Anglo-Saxon England must have been, by our present standards, impressively rich in wildlife and general bio-diversity. The human population was less than a tenth of its current size and that population used organic food and clothes. Tools, when not made of iron, were made from natural materials such as bone, horn, wood and leather. There was none of the application of chemical fertilisers, herbicides, pesticides and manipulated animal feeds that happen today, so the pollution of the Anglo-Saxon environment would have been relatively minimal.
You can read more about the countryside and the wildlife in our detailed articles: