In this illustrated nonfiction book, author Adrienne Mason compares planet Earth today to Noah's ark, as it travels through the universe carrying every living thing and whatever each one needs for its survival. As explained in the introduction: “For billions of years, life on Earth has been evolving. The result --- a rich biodiversity --- is the foundation of life on our planet. And just like a strong, leak-proof hull on an ark, a healthy biodiversity supports so much.” Notable features of Earth's unique biodiversity are described, with a focus on the delicate interdependencies between species, habitats, climate and more. And explained throughout are the ways in which “Planet Ark is sailing in troubled waters” because of threats such as climate change, extinction and overharvesting. Mason also includes a section on “modern-day Noahs” who are helping to protect and preserve life on Earth through responsible environmental stewardship. By breaking down the broad subject of biodiversity into smaller, more manageable topics, the material is never overwhelming but becomes instead personal and compelling. Illustrations by Margot Thompson further flesh out the concepts with details. Packed with statistics, definitions and facts, this would be a tremendous resource for life sciences classes in the early grades, when biodiversity is a key component of the curriculum. As with all the titles in the CitizenKid series, this book contains specific suggestions for ways young readers can help the cause. Useful tools include a table of contents, glossary and index.