World Population: Where it's thick and where it's thin

The growing population of the world, now estimated to be over 7 billion, marks a global milestone and presents obvious challenges for the planet.  There are extremely densely populated cities and sparsely populated countries.  China is the most populous country with India following closely behind. This post brings together some disparate illustrations of our world as it grows, including scenes from Mong Kok district in Hong Kong, which has the highest population density in the world, with 130,000 per one square kilometer. In Mongolia, the world's least densely populated country,  2.7 million people are spread across an area three times the size of France.  Then there's Out Skerries, a tiny outcropping of rocks off the east coast of Scotland where the population is just 65.  And doing what he can to contribute to that 7 billion global milestone is Ziona, the head of a religious sect called "Chana."  He has 39 wives, 94 children, and 33 grandchildren. The world is an interesting place. --(41 photos total)

Motorists pack a junction during rush hour in Taipei in 2009. Taiwan's capital is notorious for its traffic jams, even though many motorists choose motorcycles and scooters over cars. United Nations analysts warn that population growth increases pollution, deforestation, and climate change. 

A hot summer day in Suining, China, draws hundreds of residents to the swimming pool on July 4, 2010. China has the world's largest population, at 1.3 billion or one out of every five people on Earth.

Indians gather to get water from a huge well in the village of Natwarghad in the western state of Gujarat in 2003. Although its population growth has slowed in the past quarter century, India still has the second-highest population, at 1.19 billion, with three births for every death.

The facade of an apartment building in Shanghai, one of China's fastest-growing urban areas. The city has about 23 million people, or the equivalent of the eight biggest cities in the United States combined.

Cars cross a street in Mong Kok district in Hong Kong last month. Mong Kok has the highest population density in the world, with 130,000 in one square kilometer.

People cross a street in Mong Kok district in Hong Kong. The United Nations, which reported the world's population reached 7 billion on Oct. 31, says that while more people are living longer and healthier lives, gaps between rich and poor are widening and more people than ever are vulnerable to food insecurity and water shortages.

An elderly woman pushes a cart with paper for recycling on a street in Mong Kok district in Hong Kong, An increasing population is lessening the Earth's resources, the United Nations says.

Tourists shop in Mong Kok district in Hong Kong.

A foreign visitor walks past stalls on Tung Choi Street or "Ladies' Street" in Mong Kok district in Hong Kong in October.

A beggar looks for help at the entrance to Mong Kok subway station in Hong Kong. Mong Kok has the highest population density in the world.

A passenger rests at a bus stop in Mong Kok district in Hong Kong last month. In addition to being the world's most populous district, Mong Kok is known for its small shops and food and clothing booths.

Ulan Bator is the capital of Mongolia, the world's least densely populated country. It has 2.7 million people spread across an area three times the size of France, two-fifths in rural areas on wind-swept steppes.

Pedestrians walk along a street in the Mongolian capital Ulan Bator. 

People congregate outside a shop in the Mongolian capital Ulan Bator last month. According to a 2010 National Population Center census, every year about 35,000 people migrate from the countryside to Ulan Bator.

Young Mongolians walk through the capital Ulan Bator last month. With a population of almost one million, the capital is home to about one out of every three Mongolians.

Javzanpagma, 71, talks on the phone as her husband, Javzansuren, fixes a lantern in Shivert, northeast of Ulan Bator, last month. In addition to being the world's least densely populated country, Mongolia is known for its rich resources of copper, gold, and coal.

Cars and buses line a street in the Mongolian capital of Ulan Bator last month. Mongolia is the world's least densely populated country.

Yurts, such as this one in Shivert, are traditional homes that dot the Mongolian countryside. As the world's least densely populated country, Mongolia's economy is built on agriculture and herding. Recently, foreign investors have expressed interest in the country's rich mineral deposits.

Jugderdem, 2, stands at the door of a traditional Mongolian yurt in Shivert, about 140 miles northeast of Ulan Bator.

Javzansuren, 72, at his home in Shivert, Mongolia. Many rural Mongolians have been migrating to the nation's capital, Ulan Bator.

Javzansuren, 72, herds his sheep in Shivert, northeast of Ulan Bator. Herding remains an important part of the Mongolian economy.

Myagmarsuren, 33, walks back to his yurt in Shivert, Mongolia, the world's least densely populated country, with 2.7 million people spread across an area three times the size of France.

Alice Arthur is a firewoman on Out Skerries, a tiny, treeless island off the east coast of Scotland. The total population of the island is 65; it would take more than 100 million Out Skerries to match the world's 7 billion in population.

Owen Anderson, 12, takes a guitar lesson at Out Skerries school in September. Owen is one of only seven children on the island, making the school the smallest in the United Kingdom.

Aron Anderson, 6, plays at Out Skerries school in September. Islanders have been fighting efforts to close their one classroom high school.

Out Skerries, about halfway between Norway and Scotland, is reknowned for its birdlife and isolation, with just 65 residents.

Glasses of milk sit on top of pictures of all the pupils at Out Skerries school - all 7, Sept. 22, 2011.

A combination picture shows 28 of the wives of Ziona, the head of a religious sect called "Chana," which allows polygamy and was founded by Ziona's father in 1942. Ziona has 39 wives, 94 children and 33 grandchildren. He lives in his 4 story 100-room house with 181 members of his family.

Ziona, 67, takes a break at the construction site of a church in Baktawng village in the northeastern Indian state of Mizoram, Oct. 5, 2011.

Bikmawi, a granddaughter of Ziona, looks out from the window of her room at their residence in Baktawng, Oct. 5, 2011.

Ziona's 4 story house in Baktawng village where he lives with 181 members of his family, Oct. 6, 2011.

Sons and grandsons of Ziona play soccer inside their home in Baktawng village, Oct. 5, 2011.

A wife of Ziona prepares breakfast at their residence, Oct. 5, 2011. Meal preparation is a huge undertaking.

Villagers roast a pig for lunch for Ziona's family, Oct. 5, 2011.

Saroni, 42, and Manuni, 35, the 12th and 32nd wives of Ziona, cut banana branches to feed pigs in Baktawng village, Oct. 6, 2011.

A daughter-in-law of Ziona holds her grandson at their home. Oct. 7, 2011.

Heni, 23, a granddaughter-in-law of Ziona, lives with the other numerous members of his family in a 4 story residence, Oct. 6, 2011.

Laundry hangs out to dry on the roof of Ziona's residence, Oct. 6, 2011.

Zuali, 37, the 20th wife of Ziona, adjusts his shirt as his sons and driver wait before heading towards the construction site of a church,Oct. 5, 2011.

Family members wave from a vehicle on their way to a church construction site, Oct. 5, 2011.

Family members of Ziona pose for group photograph outside their residence, October 7, 2011.