I see. Unfortunately I've done very little research on Native American faiths apart from a brief reading on the Aztec and Mayan faiths.
My belief is that worship of nature = animism.
Worship of idols (regardless of whether they represent a natural element or not) = paganism.
In Hinduism, particularily the Vedic religion, for example, there are many Gods which represent natural elements, such as Agni for fire, Vayu for wind, Prithvi for the Earth, Saraswati for rivers, etc. As far as I can recall, Saraswati is the only one frequently worshiped till date (my best friend worships her once a week or so), while Agni is worshiped only by the Shrautha who have preserved their Vedic traditions.
In the state of Meghalaya here in India, three Tibetan-Burman tribes are dominant. Most of them have been Christianised, however, some in the rural area maintain their animist religion, which has the same rites as Hinduism minus the idols. The same is prevalent in the states of Mizoram, Nagaland, Manipur, and Arunachal Pradesh, which have small animist minorities due to most of them being Christianised.
Hence, I believe that lack of idols is pure animism. Paganism is a development over animism.