Causes of loss of biodiversity
The answer to what causes biodiversity loss in most cases is simple. Mankind. Over 99% of species currently extinction-threatened are so because of human activity.
Natural causes of biodiversity loss
Destruction of habitat: natural forces can act to destroy habitat, species and individual organisms. Obvious examples include volcanic eruptions, floods and fire.
Previous mass extinction events have been associated with asteroid impact.
The same factors can also drive habitat fragmentation. Fragmentation can isolate populations, reduce gene pools and weaken species 'fitness' or ability to survive and reproduce.
Volcanic eruption: a case study
On May 18, 1980, a major volcanic eruption occurred at Mount St. Helens, a volcano located in Skamania County, in the state of Washington, United States.
An eruption column rose 80,000 feet (24 km; 15 miles) into the atmosphere and deposited ash in 11 U.S. states. Hundreds of square miles were reduced to wasteland.More than 4,000,000,000 board feet (9,400,000 m3) of timber was damaged or destroyed, mainly by the lateral blast.
Downwind of the volcano, in areas of thick ash accumulation, many agricultural crops, such as wheat, apples, potatoes and alfalfa, were destroyed. As many as 1,500 elk and 5,000 deer were killed, and an estimated 12 million salmon were killed.
In total Mount St. Helens released 24 megatons of thermal energy, 7 of which were a direct result of the blast. This is equivalent to 1,600 times the size of the atomic bomb dropped on Hiroshima.
Natural climate change: environmental stress applied through heat loss or drought.
Invasive species and disease: species newly introduced through natural means out-compete the local species for resources.
Man-made causes of biodiversity loss
Climate change: see global warming pages. Induced through man-made activities although to provide a balanced view, species can gain environmental advantage or lose it when the climate changes.
Pollution on land, in air and water. Water systems suffer aquatic nutrient load from fertilisers and agricultural by-products. Oceans are seeing rising acidity levels caused by man-made pollutant activity.
Habitat fragmentation: fragmentation is one of the most serious causes of erosion of biodiversity. Fragmentation leads to artificially created ‘terrestrial islands’ with microclimatic effects markedly different from those that existed in the large tracks of habitats before fragmentation.
Over-exploitation: overfishing has reduced some commercial fish stocks by more than 90%.
Introduction of invasive (aka 'exotic') species: any species which is not a natural inhabitant of the locality but is deliberately or accidentally introduced into the system may be designated as an exotic species. Native species are subjected to competition for food and space due to the introduction of exotic species.
There are many well-documented extinctions caused by the introduction of exotic species.
The introduction of Nile perch to Lake Victoria, Africa’s largest lake, has driven almost half of the 400 original fish species of the lake to near extinction.
Human overpopulation: humans may be considered the 'worst-case' exotic species for most organisms. Human activity and an increasingly 'consumption-intensive' lifestyle means that future human population growth spells disaster going forward unless attitudes, behaviours and lifestyles change.
Recreational hunting and collecting: hides, skin, tusk, meat, fur, chemical content taken for monetary or aesthetic value or simply ego in the case of hunting with no purpose other than the thrill of the kill.
In the last decade, over one third of African elephants have been killed by hunters and poachers to fuel the ivory trade.
Fashion: fur clothing and reptile skins for bags and accessories are just two of the more obvious fashion-driven pressures on the natural world.
Medicinal or traditional medicinal demand: traditional medicines often drive significant demand for animal and plant material which can only be obtained by killing the providing lifeform.
Rhino horn is highly prized in Asian cultures for its claimed medicinal properties. Unfortunately, Rhino poaching is now pushing Rhino populations to the brink of extinction.
How big is the human impact on species loss
Measuring the human impact on biodiversity loss is a challenging question to answer. To provide any credible response requires a good handle on just how many species currently exist (we don't have this - we can only extrapolate based on known but incomplete data).
Biodiversity studies suggest that numbers of organsisms on the planet have declined by more than a half in the last 45 years. The Living Planet Index (LPI) shows a combined decline of mammals, birds, reptiles, fish and amphibian numbers of 58 per cent between 1970 and 2012.
That's simply jaw-droppingly staggering. So much so the LPI data gets its own section below.
The following alarming extract is taken from the World Wildlife Fund for nature website:
Just to illustrate the degree of biodiversity loss we're facing, let’s take you through one scientific analysis...
The rapid loss of species we are seeing today is estimated by experts to be between 1,000 and 10,000 times higher than the natural extinction rate.*
These experts calculate that between 0.01 and 0.1% of all species will become extinct each year.
If the low estimate of the number of species out there is true - i.e. that there are around 2 million different species on our planet** - then that means between 200 and 2,000 extinctions occur every year.
But if the upper estimate of species numbers is true - that there are 100 million different species co-existing with us on our planet - then between 10,000 and 100,000 species are becoming extinct each year.
*Experts actually call this natural extinction rate the background extinction rate. This simply means the rate of species extinctions that would occur if we humans were not around.
The natural “background” rate of extinction is estimated to be about one to five species per year.
** Between 1.4 and 1.8 million species have already been scientifically identified.
Wikipedia states that current estimates of species range between 10 and 14 million. With 1.2 million species documented, it means by this measure that 86% of current life on Earth is undocumented.