They are needed. They indicate to the reader that you are talking about specific levels (the higher ones), specific buildings (the ones in this district), and trees, grass, and people in this particular place.
You can omit the "the" only when you are talking about the entire class of things.
Dogs are popular pets.
Bicycles are becoming a popular way to get to work.
The dogs in this picture are playing poker.
The bicycles in this shop are expensive, but they come with life-time repairs.
Student or Learner