What is the fine-grained topic of the following text: Drainage of habitat; habitat invasion by weeds (in the Waikato royal fern (Osmunda regalis) seems to be the main weed threat); unrestricted wetland access by stock, and wild animals such as deer, pigs, and possums have proved to be a problem at some sites. Only return the topic.
Threats to Waikato Royal Fern's Habitat