a. Biomagnification is the increasing concentration of a substance, such as a toxic chemical, in the tissues of tolerant organisms at successively higher levels in a food chain.

The concentration of DDT is increased at successive trophic levels; say if it starts at 0.003 ppb (ppb = parts per billion) in water, it can ultimately can reach 25 ppm (ppm = parts per million) in fish-eating birds, through biomagnification.

Figure 2.Flowchart depicting biomagnification of DDT in aquatic food chain.

b. High concentrations of DDT can cause a decline in bird population. It can disturb calcium metabolism in birds, which causes thinning of eggshell and their premature breaking, eventually affecting the population of birds.

OR

a. Pyramid of energy is always upright. It can never be inverted. This is because when energy flows from one trophic level to the next, some energy is always lost as heat at each step. Only 10% of chemical energy is retained at each trophic level and this law is known as the 10 percent law. This 10% of energy is only available to the next trophic level. Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time or annually per unit area.

b. Limitations of ecological pyramids are:

i. It does not take into account same species belonging to two or more trophic levels.

ii. It assumes a simple food chain which never exists in nature. It does not accommodate a food web.

iii. Saprophytes which are so significant are not given a place