Which of the following is a functional aspect of an ecosystem?

In the following,which of these pairs represents the same ecological group?

Out of the following biogeochemical cycles, which ones are gaseous?
$I.$ Sulphur
$II.$ Phosphorus
$III.$ Nitrogen
$IV.$ Carbon
Choose the correct option.

An ecosystem which can be easily damaged but can recover after some time if the damaging effect stops will have:

The detritus food chain starts from:

Which one of the following has the largest population in a food chain?