i don't know if you know the name yip harburg. he was a great lyricist of pop song.r, it's only a paper moon, and brother, can you spare a dime, and over the rainbow, and his way of saying words make you think, music makes you feel, and a song makes it possible to feel a thought. interesting way of putting it. (crowd singing) ♪ we shall overcome. ♪ we shall overcome. ♪ we shall overcome someday. ♪ oh, deep in my heart, ♪ i do believe, ♪ we shall overcome someday. ♪ we shall overcome. ♪ we shall overcome. ♪ we shall overcome someday. funding for this program was provided by the annenberg/cpb project. this is pbs. funding for this program is provided by annenberg media. narrator: for thousands of years, our capacity to alter the environment has gradually increased. during the last century, we've reached the point where our actions affect the entire planet. demographer martha farnsworth riche examines population dynamics in communities all around the united states, a nation that has a disproportionately large impact on the ecosystem. dr. riche: population growth is head