This is my personal take on the issue:
It is generally better to keep the number of tags down, and keep tags relatively broad as categories. There is granularity limit beyond which categorization will not make searching easier. So let us try to find a good balance.
When is it a good idea to create a new tag then?
I think that one (but not the only!) criterion should be that there are already a sufficient number of questions which fit the tag. I propose that we should not create a new tag at all unless it can be applied to many past questions.
Thus when you do create a new tag, please don't just add it to a new question. Add it to a number of prominent old questions as well. If the tag doesn't fit any old questions, then please do not create it.
To take the example at hand, some people interested in ecology will want to browse old related question. When you tag the most interesting ecology related questions, you add value to the tag and you make it worth browsing.