Biodiversity is the variation of life forms within a given ecosystem, or on Earth more generally. Maintaining biodiversity is important for many reasons, not least of which are the many direct benefits to humankind. Biodiversity tends to be closely tied in the popular imagination with philosophical or purely environmentalist ethics, but there are a number of very practical reasons to support biodiversity conservation. In the field of human health, robust biodiversity has led to the development of drugs that come from plants or microbes. Around half of all drugs on the market in the United States are derived from plants, animals, or microbial organisms. Although in recent decades more research has been spent on developing synthetic drugs, many believe that the energy and money would be better spent on exploring new treatment options based on natural sources. In this case, maintaining biodiversity has an obvious benefit: the more plants, animals, and microbes that exist, the better the chances of finding treatments for a wide range of diseases and conditions. Biodiversity also helps humans in the agricultural, business, and industrial sectors. In agriculture, diversity among crops helps to reduce weakness to disease and to improve overall hardiness and crop performance. Most of the worst agricultural disasters throughout history can be linked to being overly dependent on only one strain of one kind of crop; the Irish potato famine being the foremost example of this. Within industry, various living organisms provide many of the base materials used in manufacturing. Maintaining biodiversity allows scientists to find even more ways to use these and other organic materials to push forward industrial development. Beyond these very straightforward advantages to biodiversity conservation, most ecological systems that humans need to survive also rely on a biologically diverse ecosystem to operate. This is true for purifying water, the recycling of nutrients in soil, and the pollination of flowers, among many other things. It is very difficult for people to properly replicate these intricate ecosystems with man-made alternatives. All life on earth depends on a varied and diverse natural environment, and naturally, that includes humans as well.