What, according to Erhard Berner, “are the sources of power that enable the poor to defend their pieces of precious land against global players and other strong competitors”?

The sources of power that enable the poor to defend their land are a sense of community and strong faith. The sense of community helps each member of the poor community survive, as other members may provide temporary assistance to a member in need with the understanding the favor may be returned in the future or another member of the community will help out in times of crisis. In order for the poor to improve their situations, they must first learn how to defend what they already have, to prevent loss and becoming more impoverished. However, without resources, poor members of the community must rely on the sense of community to develop ways of retaining and defending what they have.
Because of strong faith, many members of poor communities develop strong ties to churches and other non-governmental organizations in their communities. These alliances serve to represent the interests of the poor community members in times when land developers and politicians may seek to claim the land and make use of the property of the poor for profit. These politicians and developers will not only have to answer to individual community members but to the larger and more powerful alliances representing them. These alliances then become the "voice" of the people and are willing to represent the local interests of the poor, engaging in discussion and negotiations that benefit the poor members of the community.
https://link.springer.com/article/10.1080/09578819708426682