The Big Answer

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Species loss (Source: WWF/ZSL: Living Planet Report 2012)

Habitat destruction by human activities—particularly that of tropical forests—is believed to be the main reason for the unprecedented rate of species extinction we have been witnessing for the last century. Every day several species are lost—about a thousand times the natural rate—from the 5 to 30 million species thought to exist (estimates vary from 2 to 100 million).

Extinction threatens at least 25 percent of all species by the middle of the 21st century. By this time, we could have wasted a third of cultivable land, half of tropical forests, and face famine and irreversible climate change.

By the middle of the 21st century, half of tropical forests may be gone

Many attempts have been made to determine and make projections of species extinction rates in rainforests, but this is extremely difficult. First of all, we know very little about how many species exist in tropical rainforests. Only about 500,000 rainforest species have been described, although these forests hold at least two-thirds of the world’s species—the WWF says: “Forests are home to 80% of terrestrial biodiversity.” The number of insect species alone is surely one million and probably much higher, says the Rainforest Conservation Fund. Note: Think of the WWF as just that, although some say it is World Wildlife Fund or World Wide Fund For Nature.

As a very general rule of thumb, when 90% of an ecosystem has been disrupted in an isolated area, 50% of its species will ultimately be lost.

When 90% of an ecosystem has been disrupted, 50% of its species will ultimately be lost

Scientists from a wide range of disciplines came together to identify possible future outcomes for biodiversity change during the rest of the 21st century. The results are summarized below. (Source: Global Biodiversity Outlook)

• Projections of the impact of global change on biodiversity show continuing and often accelerating species extinctions, loss of natural habitat, and changes in the distribution and abundance of species, species groups and biomes over the 21st century.
• There are widespread thresholds, amplifying feedbacks and time-lagged effects leading to "tipping points", or abrupt shifts in the state of biodiversity and ecosystems. This makes the impacts of global change on biodiversity hard to predict, difficult to control once they begin, and slow, expensive or impossible to reverse once they have.
• Degradation of the services provided to human societies by functioning ecosystems are often more closely related to changes in the abundance and distribution of dominant or keystone species, rather than to global extinctions; even moderate biodiversity change globally can result in disproportionate changes for some groups of species (i.e., top predators) that have a strong influence on ecosystem services.
• Biodiversity and ecosystem changes could be prevented, significantly reduced or even reversed (while species extinctions cannot be reversed, diversity of ecosystems can be restored) if strong action is applied urgently, comprehensively and appropriately, at international, national and local levels. This action must focus on addressing the direct and indirect factors driving biodiversity loss, and must adapt to changing knowledge and conditions.

Human beings are currently causing the greatest mass extinction of species since the extinction of the dinosaurs 65 million years ago

"Human beings are currently causing the greatest mass extinction of species since the extinction of the dinosaurs 65 million years ago. If present trends continue one half of all species of life on earth will be extinct in less than 100 years, as a result of habitat destruction, pollution, invasive species, and climate change." (Source: MASS EXTINCTION UNDERWAY)