ESS 2.2.3-7

• Trophic level is the position an organism (or group of organisms in a community) occupies in the food chain.
• The number of steps an organism is from the start of the food chain indicates its trophic level. The flow of energy along the food chain is in the direction of the arrows and points towards where the energy is going not where it has come from. 
• Food chains demonstrate the first and second laws of thermodynamics. Energy is neither created nor destroyed (1st law) in the food chain and as energy passes along the food chain entropy increased (2nd law).
• Energy enters the food chain as light energy from the sun. It is transformed into chemical energy in the form of chemical bonds in organic matter. It then passes along or transfers along the food chain as chemical energy.
• the laws of thermodynamics act on the food chain and we know that energy transformations are not 100% efficient.

  • That means that only 10% of the energy moves from one trophic level to the next, not a lot. This has significant implications for the number of organisms and the amount of biomass found at each trophic level
• Ecological pyramids are the final method of showing the feeding relationships between the groups of organisms described in ecosystem basics. They are quantitative models to show information about the organisms at each trophic level.
• The pyramid of numbers may not be pyramid shaped because all organisms are counted as equal regardless of their size. This pyramid ignores the biomass of the organism and how much energy it has stored in that biomass. So a single oak tree is one individual as is a single caterpillar. 
• Biomass is measured as dry weight; this eliminates the variation in water content between organisms. To find out what the biomass of a particular trophic level is a sample is collected, dried and then weighed – this is obviously very destructive, not to mention unacceptable if you are dealing with animals.
• Biomass is the total amount of living matter in a given area so it represents the standing stock of energy storage at each trophic level. Biomass is measured in mass per unit of area or g m–2 (grams per meter squared). If the study is looking at energy storage then the units are Joules per unit area (J m-2).
• The pyramid of productivity solves some of the problems of the other two types of pyramid as it shows the turnover of biomass at each trophic level. It is not a snapshot in time as it shows the flow of energy over a period of time. Each bar in this pyramid represents the amount of energy that is generated and available as food for the next trophic level. The units are given as energy or mass per unit area per unit of time – joules per meter squared per year or J m-2 yr-1.
• Unlike the other ecological pyramids the pyramid of productivity in a healthy ecosystem is always pyramid shaped. This is due to the 10% rule and the energy efficiency discussed in "Trophic levels, food chains and food webs".
• In terrestrial food chains the average number of trophic levels tends to be four or maybe five, in aquatic food chains there can be seven trophic levels. This is because in aquatic food chains:
• The down side to aquatic food chains is that less light gets through to the primary producers because it is absorbed or reflected by the water. 
• Toxins in the food chain may be natural or man made. The biggest problems tend to be caused by the man made toxins so that is what will be discussed here. The main toxins that cause problems in the food chain are heavy metals and organic pollutants. This is because many of these toxins are relatively new to organisms so they do not have the capacity to eliminate them from their bodies.

  Dictionary
• Bioaccumulation is the increase in the concentration of a pollutant in an organism as it absorbs or it ingests it from its environment.

  Biomagnification is the increase in the concentration of the pollutant as it moves up through the food chain.
• DDT Is (Dichlorodiphenyltrichloroethane) is an insecticide used in agriculture.