Water ecosystems rely on a delicate balance of plant and animal species to survive. Each creature plays a role in keeping the ecosystem healthy. But what happens when an unwelcome invader upsets an ecosystem’s balance? Readers learn the answer to this question and more with this engaging and informative text. Bright color photographs take readers on a visual journey through watery ecosystems, showing them who is welcome–and who is not. Helpful information on protecting native species and their habitats completes this text’s comprehensive learning experience.