There's two significant things that happened to the plant populations around the world:
1. Destruction;
Lots of plants have been brought to the verge of existence or have been destroyed, and the main factor for this has been the human activity. This affects the whole ecosystem because everything in an ecosystem is connected, and the plants are in general the basis for the functioning of it, so with the destruction of the plant communities, lots of other organism suffered because of losing their natural habitat and food source, and because lots of organism that used the plants as a food source died, the predators were left without their food source so they died too.
2. Domestication;
There's tens of thousands of wild plants that have been domesticated and changed over time by the humans. This contributed to them having different characteristics, and also because of this they became competition with their wild ancestors, and people in order to give these plants the advantage are using lots of chemicals, and that destroys the wild plants, and also whole ecosystems in which they are an essential part.