So the question is a tad vague, thats plain to see!
I am curious to see if there is a figure to go alongside this question as it sounds a lot like their should be. If not, we can still tackle this question!
I am currently studying biodiversity and therefore am doing a lot based on the reduction of ecosystems such as rainforests.
One human factor that can cause reduction is deforestation- this is the process where organisations or individuals cut down trees from mainly rainforests and use them for fuel or timber. I would possibly argue that deforestation is one of the largest human factors that causes the reduction.
Areas are often also cleared of trees for agricultural use. Impacts of this can be positive and negative which you can come up with.
This website may help you slightly and give you a few more ideas:
An example of a rainforest I would use is the Daintree in Aus. With little research you can find out a lot about this rainforest- i suggest this is undertaken!
Hope I've helped! x