Food Chains follow the flow of energy from one organism to another. It is a sequence of organisms, with each organism feeding on the one below it in the food chain. Predators often top the food chain. Food chains can be simple with just two trophic levels (producer -> herbivore), or more complex with many tropic levels (producer -> herbivore/omnivore -> carnivore -> secondary carnivore).

Food Webs are similar to food chains, however they allow for an organism to be eaten by more than one type of organism, and follows species interactions up through a web-like pattern. They are many different interlinking food chains in a particular community. Food webs also have trophic levels- the same as the food chains they contain!

Trophic Levels are a way of describing the number of energy-transfer steps it has taken to get to that level.

Ecological Pyramids are just another way of describing the energy flow through a food web or chain. The pyramids show the number of organisms or their biomass in a specific trophic level. The producers are the first trophic level, and they are normally the largest in biomass, with first order consumers being the second largest in biomass, third order consumers being third largest and so on. The reason biomass must decrease, is that there is no perfect transfer of energy from one level to another. Animals use energy, wastes are deposited, and heat is expelled, which means that each organism must consume more than its own biomass in order to survive. In most cases, producers take their energy from the sun, which gives enough solar energy for unlimited growth, the limiting factor is more often the availability of water.
Sometimes Ecological Pyramids describe the number of organisms at each level. Sometimes the level does not increase, but the organisms are smaller in the higher levels. The shape of these diagrams is not always a perfect pyramid for this reason.

Keystone Species are species that have an effect, far greater than that of most species. Their actions affect a large number of species, and the removal of a keystone species can cause widespread changes to the structure of the community in which it lives. A classic example is the beaver. Beavers change the environment around them, building dams and changing the structure of the environment around them. They turn dry land into lakes and marshes, changing the types and number of species in the area. Removed from the community, the beaver’s dams eventually break down, returning the land to its original state. Cassowaries are Australian keystone species, as they spread and assist many different rainforest tree species to germinate, by eating forest fruit- which contain seeds!

Biological Control is what we call the introduction of a species/organism into a community for the sole purpose of reducing numbers of a different species. The introduction of myxomatosis and the calici virus to control rabbits was one biological control success story. One biological control failure, was the introduction of the cane toad to north Queensland cane fields in order to control cane bugs. The toads soon discovered more delectable dinners, and were disinclined to spend the effort jumping for the bugs. They have now spread well beyond the cane fields, and can be found as far south as northern New South Wales.
Biological control needs to be thoroughly investigated before it is carried out, as biological control methods have the potential to both help and harm wildlife.

Biological Magnification is the concentration of toxins up through the trophic levels of a food chain/web. Pesticides and heavy metals are two of the most common bioaccumulative agents. DDT is a very famous example of biological magnification, with the pesticide leaching into streams and running onto oceans, where plankton become contaminated. Small fish eat many plankton, and so the level of DDT in their bodies is more than that which was in the plankton. Larger fish eat many small fish, and are then eaten by even larger fish. If we eat the large fish, we may get sick, because the amount of DDT has accumulated in each level of the food chain. The amount in the plankton was small, but the amount in the large fish can be 1,000000 times greater. The same occurs on land with insects and birds being effected adversely by pesticides and other chemicals.