Use this source in lieu of creating an individual source for a single user's Public Member Tree. However, citation to Public Member Trees is discouraged generally and should be limited wherever possible.
"This database contains family trees submitted to Ancestry by users who have indicated that their tree can be viewed by all Ancestry members. These trees can change over time as users edit, remove, or otherwise modify the data in their trees. You can contact the owner of the tree to get more information." - Ancestry's description
These trees are easy to upload, easy to copy, easy to source with bad information, and do not include a user's original sources or notes (the linked sources are only those Ancestry.com databases a user individually adds to each person, a time-consuming and error-prone process). As such they are often of poor quality, and even those of good quality lose most of their supporting documentation upon upload and are therefore of limited use to later researchers. In addition, as they can only be viewed by Ancestry.com members, they are of limited use to the community as a whole.