I concur with Jenna that those that borrow documents, photos and sources should try to contact the owner of tree. After all, it is another cousin. Plus, as she mentioned, she has quite a bit more information that is not posted on her Ancestry tree, information that would be valuable to other researchers. It is just good researching to track down a source to find more sources. Each tree owner is another bibliography. What's so hard about an email?
But, as usual, I took it further. I got on my proverbial soapbox here regarding accepting trees found online as fact. As a journalist it has been hammered into my head that nothing in a news story is mine. Every sentence and thought must be attributed to someone...if not it's just an editorial. So perhaps that is why I am passionate about knowing the origination of sources for family trees. I'll be the first to admit that my trees are not 100% sourced. Even saying 70% sourced would be a bit generous, but I still work very hard to provide a link, however tenuous, between generations. Providing that link has cost me many, many hours and not some little money.
Imagine my dismay then when I continually found that my public family trees had been "chain linked" by numerous other users. Why did they accept my tree and its sources as fact? Don't they follow The Code of attributing information? That would mean they would have to write me and ask where I found the information...and I'm still waiting for an email. I was dismayed and not a little disheartened. So I changed my trees to private.
But after a while I determined to return my trees to the public world. After contemplating it long and hard, I realized that it comes down to why I do this hobby. It isn't for the recognition of a great find or to horde my information. I do it because it is something I enjoy. I am passionate about it and I am passionate about making sure it is accurate. So if someone wants to chain link my tree than so be it...I will have become a source for them and hopefully at least a 70% accurate one at that.