V.C.E. Biology Unit 2

Area of Study 2Dynamic Ecosystems

Chapter 14 – Flow of energy and cycling of matter

Key knowledge

This chapter should enable you to:

Energy

Where does energy come from?

• Plants capture the radiant energy from the sun, and transfer it to chemical energy through the process of photosynthesis

• This is used by animals for their biological processes.

• In marine environments, phytoplankton are also responsible for the input of energy via this process.

Ecosystems require an energy source

• Every ecosystem must have a constant input of energy

• The forms of energy that are particularly important in ecosystems are radiant energy from the sun and chemical energy which is stored in organic matter.

To do: Read ‘Energy: What is it?’ p442 to re-familiarise yourself with the concepts of energy transformation.

The importance of producers

• Producers capture and transform energy, so no ecosystem can survive without the presence of producers.

• Producers vary greatly in size and shape, but they all have one thing in common – the presence of chlorophyll, the light trapping pigment within their cells.

• About 1% of the incoming radiant energy of sunlight to Earth is captured by producers and converted to chemical energy.

Energy transfer

• Once converted, chemical energy is transferred from one organism to another.

- In a terrestrial ecosystem, a transfer of chemical energy occurs when herbivores eat plant matter

- In an aquatic ecosystem, a transfer of chemical energy occurs when herbivores eat phytoplakton or algae

Trophic levels

• Producers and the various consumers in an ecosystem can also be identified in terms of their ‘feeding’ level or trophic level.

• Producer organisms that make their own ‘food’ are said to be the first feeding level or the first trophic level.

Consumers at different feeding levels

• All animals are consumers.• Consumers can be classified into groups

depending on their major source of nutrition.e.g. Carnivores eat only meat and herbivores eat only plant matter.

Consumers at different feeding levels

• Consumers that mainly feed on the organic matter found in plants are known as primary consumers.

• Consumers that mainly feed on the organic matter found in primary consumers are known as secondary consumers.

• Tertiary consumers are the highest order consumers, also known as the top predators.

Decomposers cannot easily be classified in terms of their major source of nutrition since they feed on the remains of dead plants and

animals.

Comparison of producers and consumers in a terrestrial and an aquatic ecosystem

Answer ‘Quick-check’ questions

1 & 2 on page 446

Energy flows through an ecosystem

• We now know that energy is transferred through an ecosystem through feeding behavior.

• However, this process is not 100% efficient; some energy is ‘lost’ to the environment as heat energy.

As you can see, there is a great amount of the chemical energy ‘made’ by producers that is ‘lost’ as heat when it is transferred through the ecosystem.

The chemical energy ‘made’ by producers is mainly used for their own cellular processes (cellular respiration). It is estimated that producers use between 15 and 70 percent. Whatever is left over is available to herbivores.

Energy flows through an ecosystem

• Likewise, most of the chemical energy taken in by a herbivore when they eat plants is used for their own maintenance.

• This is why such a small amount is available to carnivores (2 units of 120!)

• But where does this ‘lost’ energy go?

Not lost, utilised!

• Some of the energy is not actually lost at all, it is utilised by the animal for everyday maintenance and cellular requirements such as;

Active transport, muscle contraction, digestion, nerve transmission and hormone production

Lost via excretion

• Some is lost as organic matter in the faeces, urine and tissue that the animal or plant egests, excretes or loses

e.g. leaves that fall from deciduous treesskin cells shed by animals

Comparing energy needs

• Mammals, being homeothermic (warm-blooded) require more energy to maintain their constant internal environment.

• Therefore, the production of new tissues from food by mammals is less efficient than ectothermic animals such as fish, insects and reptiles.

The rough ‘rule of thumb’

A rough ‘rule of thumb’ figure is used by ecologists for the transfer of chemical energy between trophic levels is the 10 percent rule; that is

about 10 percent of the energy going into one trophic level is available for transfer to the

next trophic level in the form of organic matter in tissues for tissue production

Showing energy transfers

Chemical energy transfers in an ecosystem can be shown in various ways, such as:

• food chains• food webs

Food chains and food webs show the energy transfer or the energy flow in an ecosystem by simply specifically indicating who eats whom. Although, neither show the relative amounts of chemical energy at each transfer.

Food chains

• Food chains depict the one-way energy transfer from prey to predator in a simple drawing as shown below

• The energy flow in an ecosystem is more complex than can be shown with a food chain.

• Explain the limitations of a food chain (refer to page 451)

Food webs

• A food web includes may food chains within it, so in this way it is generally viewed as a more accurate indicator of energy transfer in an ecosystem.

Putting it together

• Answer Quick-check questions 3-6 on page 452

Ecological pyramids

• Information about the number of organisms at each trophic level, and other aspects can be shown for an ecosystem through ecological pyramids.

• There are three main types of ecological pyramids:

- Pyramid of numbers - Pyramid of biomass- Pyramid of energy

Pyramid of numbers

• A pyramid of numbers shows the number of organisms at each trophic level per unit area of an ecosystem.

• A pyramid of numbers does not take into account the relative sizes of the organisms.

Pyramid of biomass

• In an ecosystem, the total amount of matter present in organisms at each trophic level at a given time is referred to as biomass.

• Biomass is measured in units of mass.• A pyramid of biomass records the total dry

organic matter of organisms (biomass) at each trophic level in a given area of an ecosystem.

Pyramid of energy

• A pyramid of energy shows the amount of energy shows the amount of energy input to each trophic level in a given area of an ecosystem over an extended period of time, often one year.

• What will you notice about this pyramid that will always be consistent, no matter what the ecosystem?

• Answer Quick-check questions7-8 on page 454.

Ecosystems differ in productivity

• We can look at ecosystems in terms of their productivity.

• Productivity is the rate at which chemical energy is produced in an ecosystem.

• Productivity is identified through questions such as:- How fast do the producers make the chemical energy?- What is the rate of the production organic of matter by the producers?

Production: gross and net

• In an ecosystem, the chemical energy produced in the form of organic matter by photosynthesis over a given time is termed gross primary production (GPP).

• Some of this organic matter is used by the producers themselves as part of their own cellular respiration as part of being alive; we refer to this fraction as R.

Production: gross and net

• What is left after this is called net primary production (NPP).

• The NPP is the amount of chemical energy available to the consumers in an ecosystem.

• GPP and NPP are expressed in units such as ‘mass of organic matter per unit area per time’.

• We can show this by the following equation:

NPP = GPP - R

Production: gross and net

• In general, ecosystems with the highest net productivity are those with the greatest accumulated biomass:

• The higher the net primary production of an ecosystem, the more food is available for consumer organisms and the greater the biomass of consumers living in that ecosystem.

Why do production rates differ?

• Productivity of an ecosystem falls when any of the requirements for photosynthesis are in short supply.- Temperature - Carbon dioxide - Water - Sunlight

• We refer to these as limiting factors as they limit productivity.

The factor that is in short supply is the limiting factor.

Answer quick-check questions9-12 on page 458

Producers at ocean depths

• For almost all ecosystems, sunlight is the external energy source that is captured by producers and converted to chemical energy which is relied upon by consumers.

• Hydrothermal vents on the depths of the ocean floor receive no sunlight.

• So if all producers require sunlight, who are the producers in these ecosystems???

Special autotrophic bacteria

Chemosynthetic bacteria

• Bacteria that can produce organic matter using the energy released from certain chemical reactions are said to be chemosynthetic.

• These chemosynthetic bacteria are the producers of hydrothermal vent systems.

• In this ecosystem, all consumers feed either directly or indirectly on the organic matter produced by these bacteria.

Answer quick-check questions13-15 on page 460

Matter cycles within ecosystems

• We can show the basic cycling of matter using the simple diagram below;

TASK: In words, explain this diagram.

Biogeochemical cycles

• As we have discussed, matter cycles through n ecosystem.

• There are three elements in particular which we will be looking at, and how they cycle within ecosystems:- Carbon- Nitrogen- Phosphorus

• These cycles are called biogeochemical cycles.

The Carbon cycle

• The cycling of carbon atoms is depicted in the diagram below;

TASK: Using words, explain the Carbon cycle.

The Nitrogen cycle

• The cycling of nitrogen atoms is depicted in the diagram below;

TASK: Using words, explain the Nitrogen cycle.

The Phosphorus cycle

• The cycling of phosphorus atoms is depicted in the diagram below;

TASK: Using words, explain the Phosphorus cycle.

The water cycle• Water does not change chemical form during

its cycling, it only changes physical state (solid, liquid and gas).

• The energy source for the water cycle is the Sun.

Bioaccumulation

• Chemical pollutants enter waterways mainly via runoff.

• Often these chemical pollutants enter the foodchain.

• If the pollutant is not broken down by organisms, it will accumulate and become progressively more concentrated as it enters organisms at higher trophic levels.

• This is termed bioaccumulation or biological magnification

Answer Quick-check questions 16-19 on page 467