"main source of livelihood" is a correct and usable phrase in written English.
You can use it to describe an activity or job that provides a person with their regular income. For example, "Fishing is the main source of livelihood for many residents of the coastal town."
I don't have much to survive on because there are only four acres of land on which my home is and that is our main source of livelihood.
Their cattle are the main source of livelihood.
Some did not even engage in farming as a main source of livelihood.
Agriculture is the main source of livelihood for rural masses and the most of the living population is tribal community.
Additionally, their main source of livelihood is climate dependent, and this exposes their income to vagaries of climate.
Subsistence nomadic pastoralism is the main source of livelihood for the Turkana and Pokot communities in the study area.
The major inhabitants of the state are the Hausa and Fulani ethnic groups whose main source of livelihoods is livestock rearing, farming, hunting, and fishing, although hunting and fishing as livelihood activities cause little or no harm to the forest ecosystem if carried out in a conventional way.
As well as the major loss of life, the cyclone and accompanying storm surge caused huge damage to houses and the agricultural sector – the main source of livelihoods for the majority of the people of the delta.
As well as a major loss of life, the cyclone and accompanying storm surge caused massive damage to houses and the agricultural sector – the main source of livelihoods for the majority of the Delta's residents.
Typhoon Haiyan has devastated two of the main sources of livelihood in Leyte: coconut and fisheries.
The main sources of livelihood, other than hemp and poppies, are grapes and pomegranates, and, during the summer fighting season, foliage in fields and orchards provides cover for insurgents.
