Denver had a few more U.S. Top 30 hits as the 1970s ended, but nothing to match his earlier success. He began to focus more on humanitarian and sustainability causes, focusing extensively on conservation projects. He made public expression of his acquaintances and friendships with ecological design researchers such as Richard Buckminster Fuller (about whom he wrote and composed "What One Man Can Do") and Amory Lovins, from whom he said he learned much. He also founded the environmental group Plant-It 2020 (originally Plant-It 2000). Denver had a keen interest in solutions to world hunger. He visited Africa during the 1980s to witness firsthand the suffering caused by starvation and work with African leaders toward solutions.