5.1.1 Definitions.
5.1.2 Autotrophs and heterotrophs.
5.1.3 Consumers, detritivores and saprophytes.
5.1.4 Food chains.
5.1.5 Food webs.
5.1.6 Trophic level.
5.1.7 Determining trophic levels in food chains and food webs.
5.1.8 Constructing a food web.
5.1.9 Light and food chains.
5.1.10 Energy flow in a food chain.
5.1.11 Efficiency of energy transformations.
5.1.12 Shape of energy pyramids.
5.1.13 Energy and matter in ecosystems.
5.1.14 Decomposers.
TOKBIT: Students should be aware that these are IB Biology definitions. These definitions are controversial and in many journals and textbooks they appear in alternative forms. As an example the definition of species is far from certain and still occupied much debate and research. This may surprise students after all is the above definition not clear enough? Just imagine being in a forest, there are birds flying all around you, you identify one at the species level from your identification book. Now consider why the population of birds within the forest to which you could attach this species identification may or may not constitute a species. An authoritative review of the species problem can be found in:
Mayr, E (2004) What Makes Biology Unique? (Chapter 10.) Cambridge University Press: Cambridge
Food chains show a simple linear flow of' who eats who' and therefore the energy and matter flowing through the links in the chain
Carrot plant ---> Carrot fly ---> Flycatcher -----> Sparrow hawk
Bushgrass---> Impala ---> Cheetah----> Lion
buckwheat ---> Gopher ---> Gopher snake ----> Red Tailed Kite
As you view each food chain try to focus on the processes in which energy and matter are transferred along the chain from one organism to the next.
Consider at each stage how much of the available energy in the food is actually captured by the consumer.
What kind of processes will create losses from one link to the next.
Consider the question: Why are big, scary predator so rare?
Definition: The food web is a diagram that shows how food chains are linked together into more complex feeding relationships
The food web has a number of advantages over a food chains including:
Shows the much more complex interactions between species within a community/ ecosystem
More than one producer supporting a community
A single producer being a food source for a number of primary consumers
That a consumer may have a number of different food sources on the same or different trophic levels
That a consumer can be an omnivore, feeding as a primary consumer and as a consumer at higher trophic levels
There are certain problems in drawing a complete food web as this would in most cases require a very complex study and identification of species. For this reason, food webs often reflect the interests of its author. The author will detail the species of interest by name but group other less interesting/ important species into larger family. order groups.
Examples:
The trophic level of an organism defines the feeding relationship of that organism to other organisms in a food.
In a food web a consumer can occupy a number of different trophic levels depending on which organism is the prey.
Using the simple food web to the left.
Determine the trophic level of each organism in the diagram.
Check your assessment by placing the mouse-over the diagram.
The red numbers provide the correct answers.
The following images are of organisms found in a swamp region of Australia.
With the image you are provided with its feeding preferences.
Construct a food web to illustrate the information supplied
Where possible identify trophic levels for each organism.
What is the trophic level of a decomposer or a saprophyte?
To maintain food chains, food webs, communities and all their interactions requires energy.
Sunlight is the source of this energy for most communities both aquatic and terrestrial.
The principle trap of sunlight energy is the protein molecule chlorophyll found in the chloroplasts of producers cells
There are other energy sources for deep ocean communities based on geothermal energy. These are not studied on this course but can be read about on this external link
a) Not all solar energy will come into contact with chlorophyll and will therefore not be trapped in the synthesis of organic compounds
c) Consumers feeding and passing on energy in the food
d) Loss of energy as heat from respiration
e) death and the consumption of dead organisms by detritivores. Or as food not assimilated because of incomplete digestion.
Energy Loss
loss of energy in undigested food which will then be used by saprophytes/ decomposers
loss of heat energy in the reactions of respiration
ultimately all energy will be lost has heat
The transfer of energy from one trophic level to the next is inefficient
Approx 10-20 % of the energy on one trophic level will be assimilated at the next higher trophic level
This model shows the typical loss of energy from solar radiation through the various trophic levels.
Note how this causes a tapering of the model
The volume of one layer is 10% of the layer below.
It is this loss of energy which in part makes food chains relatively short.
In extreme environments like the arctic the initial trapping of energy by producers is low. Thus the food chains are short.
In a tropic rainforest the trapping of energy is more efficient and therefore food chains are longer, webs are more complex.
Why are big scary predators rare?
The reason of course is that energy and matter are lost at each stage of the food chain. Therefore the number of organisms generally reduces at each step or link in the chain. Organisms at higher trophic levels become less and less common. The 'top carnivores' will have to feed over a wide area or territory simply to find food.
Another reason is that as an organism population gets smaller it becomes more vulnerable to 'catastrophes' such as environmental changes or disease. Therefore 'super top carnivores' are unlikely since they would be so prone to extinction.
This is a more typical pyramid of energy. Note that the initial solar energy is not shown.
The narrowing shape illustrates the gradual loss of energy progressing along the links of a food chain to higher tropic levels (see above for detail).
The base of this pyramid would have a scale = energy/ area/unit time e.g. kJ m-2 yr-1
Unlike pyramids of number (of organisms) a pyramid of energy cannot invert due to the second law of thermodynamics,'energy cannot be created nor destroyed' .
(a) Energy flows: this diagram is a simple version of the pyramids of energy. At each trophic level energy is lost as heat. At the top of the pyramid of energy it tapers to a point showing how all energy is ultimately radiated to space as heat.
(b) Matter cycles: new matter is not created, no new carbon, hydrogen or oxygen. Producers (autotrophs) take inorganic molecules and convert them to organic compounds. Consumers feed at different trophic levels taking in organic matter and using it for their own growth. This cycling of matter is the subject of the carbon, nitrogen and water cycle.
Saprotrophic bacteria and fungi recycle the nutrient (organic molecules) of dead organisms.
Decomposition is a complex process and serves many functions, including the formation of soil, the recycling of nutrients stored in the organic materials, and the reduction of high energy carbon compounds.
Decomposition is a biological process begins with the secretion of extra-cellular digestive enzymes
These enzymes are produced by the saprophytic bacteria and fungi
They secrete the enzymes onto the dead organism
The enzymes hydrolyse the biological molecules of which the dead organism is composed
The hydrolysed molecules are soluble and will then be absorbed by the fungi or the bacteria
Organic molecules are oxidised to release carbon dioxide back to the atmosphere
Organic molecule are oxidised to release nitrogen in form of nitrate, nitrite and ammonium.
The oxidation of these organic compounds produces energy for the saprophyte but returns the various forms of matter to the abiotic environment.