In conservation and wildlife photography I think there are at least two primary ethical issues to consider:

A) What impact does taking the photograph have on the animals  – both at the actual time of capture (e.g. stress, changing behaviour), and in the future (e.g. do the images use help the animal or the species etc.).
B) Is the photograph honest

The answers to these questions are not always as simple as they appear, however I think a good place to start is by always keeping in mind the four virtues of:
> Empathy
> Gentleness
> Compassion (or some might say kindness)
> Transparency (in what you have done and created)

Or if you forget those, maybe just ask ‘What would a good person do in this situation?’