Biodiversity refers to the variation and richness of living organisms. Biodiversity provides many important services to humanity and is also often considered to be valuable for its own sake. Biodiversity tends to be concentrated in certain regions, with rainforests having more biodiversity than any other type of ecosystem on land. While there have been species extinctions throughout history, both because of human activities and for other reasons, today, human activities are threatening extinctions at unprecedented rates. To be sure, humans are also active in protecting biodiversity, such as through protected areas and other projects focused on conservation. But the overall threat to biodiversity loss is so great that conservationists face conservation triage, in which they must decide which species to protect. If too much biodiversity is lost, ecosystems could collapse, threatening the extinction of one more species: humans. The threat of human extinction, whether due to biodiversity loss or other events, raises some very profound ethical issues, issues which connect deeply to the topics in this course.