8.5- Local Environment

Food Chains and Food Webs

Food Chains

Trophic levels

Organisms in food chains are grouped into trophic levels. The trophic level of an organism is its position in a food chain, trophic levels consist of either a single species or a group of species that are presumed to share both predators and prey, its level determines what it eats and what eats it

The trophic levels in this basic food chain in Figure 1 below are producers, primary consumers, secondary consumers, tertiary consumers, quaternary consumers and decomposers.  The quaternary consumers are those that eat the tertiary consumers as shown on the terrestrial and marine food chain diagrams.

Decomposers

The decomposers are the final level or stage in the food chain. Decomposers are organisms that consume dead plants and animals, and, in doing so, carry out the natural process of decomposition. Decomposers use deceased organisms and non-living organic compounds as their food source. The primary decomposers are bacteria and fungi. The decomposers then return some of the energy, accumulated through the food chain back to the soil as nutrients.



Figure 1:  Food Chain
(Image source http://en.wikipedia.org )

Food chains don’t always make sense!

It would be easy to assume that food chains follow a logical sequence with smaller animals being consumed by larger ones and so one up the chain. This is not necessarily the case, for example the whale shark is the largest fish in the world, but it feeds on tiny krill and plankton. This is shown in the marine food chain below (Figure 2).



Figure 2:  Marine Food Chain
(Image source http://en.wikipedia.org )

The direct pathway shown in a food chain is very simplistic as it assumes that the animals shown in the diagram only consume one type of food and are in turn consumed by only one species. Therefore a food web is a more accurate way of showing the feeding relationships between plants and animals.

Food Webs

Trophic levels in food webs.

Trophic levels exist in food webs also, although more complex than the simple food chain. For example in the aquatic food web shown below (Figure 3) on an Australian river ecosystem, the primary producers in this food web are the algae and the sedge. The algae is then consumed by the primary consumers, the tadpole and daphnia in this example. The secondary consumers are the damselfly and back swimmer. The sedge is consumed by the maned duck, moth, dragonfly, and ant, these animals being the primary consumers. The secondary consumers are the frog, centipede and lizard and the tertiary consumer is the kookaburra. The decomposers (bacteria) are also shown on this food web, breaking down the decayed material and returning the nutrients to the soil to then be used by the sedge for growth.

Light energy is used by primary producers (plants and phytoplankton) to synthesize organic molecules through the process of photosynthesis. Energy then flows to higher trophic levels through the consumers in the food web. It is often the case that the biomass (plant and animal material that can be used for energy) of each trophic level decreases from the base of the chain to the top. This is because energy is ‘lost’ to the environment with each transfer, this loss can be in the form of waste (e.g. carbon dioxide, faeces), and heat and kinetic energy (e.g. constant body temperature of mammals, the energy used to move). Up to 90% of matter and energy can be lost at each level. Therefore only 10% of the organism's energy is passed on to its consumer in the next level of the food web. For example grass has a biomass of 400g/m², this is then consumed by a grasshopper with a biomass of 50g/m² and finally the grasshopper is consumed by the kookaburra with a biomass of 10g/m².


  
Figure 3:  A simplistic diagram showing the food web of a basic Australian river ecosystem
(Image source: http://en.wikipedia.org )
  

A food web obviously shows the relationships between animals and plants in an ecosystem but can also be expanded to show the input of solar energy and carbon dioxide. The process of photosynthesis is the conversion of light energy into chemical energy by living organisms.

Respiration

Plants and bacteria use the energy from sunlight to produce sugar, which cellular respiration converts into ATP (adenosine triphosphate). ATP is the "fuel" used by all living things. The conversion of unusable sunlight energy into usable chemical energy is associated with the actions of the green pigment chlorophyll. Chlorophyll is found in most plants and algae and is vital for photosynthesis. The raw materials in this process of photosynthesis are carbon dioxide and water (inputs), the energy source is sunlight, and the end-products include glucose and oxygen. It is of vital importance as nearly all life depends on it.

Respiration is the opposite of photosynthesis. To unlock the energy in the sugars produced in photosynthesis, green plants need to respire, just as animals do. Respiration takes place in the plant's cells, using oxygen to produce energy and giving off carbon dioxide as a waste product. The result is that during the day when the plant is both respiring and photosynthesising there is a two-way traffic of oxygen and carbon dioxide both into and out of the plant. During the night when the plant is respiring but not photosynthesising, oxygen is being taken in but not given out - and carbon dioxide is being given out but not taken in.


Australian Curriculum links

Science Understanding  - Energy for Earth processes

(ACSES045)  Processes within and between Earth systems require energy that originates either from the sun or the interior of Earth

(ACSES046)  Thermal and light energy from the Sun drives important Earth processes including evaporation and photosynthesis

Science Understanding  - Energy for biogeochemical processes

(ACSES053)  Photosynthesis is the principal mechanism for the transformation of energy from the sun into energy forms that are useful for living things; net primary production is a description of the rate at which new biomass is generated, mainly through photosynthesis

 

Food chains and food webs

Science Understanding  - Describing biodiversity

(ACSBL019)  Ecosystems are diverse, composed of varied habitats and can be described in terms of their component species, species interactions and the abiotic factors that make up the environment

(ACSBL020)  Relationships and interactions between species in ecosystems include predation, competition, symbiosis and disease

 

Science Understanding  - Ecosystem dynamics

(ACSBL022)  The biotic components of an ecosystem transfer and transform energy originating primarily from the sun to produce biomass, and interact with abiotic components to facilitate biogeochemical cycling, including carbon and nitrogen cycling; these interactions can be represented using food webs, biomass pyramids, water and nutrient cycles

(ACSBL024)  Keystone species play a critical role in maintaining the structure of the community; the impact of a reduction in numbers or the disappearance of keystone species on an ecosystem is greater than would be expected based on their relative abundance or total biomass

(ACSBL029)  Models of ecosystem interactions (for example, food webs, successional models) can be used to predict the impact of change and are based on interpretation of and extrapolation from sample data (for example, data derived from ecosystem surveying techniques); the reliability of the model is determined by the representativeness of the sampling

 

Impacts on food chains and food webs

(ACSBL024)  Keystone species play a critical role in maintaining the structure of the community; the impact of a reduction in numbers or the disappearance of keystone species on an ecosystem is greater than would be expected based on their relative abundance or total biomass

(ACSBL025)  Ecosystems have carrying capacities that limit the number of organisms (within populations) they support, and can be impacted by changes to abiotic and biotic factors, including climatic events

(ACSBL026)  Ecological succession involves changes in the populations of species present in a habitat; these changes impact the abiotic and biotic interactions in the community, which in turn influence further changes in the species present and their population size

(ACSBL028)  Human activities (for example, over-exploitation, habitat destruction, monocultures, pollution) can reduce biodiversity and can impact on the magnitude, duration and speed of ecosystem change

(ACSBL029)  Models of ecosystem interactions (for example, food webs, successional models) can be used to predict the impact of change and are based on interpretation of and extrapolation from sample data (for example, data derived from ecosystem surveying techniques); the reliability of the model is determined by the representativeness of the sampling