1. Rarity and extinction Why do species become rare and extinct?
The answer is people. The main reasons for the rarity and extinction of species are:
uncontrolled commercial hunting.
loss of habitats.

2. Rarity and extinction How species become depleted and extinct?
Introduction
In early 1800s, a single migrating flock of passenger pigeons, a North American bird, can darken the sky for over four hours.
This flock was estimated to be more than 2 billion birds.

3. Rarity and extinction How species become depleted and extinct?
Introduction
By 1850s, the mass killing of passenger pigeons became a big business.
Shotguns, traps and even dynamite were used.

4. Rarity and extinction How species become depleted and extinct?
Introduction
By 1880s, commercial hunting ceased because only several thousand birds were left.
Many of the remaining birds died from infectious diseases and from severe storms during their annual fall migration to Central and South America.
However, uncontrolled commercial hunting continued.
By 1914, the passenger pigeon had disappeared forever.

5. Rarity and extinction How species become depleted and extinct?
Rise and fall of species
Extinction is a natural process. As the planet’s surface and climate have changed over million years, species have disappeared and new ones have evolved to take their places.
Evidence indicates that over the past 600 million years, there have been:
periods of mass extinctions that have reduced Earth’s biodiversity.
periods of radiations that have increased the diversity of life.

6. Rarity and extinction How species become depleted and extinct?
Extinction of species today
Past mass extinctions, 600 million years ago, took place slowly enough to allow new forms of life to arise as adaptations to an ever-changing world.
Since agriculture began about 10,000 years ago, the rate of species extinction has increased sharply as human settlement have expanded worldwide.

7. Rarity and extinction How species become depleted and extinct?
Extinction of species today
Scientists warn that if deforestation and desertification, and destruction of wetlands and coral reefs continue at their present rate, within the next few decades we will lose at least 2% and perhaps 25% of Earth’s species forever.

8. Present mass extinction
Rarity and extinction
How species become depleted and extinct?
Differences between the present mass extinction and those in the past
Such rapid present extinction cannot be balanced by speciation because it takes between 2,000 and 100,000 generations for new species to evolve.
Point of differences
Present mass extinction
Past mass extinction
Cause
It is caused mainly by human activities.
It is caused by catastrophes.
Length of time
It takes place in only a few decades.
It takes place in thousands to millions of years.

9. Rarity and extinction How species become depleted and extinct?
Endangered and threatened species today
Point of differences
Endangered species
Threatened species
Number
is one having so few individual survivors, so that the species could soon become extinct .
is still abundant in its natural range but is declining in numbers and likely to become endangered.
Examples
Javan rhino (60 left), California condor in the US (only 2 in the wild), giant panda in china (1,000 left), snow leopard in central Asia (2,500 left).
Bald eagle, grizzly bear.

10. How species become depleted and extinct? Threatened species today
Rarity and extinction
How species become depleted and extinct?
Threatened species today
Bald eagle
Grizzly bear

11. How species become depleted and extinct? Endangered species today
Rarity and extinction
How species become depleted and extinct?
Endangered species today
Javan rhino
California condor
Giant panda
Snow leopard