Answer:Because :-
Step-by-step explanation:
1-Plants have the capability to adapt to environmental changes which can be seen in the form of different types of plants coming out of the same species.
2-Due to the continuously changing environment at rivers and streams the types of plants also different.
3-Some can adapt themselves in dry areas some can survive in the areas where the level of pollution is high, some grow because of the different minerals present near rivers and streams.
4-The Soil fertility helps the plants which grow in that environment.
5-Over time, the earth atmosphere has changed many times and some plants are better at adapting than others.