P.S. I should have mentioned that there are some exceptions to the pattern I described above. These are links that have been on the lists for two years or more and appear to be permanent. They include some links from Wikipedia, the Open Directory (dmoz), an arboretum, a few university websites, and a newspaper article. I assume that they stay on the lists indefinitely because they are higher-quality links, even though some (such as Wikipedia) have a nofollow tag. Anyway, these are exceptions to the pattern I described above.