$A$ food web:

Time is a crucial factor for . . . . . . .

The phosphates remain outside the natural cycle for a long time:

Sparrow eats seeds. Afterwards, it eats insects. In which trophic level does the sparrow belong in both situations, respectively?

In a food chain of a grassland ecosystem,the top consumers are

Consider the following statements concerning food chains.
$I$. Removal of $80\; \%$ tigers from an area resulted in greatly increased growth of vegetation.
$II.$ Removal of most of the carnivores resulted in an increased population of deers.
$III.$ The length of food chains is generally limited to $3$ to $4$ trophic levels due to energy loss.
$IV.$ The length of food chains may vary from $2$ to $8$ trophic levels.
Which two of the above statements are correct?