See Copyright Timeline for copyright rules that apply to works published at various times.
"§ 103. Subject matter of copyright: Compilations and derivative works "(b) The copyright in a compilation or derivative work extends only to the material contributed by the author of such work, as distinguished from the preexisting material employed in the work, and does not imply any exclusive right in the preexisting material. The copyright in such work is independent of, and does not affect or enlarge the scope, duration, ownership, or subsistence of, any copyright protection in the preexisting material." 
Other peoples ideas:
Restrictive (limited) licenses
Hoping that this is not a copyright infringement, some thoughts from Dallan from the wiki-genpages mailing list, prefaced by the disclaimer "Disclaimer: I am not a lawyer. These views are just my opinion."
Question: YOu say that you retain copy right for material you published on a wiki. What "right" do you retain in the sense that you have a copyright?
Answer:You have the right to re-publish your material under another license if you want to. You don't have to abide by the wiki license requirements for any work that you personally submit to the wiki. You could for example create a "derivative work" by adding some new material and publish it in an all-rights-reserved book.
Question: If you write some text and place it on the wiki, then decide to use that text in a later work, perhaps modifying it in some way, but with the essential ideas more or less intact, and inserting in the middle of a larger context, wouldn't that later work be "derivitive".
Question: And you'd be obligated to distribute that under whatever copyleft applied?
If you include others' work from the wiki in your derivative work, then yes. You'd be obligated to abide by the wiki license. But if you're using just your own work that you submitted to the wiki, then no. Since you retain copyright, you're free to re-publish your work however you want.
From US Copyright Circular 1 For Works Originally Created and Published or Registered before January 1, 1978
Under the law in effect before 1978, copyright was secured either on the date a work was published with a copyright notice or on the date of registration if the work was registered in unpublished form. In either case, the copyright endured for a first term of 28 years from the date it was secured. During the last (28th) year of the first term, the copyright was eligible for renewal. The Copyright Act of 1976 extended the renewal term from 28 to 47 years for copyrights that were subsisting on January 1, 1978, or for pre-1978 copyrights restored under the Uruguay Round Agreements Act (URAA), making these works eligible for a total term of protection of 75 years. Public Law 105-298, enacted on October 27, 1998, further extended the renewal term of copyrights still subsisting on that date by an additional 20 years, providing for a renewal term of 67 years and a total term of protection of 95 years.
Public Law 102-307, enacted on June 26, 1992, amended the 1976 Copyright Act to provide for automatic renewal of the term of copyrights secured between January 1, 1964, and December 31, 1977. Although the renewal term is automatically provided, the Copyright Office does not issue a renewal certificate for these works unless a renewal application and fee are received and registered in the Copyright Office.
Public Law 102-307 makes renewal registration optional. Thus, filing for renewal registration is no longer required to extend the original 28-year copyright term to the full 95 years. However, some benefits accrue to renewal registrations that were made during the 28th year.
For more detailed information on renewal of copyright and the copyright term, request Circular 15, Renewal of Copyright; Circular 15a, Duration of Copyright; and Circular 15t, Extension of Copyright Terms.