The pioneer community established on a bare rock is:

$A$ frog,which feeds on insects,is a

Match the following:

Trophic levels	Examples
$A$. First	$a$. Man
$B$. Second	$b$. Wolf
$C$. Third	$c$. Cow
$D$. Fourth	$d$. Phytoplankton

Pisaster is a . . . . . . .

Biological equilibrium is an equilibrium among the

$A$: Estuary constitutes one of the most productive ecosystems.
$R$: It represents a transition zone between two populations of a species.