Enzymes are regulated by more than the binding of small molecules. A second method that is used all the time by eucaryotic cells to regulate a protein's function is the covalent addition of a phosphate group to one of its amino acid side chains. These phosphorylation events can affect the protein in two important ways.
I believe they cannot start with big trees and bushes because the there is not enough nutrients in the soil, so instead pioneer species pave the way for primary succession.
