Here are a few possibilities:
* The moist riverbank where a fern lives is a haven for other plants and animals. This emphasizes the biodiversity of the area.
* The moist riverbank where a fern lives is a delicate ecosystem. This highlights the importance of maintaining its fragile balance.
* The moist riverbank where a fern lives is a place of constant change. This emphasizes the dynamic nature of the environment.
To get a more accurate answer, please provide more context or the full sentence.