Since the dawn of time, there is a well guarded secret, that even the elves do not know. Some areas of Eredane were never settled by man or fey, in spite of their natural generosity. Whys is this so? Because these areas are home to powerful nature spirits. And, yes, contrary to the standard D&D worlds, these spirits are sentient. MOst of the time, these areas are also home to powerful magic nexuses that strengthen the spirit's powers.
Since Izrador's invasion, a few humans or fey people tried to flee in these remote zones, hoping to elude the overwhelming evil forces, and find shelter there.
Those who would not adapt themselves to Nature would be mercilessly crushed; they were never again heard of, and those who managed to escape the spirit's wrath claimed these were indeed haunted areas, which had bo doubt already succumbed to the Shadow.
Those who adopted the Nature's ways were tolerated. After long years, they eventually discovered the spirits' existence. THey became their servitors, bound to protect these lands which had become their homes. They are the land's protectors, the Druids.
The druids (or animists) have a duty: protection of their homeland. This means surveillance most of the time, but it can also take a more proactive approach (like going outside to find some allies, destroying an orc encampment that settled nearby, etc.). They do not feel restricted to the means to achieve this end: spirits are not concerned by the protection of Good, only of their environment. However, no doubt they would ally with the resistance should they ever learn of Izrador's plan to use the land's energy to make his celestial ascension. Each spirit is isolated, and they rarely communicate (only if they are in great need).
All Protectors must meet the criteriae to become a PrC Druid.
They also have to be accepted by the local spirit, and such confidence is usually gained by a spectacular dedication to the protection of the land.
Protectors can also recruit "assistants" to help them in their task (usually wildlanders).
Whenever a protector is "at home", he gains a spell energy bonus equal to +1 per level as Druid.
Spirits also sometimes grant their servitors (Druids or wildlanders) a "natural" lycanthropy (with shape control), only if they feel particularly threatened and need strong agents (maybe this could become a PrC? I was thinking of the Beornids in "The Hobbit" and thought they would fit very well). As a side note, this has nothing to do with the disease that is spread by Izrador's agents (evil wildlanders) in order to scare the people (the animals "granted" should be different: bear or boar for the Protectors, and wolves or rats for the bad guys).
Spirits don't want their existence to be known, as it would mark their death sentence. So, Izrador's agents know there are druids, but ignore that they actually are servants of other beings. These spirits are actually "nature gods" whose power is very restricted, to a particular area (usually a few square kilometers). This explains how a Druid gets his powers without resorting to arcane magic.
To keep their existence a secret, they rely on stealth and ruse. A protector will try and convince potential invaders that the area is completely devoid of any interest for them, thus encouraging them to leave. If this does not succeed, he will try to scare them (haunted woods, etc.), and only when this fails too will he destroy his enemies, usually seeking allies in order to achieve this goal.
When an area's protectors are not able to protect it, they are usually sent to meet another powerful allied spirit and beg him to "lend" his Protectors for a limited time.
Spirits often communicate with each other using small animals or Leapers (as they can speak, see MoS page 41). They only know about their closest "neighbors", that is, geographically close other "spirits", though. In this regard, they SOMETIMES (once in a decade, maybe?) communicate, when there is a great need. Note that these rare spirits who know of each other are all individuals who are not necessarily on good terms. It would make for a nice campaign just to discover their existence and convince them to join the resistance (and uniting them would be a considerable accomplishment).
As for the Spirit's physical manifestation, I think they would be powerful sentient elementals.