Definitions
Food chains and food webs are representations of the predator-prey relationships between species within an ecosystem or habitat.
An ecological pyramid / energy pyramid is a graphical representation designed to show the biomass or productivity at each trophic level in a given ecosystem.
A trophic level which describes the position that an organism occupies in a food chain: what an organism eats, and what eats the organism.
Basics
A food chain represents a series of events in which food and energy are transferred from one organism in an ecosystem to another. Food chains show how energy is passed from the sun to producers, from producers to consumers, and from consumers to decomposers. They also show how animals depend on other organisms for food.
In any ecosystem, many food chains overlap. Different food chains may include some of the same organisms. Several consumers may eat the same kind of plant or animal for food. When this happens, the food chain forms a food web. A food web shows how food chains are related within an ecosystem.
A food web is similar to a food chain; only bigger. Food webs show how plants and animals are connected in many ways to help them all survive, unlike food chains that just follow one path.
For example, a tree produces acorns - this is called the producer. Mice and other insects eat these acorns and because there are many mice, the weasels, snakes and racoons have food. With insects in the acorns, other predators would be attracted (like skunks or opposums) and therefore, foxes, hawks or owls could all find food.
Topics of Interest
A food chain is the flow of energy from one organism to the next. There is one organism per trophic level, and trophic levels are therefore easily defined. They usually start with a primary producer and end with a top predator. Here is an example of a food chain:
phytoplankton → copepod → fish → squid → seal → orca
This "chain" can be described as follows: Orca (also known as "killer whales") feed upon seals, that feed upon squid, that eat small fish, that feed on copepods, that feed on microscopic algae. In this example, algae—autotrophs by virtue of their ability to photosynthesize—are the base of the food chain. It is always the case that numbers—or at least biomass—decreases from the base of the chain to the top. In other words, the number and mass of phytoplankton cells are much greater than the number and mass of copepods being supported by the phytoplankton. Viewed another way: to support one orca requires many seals, large numbers of squid, huge numbers of fish, and so on down the chain (see energy pyramid below). This is because, with each transfer, some of the energy is lost to the environment. On average, only 10% of the organism's energy is passed on to its predator.
Food chains are overly simplistic as representatives of what typically happens in nature. The food chain shows only one pathway of energy and material transfer. Most consumers feed on multiple species and are, in turn, fed upon by multiple other species. The relations of detritivores and parasites are seldom adequately characterized in such chains as well.
A food web or food network extends food chain concept from a simple linear pathway to a complex network of interactions. The direct steps as shown in the food chain example above seldom reflect reality. This "web" makes it possible to show much bigger animals (like a whale) eating very small organisms (like plankton). Food sources of most species in an ecosystem are much more diverse, resulting in a complex web of relationships.
An ecosystem network adds an additional amount of information to a food web, measuring the quantities of energy or nutrients moving from one organism to another. Since this data can be difficult to obtain, and since the mathematics of such networks are more complex, this approach developed more recently and is less widely used. However, one advantage of such approach is that one can quantify the degree to which various species fulfill similar or different roles in the flow of nutrients or energy within an ecosystem.
An Energy Pyramid / Ecological Pyramid is a graphical representation designed to show the relationship between energy and trophic levels of a given ecosystem. Most commonly, this relationship is demonstrated through the number of individuals at a given trophic level, the amount of biomass at a given trophic level, or the amount of energy at a given trophic level. It is worth noting that all Ecological Pyramids begin with producers on the bottom and proceed through the various trophic levels, the highest of which is on top.
An Ecological Pyramid of Biomass shows the relationship between energy and trophic level by quantifying the amount of biomass present at each trophic level (dry mass per trophic level). As such, is assumed that there is a direct relationship between biomass and energy. By doing this, the earlier discrepancy is avoided because even though there is only one tree, it is much more massive than the next trophic level.
The main problem with this type of Ecological Pyramid is that it can make a trophic level look like it contains more energy than it actually does. For example, all birds have a beak and skeleton, which despite taking up mass are not eaten by the next trophic level. In a Pyramid of Biomass, the skeleton and beak would still be quantified even though it does not contribute to the overall flow of energy into the next trophic level.
An Ecological Pyramid of Energy is the most useful of the three types, showing the direct relationship between energy and trophic level. It measures the number of calories per trophic level. As with the others, this graph begins with producers and ends with a higher trophic level.
When an ecosystem is healthy, this graph will always look like the standard Ecological Pyramid shown at the top of the page. This is because in order for the ecosystem to sustain itself, there must be more energy at lower trophic levels than there is at higher trophic levels. This allows for organisms on the lower levels to maintain a stable population, but to also feed the organisms on higher trophic levels, thus transferring energy up the pyramid.
When energy is transferred to the next trophic level, only 10% of it is used to build bodymass, becoming stored energy (the rest going to metabolic processes). As such, in a Pyramid of Energy, each step will be 10% the size of the previous step (100, 10, 1, 0.1, 0.01, 0.001 etc.).
The advantages of the Pyramid of Energy:
- It takes account of the rate of production over a period of time because each rectangle represents energy per unit area / volume per unit time. An example of units might be - kJ/m2/yr.
- Two species weight for weight may not have the same energy content therefore the biomass is misleading but energy is directly comparable.
- The relative energy flow within an ecosystem can be compared using pyramids of energy; also different ecosystems can be compared.
- There are no inverted pyramids.
- The input of solar energy can be added
The disadvantages of the Pyramid of Energy:
- The energy value for a given mass of organism is required, which involves complete combustion of a sample.
- There is still the difficulty of assigning the organisms to a specific trophic level. As well as the organism in the food chains there is the problem of assigning the decomposers and detritivores to a particular trophic level.
The best way of showing what is happening in the feeding relationships of a community is to use Energy Pyramids.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)
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