The combination of human population growth and resource use is the core threat to biodiversity on Earth, as it leads to habitat loss, overharvesting, and introduction of exotic species. Anthropogenic climate change is also predicted to become a significant cause of extinction during this century. These environmental issues, along with toxic pollution, have targeted effects on species but are not generally seen as threats at the magnitude of the others. It is imperative to address these challenges to preserve biodiversity and human welfare.
Habitat loss due to human destruction of ecosystems is a major threat to biodiversity. Species rely on their specific habitats for survival, and the elimination of these habitats leads to their extinction. The accelerated destruction of habitats in the latter half of the twentieth century has caused exceptional biodiversity hotspots such as Sumatra and Borneo to lose half of their forests, leading to the extinction of numerous species. The primary causes of habitat loss are timber extraction and land use change to plantations. Habitat loss also occurs due to river damming and bank modifications, which have caused declines in fish populations and amphibian species. Amphibians, which must carry out parts of their life cycle in both aquatic and terrestrial habitats, are particularly at risk of extinction due to habitat loss.
Overharvesting, particularly of aquatic species, poses a significant threat to many species. The western Atlantic cod fishery is a prime example of a regulated fishery that ultimately collapsed due to economic and political pressures. The tragedy of the commons is a common economic pressure that leads to overexploitation of common resources. While fishery extinction does not equate to biological extinction, some species, such as whales and sharks, are at risk of complete extinction due to hunting and overfishing. Coral reefs, which are home to a third of the world's marine fish species, are threatened by harvesting for the pet trade, while bush meat hunting in Africa and Asia endangers several species, including monkeys and great apes in the Congo basin.
Exotic species are species that have been introduced into an ecosystem in which they did not evolve, either intentionally or unintentionally by humans. While many introductions fail due to low numbers or poor adaptation to the new ecosystem, some exotic species can become invasive and threaten native species through competition for resources, predation, or disease. Lakes and islands are particularly vulnerable to extinction threats from introduced species, as they often contain a disproportionate number of endemic species due to their isolation. Aquatic species can also be introduced through ship ballast water, which can contain living organisms such as plant parts, microorganisms, eggs, larvae, or aquatic animals. Invasive species can also be disease organisms, such as the fungus Batrachochytrium dendrobatidis, which causes the disease chytridiomycosis and has been spread throughout the world by transport of the African clawed frog, Xenopus laevis. Another fungal pathogen, Geomyces destructans, introduced from Europe, is responsible for white-nose syndrome, which infects cave-hibernating bats in eastern North America and threatens the extinction of endangered species such as the Indiana bat, Myotis sodalis, and potentially the Virginia big-eared bat, Corynorhinus townsendii virginianus.
Climate change is a complex and multifaceted issue with a wide range of potential impacts on both natural and human systems. The scientific consensus is that the current warming trend is caused by human activities, primarily the burning of fossil fuels and deforestation, which release greenhouse gases into the atmosphere and trap heat. The impacts of climate change on biodiversity are expected to be significant, and estimates of the percentage of species committed to extinction by 2050 range from 15 to 40 percent. Climate change is expected to alter regional climates, including rainfall and snowfall patterns, making habitats less hospitable to the species living in them. This will result in range shifts, as species move with their adapted climate norms, but also face habitat gaps along the way. Climate gradients will also move up mountains, eventually crowding species higher in altitude and eliminating the habitat for those species adapted to the highest elevations. Some climates will completely disappear, and global warming will raise ocean levels due to meltwater from glaciers and the greater volume occupied by warmer water.
The impacts of climate change on biodiversity are already being observed, with range shifts observed in plants, insects, freshwater fishes, reptiles, amphibians, and mammals. Climate change is also expected to cause mismatches between the timing of seasonal food resources and breeding times, as well as unexpected species contacts and shifts in competitive regimes. These impacts could have significant consequences for ecosystem functioning and the provision of ecosystem services.
Overall, it is clear that climate change poses a significant threat to biodiversity, and urgent action is needed to reduce greenhouse gas emissions and limit the extent of future warming.
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