times 100 = 60\,\%$ . $P = 12\,000 - 11\,400 = 600\,\mathrm{kJ}$ : net production efficiency $= \frac{600}{12\,000} \times 100 = 5\,\%$ .
Energy is lost at each transfer, so each level must receive less energy per year than the one below. Biomass is a snapshot at one time: a small mass of fast-reproducing phytoplankton can support a larger standing mass of zooplankton.
$\frac{96}{1\,600} \times 100 = 6\,\%$ . Reasons: the secondary consumers are endotherms that lose much energy as heat; not all primary consumers are caught; bones, fur or exoskeletons are not digested.
Feeding maize to pigs adds a trophic level: the pigs lose most of the maize's energy in respiration, heat and faeces (often around 90 %), so the pork contains only a small fraction of the maize's energy. Eaten directly, all of the maize's energy is available to people.
$\frac{18}{0.002} = 9\,000$ times. It is persistent (not broken down) and fat-soluble (stored in fat, not excreted).

Key points

$\text{NPP} = \text{GPP} - R$ ; only about 1 % of sunlight becomes GPP in most ecosystems.
For a consumer, $I = F + R + P$ ; endotherms have low net production efficiency.
Transfer efficiency is often near 10 %, so food chains are short and top predators are few; the pyramid of energy is always upright.
Persistent, fat-soluble poisons are concentrated up food chains (biomagnification).

Sources and credits

8.3.3: Energy Flow through Ecosystems, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for productivity, net production efficiency, ecological pyramids (including the English Channel biomass values) and biological magnification. Adapted by Ganzaa.
Stockholm Convention on Persistent Organic Pollutants, Wikipedia contributors, licensed under CC BY-SA 4.0. Used for the date and scope of the DDT restriction. Adapted by Ganzaa.
Photo: A lion and a lioness in Akagera National Park, Gilson1111, CC BY-SA 4.0, via Wikimedia Commons.
Photo: Gisenyi fishermen, François Terrier, CC BY 2.0, via Wikimedia Commons.
Video: Ecosystem Ecology: Links in the Chain - Crash Course Ecology #7, CrashCourse, embedded from YouTube.

Figures 1 and 2, the tables, the worked examples and the practice questions are written by Ganzaa. The data in Figure 1, the worked examples and the insecticide table are illustrative.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Why are lions rare?

2Following the energy

3The energy budget of a consumer

4Ecological pyramids

5Bioaccumulation and biological magnification

Why are lions rare?

Following the energy

The energy budget of a consumer

Ecological pyramids

Bioaccumulation and biological magnification