We've all heard, and many of us have said, that the cultures predating Christianity knew how to live in harmony with the Earth. In fact, environmentalism is often described as part of how we honor the ways of our ancestors. Surely they wouldn't have wrecked the land around them in order to achieve a selfish end. The more I look at Irish myth and ancient history, though, the more I see that as a lovely fantasy.
The myth that points to a contradiction between the "go green" approach of neopagans and our ancestors is the history of Lughnasadh. Lugh wanted to grow grain. The best land was forested. Did he decide to abandon the idea of bread in favor of harvesting acorns? No. Tailltiu, Lugh's foster mother, single-handedly wiped out the Forest of Breg as a personal favor to her darling boy. The exertion killed her, and Lugh founded the feast day in honor of her achievements and what it provided the TdD. The main thrust of the holiday is the start of harvest. It wouldn't have been possible without one serious act of clear-cutting.
I realize the myth wasn't written down until after the conversion process had been in full swing for several centuries. But unlike the mythical and misinterpreted snakes, we know Ireland used to be very heavily forested. The peat bogs which are part of the current definition of the Irish landscape resulted from Neolithic forest-clearing practices. The Tailltiu myth is thus, if nothing else, an allegory for the rise of farming as brought in by settlers from elsewhere in Europe.
This is not to say that modern pagans should abandon environmentalism. Hardly. It's a very important approach, and more necessary now than it ever has been. The reasons, however, are not easily supportable by claiming "our ancestors would want it this way." Our ancestors used the technology they had to make their lives more comfortable and developed more efficient means of getting there. If they could have predicted the results of their deforestation, would they have done it anyway? I personally suspect that can only be answered with a question. Were they human beings?