This is a timely question as Jim (icephoto) and I have been having a conversation about whether San Antonio could/would support a Nature Photographer's group.
As well as the groups that Melissa posted, there is the
Greater San Antonio Camera Club which is a good group but more wide-ranging in their photo interests than just nature. They meet twice a month.
We should be able to generate a Nature Photographer's group here. We certainly have the population and interest. Maybe it is time to get organized?
Chris