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Unit: A Local Ecosystem Topic 6 : Trophic Interactions . Part of the Local Ecosystems Module Biology in Focus, Preliminary Course Glenda Childrawi and Stephanie Hollis. DOT Point.
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Part of the Local Ecosystems Module
Biology in Focus, Preliminary Course
Glenda Childrawi and Stephanie Hollis
Ecological interactions are the exchanges and flows of energy and matter, and these interactions are determined by the ways in which organisms obtain their food. Ecosystems are often described in terms of their trophic or feeding relationships.
Autotrophs: (producers) are organisms that make their own food by converting inorganic molecules to organic compounds.
Heterotrophs: (consumers) must consume other organisms in order to gain the organic molecules they need for life.
There are several types of consumer organisms:
Primary consumers are eaten
by secondary consumers, and
secondary consumers are eaten
by tertiary consumers.
Among the heterotrophs there are also organisms that feed on dead organisms and organic waste from different trophic (feeding) levels. These are calleddegraders.
■ Scavengers—animals that eat dead organisms
■ Detritivores—animals that ingest organic litter or detritus (and then digest it)
■ Decomposers—fungi and
bacteria that cause chemical
decay of organic matter and
absorb the broken-down
Foodchains: show the energy movement from one living thing to another.
producer → herbivore → carnivore
The trophic level of an organism is its position in a food chain, the sequence of feeding and energy transfer through the environment.
Food chains are not isolated in ecosystems; they are more realistically shown as a food web.
Food webs: show complex food interactions in an ecosystem. They are made up of two or more food chains.
An ecological pyramid is graphical representation designed to show the biomass or biomass productivity at each trophic level in a given ecosystem. They begin with producers on the bottom (such as plants) and proceed through the various trophic levels (such as herbivores that eat plants, then carnivores that eat herbivores, then carnivores that eat those carnivores, and so on). The highest level is the top of the food chain.
Pyramid of numbers: shows graphically the population of each level in a food chain.
A Pyramid of biomass: shows the amount of biomass through each level of the food chain. biomassis the total amount of living material present at any one time.
At each level, energy (heat) and matter (food and wastes) are lost (90 per cent). Only 10% is passed on to the next trophic level.
When an ecosystem is found to be unstable (where biomass from one level cannot support the next) then the biomass pyramid shape moves away from the pyramidal shape. For example, if only 4 g of eucalypt leaves has to support a 21g leaf beetle, then the biomass pyramid is unstable; the lower trophic level must have a larger biomass than the higher levels.
Pyramids of Energy: indicates the flow of energy (food) through each trophic level. This is more accurate than either numbers or biomass. We can therefore represent this energy flow diagrammatically in pyramids of energy flow. A pyramid of energy is never inverted.
Some ecologists illustrate ecological pyramids as a stepped shape (as represented in Figs 2.15, 2.16 and 2.17); however, others prefer to simplify it further to a triangular non-stepped shape. The following diagrams summarise the three different types of ecological pyramids illustrated as the simplified, triangular non-stepped shape.
-Students to complete Constructing Food Chains and Food Webs