According to the University of Arizona's Center for Insect Science Education Outreach, isopods are omnivores that feed on decaying plants and animals. Some isopods eat live plants, including crops grown by humans.
The Tree of Life Web Project states that there are at least 4,500 species of isopods that live in the ocean. Around 500 species of isopods have also colonized freshwater habitats, including lakes, rivers, streams, underground waters, thermal springs and caves. Another 5,000 species, such as sowbugs and pillbugs, live exclusively on land. Even these land-based isopods use gills to breathe, so their living environments are generally areas with high humidity.