There are a number of reasons for the overall loss of biodiversity that we now face, including:
Climate change: Human-induced climate change is altering temperature, rainfall patterns, water availability, drought and similar factors that affect the distribution of plant and animal species throughout the world. According to the Intergovernmental Panel on Climate Change, 20 to 30 percent of plant and animal species assessed would be at risk of extinction if average global temperatures rise by more than 1.5-2 degrees Celsius.
Many species are already affected by warmer global temperatures: for example, more frequent droughts are threatening wildlife in Africa and frequent storms and rising ocean temperatures are damaging and even killing corals around the world, while in the Arctic, polar bears are finding it more difficult to feed as the sea-ice breaks up earlier each year.
Habitat conversion: Through land-use changes and inappropriate occupation, physical modification of rivers or water withdrawal from rivers, loss of coral reefs, and damage to sea floors from trawling, about half of the Earth's land surface has already been transformed or degraded by human activity. The costs of decades of habitat conversion are now becoming all too apparent: for example, where forests have been cleared for timber and agriculture, or for infrastructure development, soil erodes faster and rivers flood more frequently. The loss of critical habitats also adversely impacts many plant and animal species.
Invasive species: Non-native species introduced accidentally or deliberately into an ecosystem can cause major damage to ecosystem functions and populations of indigenous species through predation or by out-competing for key resources such as food, water or nesting sites. One dramatic example of an invasive species is the cane toad: introduced into Australia to control beetles that were destroying sugar cane crops, the cane toads failed to control the beetles, but have caused major damage to indigenous Australian wildlife by eating small animals and poisoning larger predators that try to eat them.
Overexploitation: The use of species, nutrients, water and other biological resources faster than they can be replenished by natural cycles of reproduction or replenishment can cause serious declines in species populations and resource availability. The overuse of water resources in cities and tourism resorts and for intensive agriculture is threatening natural wetlands and groundwater levels. Over-fishing has damaged stocks of fish in most of the world's major fisheries. On land, hunting has contributed to a specific number of species declines or extinctions in the last century, while many others continue to be threatened by illegal hunting today.
Pollution: Chemicals, fertilisers and pesticides, air pollutants, wastewater and solid wastes can all cause damage to individual species and overall ecosystem functioning. Pollution from fertilisers and sewage can result in high nutrient concentrations in water, triggering algal blooms, a sequence of events which begins with rapid growth of algae that then die and decay, leaving behind dead zones in rivers, lakes and coastal waters and adversely impacting fauna and flora.
Other types of pollutants can mimic animal hormones and seriously affect the health and development of fish and amphibians. Many of our landfills ooze polluted wastewater and produce gases that contribute to global warming. Plastic debris and other wastes are found in most of the world's oceans, where species may eat them by mistake or become tangled in them, with often fatal consequences.
Despite this range of threats, there are solutions. The causes of biodiversity loss need to be addressed by society as a whole, and we each have a part to play in meeting this challenge. While much has been done to protect the environment and biodiversity in the last decades, there is still very much to do. The key is to keep our uses of biodiversity and ecosystem services within sustainable limits.
Sustainable use means meeting human needs while not threatening the health of the environment or the stocks of resources that will be available for future generations to meet their needs. In practice, maintaining stocks of natural resources for the future will require not using them up faster than they can be replaced by reproduction and natural process, such as rainfall, which replenishes freshwater stocks.
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