The world's biodiversity is under threat from various dangers, the majority of which have been caused by humans.
Habitat loss and fragmentation is considered by conservation biologists to be the primary cause of biodiversity loss. Clearance of native vegetation for agriculture, housing, timber and industry, as well as draining wetlands and flooding valleys to form reservoirs, destroys these habitats and all the organisms in them. In addition, this destruction can cause remaining habitats to become fragmented and so too small for some organisms to persist, or fragments may be too far apart for other organisms to move between.
Invasive alien species are the second greatest threat to biodiversity worldwide. Whether introduced on purpose or accidentally, non-native species can cause severe problems in the ecosystems they invade, from affecting individuals to causing huge changes in ecosystem functioning and the extinction of many species. Virtually all ecosystems worldwide have suffered invasion by the main taxonomic groups. This problem will probably get worse during the next century driven by climate change, and an increase in global trade and tourism. As well as the risks to human health, alien species inflict massive economic costs to agriculture, forestry, fisheries and other human activities.
Pollution is currently poisoning all forms of life, both on land and in the water, and contributing to climate change (see below). Any chemical in the wrong place or at the wrong concentration can be considered a pollutant. Transport, industry, construction, extraction, power generation and agroforestry all contribute pollutants to the air, land and water. These chemicals can directly affect biodiversity or lead to chemical imbalances in the environment that ultimately kill individuals, species and habitats.
Climate change, brought about by emissions of greenhouse gases when fossil fuels are burnt, is making life uncomfortably hot for some species and uncomfortably cold for others. This can lead to a change in the abundance and distribution of individual species around the globe and will affect the crops we grow, cause a rise in sea levels and problems to many coastal ecosystems. In addition, the climate is becoming more unpredictable and extreme devastating events are becoming more frequent.
Over exploitation by humans causes massive destruction to natural ecosystems. Exploitation of biodiversity occurs for food (e.g. fish), construction (e.g. trees), industrial products (e.g. animal blubber, skins), the pet trade (e.g. reptiles, fish, orchids), fashion (e.g. fur, ivory) and traditional medicines (e.g. rhino horn). Selective removal of an individual species can unbalance ecosystems and all other organisms within them. In addition, the physical removal of one species often harms other (e.g. fishing by-catches)
Human populations are growing at an exponential rate, resulting in the problems above. There are more than 7 billion people in the world, and although natural disasters, disease and famines cause massive human mortality, we are getting better at surviving and the population just keeps growing. Human population numbers tripled in the twentieth century and although growth is slowing, one estimate predicts it will take until the twenty-third century for them to level out at around 11 billion.