1. what type of succession takes place that increases the biological diversity of an ecosystem? for example, if we started with small weeds and trees, and ended with a tropical rainforest.

There are two types of succession that lead to the increasment of the ecosystem:

1.Primary succession is the change in the structure of an ecosystem represented with the increase of the ecological community on an area that has not been previously occupied by an ecological community such as area after the lava flow or glacial lake. The first organisms of primary succession are pioneer plants usually, lichens and mosses.

2.Secondary succession includes the step of removal of pre-existing community and after that, colonization of the new one.  

littering streams           is the answer