* Trip is first in South America to countries he chose
* Will visit Ecuador, Bolivia and Paraguay
* Expected to defend poor, native peoples
* 1st trip abroad since encyclical on environment, the poor
By Philip Pullella
VATICAN CITY, July 2 (Reuters) - Pope Francis sets out on Sunday to three of South America's poorest and smallest countries, taking his message of solidarity with the downtrodden to prisoners, peasants, garbage pickers and indigenous people.
The July 5-13 trip to Ecuador, Bolivia and Paraguay will be his first abroad since he made a clarion call for defence of the environment and the poor in his landmark encyclical "Laudato Si" last month.
It is also the Argentine pope's first trip to Spanish-speaking South America - he made a trip to Brazil for a youth festival in 2013 to substitute for his predecessor Benedict after his sudden resignation. He is expected to improvise as he delivers some 25 speeches.
Because he chose the three countries himself, Vatican aides say this is the real "homecoming" to his native continent. He won't visit his native Argentina until next year.
He will enter Bolivia's notoriously violent Palmasola prison, a virtual city with its own rules. In Ecuador, he will comfort elderly patients in a hospice and visit sick children. In Asuncion, the capital of Paraguay, he will enter one of the country's poorest shantytowns, the flood-prone Banado Norte.
The health of the 78-year-old pope, who lost part of a lung to sickness when he was a young man, will be in the spotlight as he confronts possible altitude sickness during a quick stop in La Paz, Bolivia, which has the world's highest international airport.
Just as he chose Albania as the first European country outside Italy to visit instead of Europe's economic and political powerhouses, his choices this time again show his concern for people and nations on the margins.
"He wants to show that you can see issues clearer from the periphery instead of from the centre," said an adviser close to the pope who spoke on condition of anonymity.
"He chose the countries in South America with the heaviest history of poverty, inequality and difficulties but which are now emerging peripheries with impressive rates of development and modernisation in recent years," he said.
While he will make references to the encyclical, a papal letter addressed to people around the world, Vatican officials say Francis will most likely save any defence of it for his trip in September to the United States, the source of most of the criticism from conservatives.
"CRY OF THE POOR"
In the encyclical Francis demanded swift action to save the planet from environmental ruin and urged world leaders to hear "the cry of the earth and the cry of the poor," who he said were most affected by climate change.
Ecuador, where he arrives on Sunday, is heavily reliant on oil and mining while boasting some of the world's greatest biodiversity including the Galapagos Islands, on which Charles Darwin formulated his ideas on evolution.
The leftist government of President Rafael Correa, which introduced austerity measures after a major drop in oil prices, is walking a tightrope between business and protecting the environment.
In Ecuador, as later in gas-rich Bolivia, Francis will face a dilemma inherent in the encyclical about the relationship between economic growth and environmental protection.
"He is not going to speak out against Ecuadorian oil or Bolivian gas," the papal confidant said, noting that the encyclical acknowledges a need for a transition period in which fossils should be phased out and replaced by renewable energy.
In Bolivia, he will hold talks with President Evo Morales, a prominent member of the bloc of socialist and anti-U.S. Latin American leaders who has won widespread support with his folksy charm and prudent spending of funds from a natural gas bonanza to cut poverty.
Under Morales, 55, who became Bolivia's first indigenous leader in 2006, the economy has grown more than 5 percent annually while he has nationalized key industries such as oil and gas to finance welfare programs and build new roads and schools. Still, one in five Bolivians live in extreme poverty.
Francis will spend only about four hours in the Bolivian capital La Paz before moving on to the city of Santa Cruz, one of the country's fastest-growing cities, for the rest of his two-day stay.
The airport in El Alto, near La Paz, sits at over 4,000 metres (13,000 feet) above sea level. La Paz is below, at about 3,650 metres (11,975 feet).
The Vatican has said Francis will decide for himself if he chews coca leaves, as many locals do, to ward off altitude sickness when he lands..
Landlocked Paraguay, the last stop, is notorious for contraband smuggling and illicit financing. It ranks as the second most corrupt in South American, behind Venezuela, according to nongovernment organization Transparency International's Corruption Perception Index.
He will meet several groups of social activists while he is there. (Additional reporting by Girish Gupta in Caracas, Daniela Desantis in Asuncion, Daniel Ramos in La Paz and Alexandra Valencia in Quito; Editing by Mary Milliken and Jeffrey Benkoe)
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