What are the two main components of our environment?

Aquariums need to be cleaned once in a while,whereas ponds or lakes do not require any cleaning. Explain.

$(i)$ What is the height of the ozone layer from the equator?
$(ii)$ Name the rays against which the ozone layer provides protection.
$(iii)$ Name one effect of the depletion of the ozone layer.

Name two gases which are considered to be better than $CFCs$ and cause less damage to the ozone layer.

In the given figure,the various trophic levels are shown in a pyramid. At which trophic level is maximum energy available?

What would be the consequence if all the herbivores are removed from a grassland ecosystem?