I think it's important to point this out, because in their marketing, they make it sound as if searching alone is enough. There's also this little counter in the upper right corner. 45 searches (on average) are required to plant a tree. Of course, only if you click on an ad.
You could even argue that you cause more environmental damage, because Ecosia is then just an unnecessary wrapper around Bing consuming energy (although it's probably not much).
Edit: Added second paragraph
Edit2: Source is their FAQ: https://ecosia.zendesk.com/hc/en-us/articles/206019452-How-d...