the role of green belt, insects and more in agriculture,gardens&health

I would like to ask for some clarity, a kind of brief "idiot's guide" if you like, to the role of wildlife, the importance of wildlife corridors and urban green belt, and the significance of individual animals, in the overall environment, and the growing of crops.

This is not an attempt to get others to help me out with a course or anything similar. It is because I have for many years, like most on this site I imagine, been concerned with wildlife-friendly practises, organic growing and so on. I will summarise my questions below.

* Its often said that if we lose much more green belt, and if agriculture becomes any more intensive, disaster will result. How does this work in practise? It is tragic that so many natural areas, animals and plants have been lost, but in what particular negative ways does this cause an impact on the remaining environment? I believe the fewer trees, the less co2 is absorbed and the less oxygen produced, but what are some of the wider, further problems?

* In what way(s) does the loss of a particular field or wood impact badly on the wider world? I know it is bad for the immediate environment, and it is now known that children growing up deprived of trees and open space are disadvantaged in terms of health and other matters.

* How does agriculture rely on healthy, large populations of bees and other pollinators? I often hear it said that if we lose the bee, food production will decline. Obviously I support this argument, but are agricultural and other food producers genuinely reliant on the hope that insects will visit their crops? If they do not, does this mean cultivation of vegetables will diminish? Do not farmers employ various techniques to ensure next years crop?

* How does my encouragement of, say, bees, help their population and the wider environment? For instance, I like too grow flowers that attract bees. If more people do this, and if more flowers that they thrive on are abundant, does the bee population locally - like that of other animals in similar situations - increase? And does this then lead to more pollination of other plants, hence the benefit to agriculture and so on? The bees that visit my garden are wild bees. Will these same bees also travel to farms, plant nurseries and so on? What about bees raised in hives - what is their role in pollination (generally, that is: I know several people who raise bees then let them fly away)?

Thanks.