Issue link: https://wardsworld.wardsci.com/i/1499797
3 Biodiversity (continued) + ward ' s science soil chemistry and prevailing climate, greatly influence biodi- versity. The high temperatures and abundant rainfall in tropical rainforests support a far greater biodiversity than do the colder, drier conditions in tundra ecosystems. The vast majority of spe- cies is concentrated in the tropical and subtropical regions. Human activities, such as direct harvesting of species, introduc- tion of alien species, habitat destruction and fragmentation, and various forms of habitat degradation (including environ- mental pollution), have caused dramatic losses of biodiversity (Fig. 1). The sixth major extinction event in geologic history, termed the Anthropocene extinction, is currently in prog- ress. Indeed, present-day extinction rates are estimated to be 100–1000 times higher than prehuman extinction rates. Many, if not most, plant and animal species alive today remain to be identified, but some are becoming extinct without having been discovered. This rapid loss of species has spurred a great deal of scientific interest in the topic of biodiversity. Currently, many biologists are working to catalog and describe extant species before they are lost. For example, the Great Smoky Mountains National Park in Tennessee and North Carolina comprises more than a half-million acres (more than 200,000 hectares) of wilderness and serves as a refuge for some of the richest and most diverse communities of life in the temperate world. This living wealth led to the park's designation as an International Biosphere Reserve in 1976 and a World Heritage Site in 1983. A large-scale biodiversity survey known as the All Taxa Biodiversity Inventory was launched in 1998 and seeks to locate and identify every species (estimated to be 100,000) living in the park. This ongo- ing inventory provides a baseline record for the examination of a variety of global factors, including acid rain, climate change, and pollution—knowledge that is essential for the park's biodi- versity to be preserved for future generations to enjoy. Importance Ethical and esthetic arguments have been offered regard- ing the value of biodiversity and why it is necessary to guard against its reduction. Scientists, however, focus on issues such as the biological or ecological functions of biodiversity that can be addressed with experiments rather than debates about val- ues. Certainly, some measure of biodiversity is responsible for providing essential functions and services that directly improve human life. Biodiversity is nature's only blueprint of itself. It is the ultimate gene bank. The rich variety of genes, species, and ecosystems provides humans with food, wood, fibers, energy, raw materials, industrial chemicals, and medicines. The Earth's life forms and ecosystems also provide recycling of nutrients and waste products, generation and maintenance of soils, purification, and natural control of pest species. Every living species contains genetic information that represents thou- sands to millions of years of adaptation to the Earth's changing environmental conditions and is the raw material for future adaptations. Ecosystems surely would not function if all species were lost, although it is unclear just how many species are necessary for an ecosystem to function properly. Thus, the current extinction crisis has provoked many scientists to ask how many species can be lost from an ecosystem before the system is negatively affected. Ecosystem function Because species are the key working parts of ecosystems, biodi- versity must be related to ecosystem function. Studies have as- sessed this relationship in various ecosystem functions, includ- ing biogeochemical processes, the processing of energy, and the flow of nutrients, water, and atmospheric gases. Evidence of the importance of biodiversity for ecosystem function is derived from comparing ecosystems that differ in the number of species present. Ecologists also have undertaken manipula- tive experiments in which the number of species has been directly varied. In general, these studies have demonstrated that various measures of ecosystem function, such as produc- tion of biomass and nutrient uptake, increase as the number of species present increases. However, some studies report no effect or even negative relationships between biodiversity and ecosystem processes. Although some evidence supports the hypothesis that bio- diversity increases or improves the overall functioning of ecosystems, the underlying mechanisms remain unclear. For example, a positive relationship between species diversity and productivity could result because including more species in- creases the chance of encompassing particularly productive or fast-growing species. Alternatively, a diverse group of species may use the available resources more efficiently because each species has slightly different requirements, resulting in higher overall growth. It is unclear whether the number of species or the number of different functional types of species is driving these effects. This distinction is important. On the one hand, if the number of spe- cies matters most, every species that is added to an ecosystem should cause an improvement in ecosystem function [panel (a) of Fig. 3]. In contrast, if the diversity of functional types is more important than the number of species per se, there will be initial increases in ecosystem function as the number of species rises, but these effects should level off once all of the functional types are represented [panel (b) of Fig. 3]. Indeed, a nonlinear