Broad-leaved forests of oak are found in

$A$: Niche becomes general and broader during biotic succession.
$R$: Species diversity increases but organic matter is decreased.

Complete the following model of the carbon cycle by filling in $A, B, C, D, E,$ and $F$.

The importance of an ecosystem lies in

What are the names of the successions occurring in saline water and on bare rocks,respectively?

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?