One other thing about agriculture, is that the american landscapes were intensively managed for thousands of years to produce what humans needed. Europeans were often oblivious to the sophisticated agricultural technology, as it did not resemble the "farming" they were accustomed to. So they didn't recognize the extent of the interventions which had produced to the "garden of eden" they conquered. While things eventually unraveled due to the maintainers being murdered, displaced, or otherwise prevented from keeping things up, the europeans often wandered into environments which "nature" had provisioned with a bounty of goods, there for the picking.

The Origin of Capitalism: A Longer View talks about how central the new Capitalist concept of "improvement" was to property rights. In France, still operating under a purely Feudalist mode of production, the job of a land speculator was to find or fabricate claims to land; in proto-Capitalist Britain, a land speculator's job was to calculate how much profit could be wrung out of a parcel of land. Under this new conception, the indigenous Americans had not squeezed every bit of utility out of the soil (depleting it of nutrients, of course) and thus had not "improved" the land and had no claim to it.