Diatoms are important as food for other organisms living in water, but also serve other purposes for humans. For instance, diatomaceous earth, a substance composed of fossilized diatoms, is used in filters, insulation, abrasives, paints, and varnishes. Diatoms are also employed for scientific purposes. Although they are found in all of the world's aquatic environments, most species are habitat specific, living only in areas with certain physical, chemical, and biological characteristics. Therefore, by collecting and analyzing individual species and communities, ecologists can employ diatoms to help determine the condition of an aquatic habitat. Long-term monitoring of habitats and analysis of the fossil record provide scientists with a unique long-term perspective on marine ecosystems.