The biodiversity increases when one moves from the pioneer

The biodiversity increases when one moves from the pioneer to the climax stage. What could be the explanation?

During ecological succession biodiversity increases or changes from pioneer to climax stages. Following are the effects of ecological succession:
(i) It leads to changes in vegetation that affect food and shelter for various types of animals.
(ii) As succession proceeds, the numbers and types of plants, animals and decomposers also change.
(iii) At any time during primary or secondary succession, natural or .human induced disturbances (fire, deforestation, etc.) can convert a particular serai stage of succession to an earlier stage.
Also such disturbances can create new conditions that encourage some species and discourage or eliminate other species of producers, consumers and decomposers.
(iv) Over the time, they are succeeded by bigger plants and ultimately a stable climax forest community is attained.
(v) The climax community remains stable if the environment remains unchanged.
(vi) With time, xerophytic habitat may get converted into a mesophytic one.