This abstract Keynote theme is perfect for your nature themed presentations. It will help you to make sure that you are ready and your audience stays tuned into what you are trying to communicate. This template will help you while you make a stand for Mother Nature.
I found this design on DeviatantArt. There are many different designs there that you could make your own Keynote themes. To add another slide with the background. Add a new template and use the Image Fill background option and the background will fill itself in.