It is because of different arrangements of nucleotide bases.
<h2>Explanation</h2>
There are only five types of nucleotide basis are present in the genetic material which makes up the all the genetic information of the individuals. These nucleotides are named as Adenine, Thymine, Guanine, Cytosine and Uracil. In all the living beings on the earth, these only five nucleotides are present which decides their behaviour and functions on the earth. These five bases have features to get any arrangements according to the situation. Their different arrangement at the heredity material makes the difference in all living beings regarding their behaviour.
Secondary succession usually occurs faster than primary succession because the substrate is already present. Compared to the image below, secondary succession does not need to have pioneer species colonize the land nor does decomposition need to occur to create a layer of topsoil.
