Up to 75 percent of the seeds needed to produce the world's diverse food crops are held by small farmers, researchers said following a review of international census data.

Growers with farms of less than seven acres preserve diversity through "networks of seed and knowledge exchanges", Karl Zimmerer, a Penn State University geography professor who led the research, told a conference of the American Association for the Advancement of Science on Friday.

Some 75 percent of the world's plant genetic diversity has been lost since the 1900s, the U.N. Food and Agriculture Organization (FAO) has reported, as farmers shift from local varieties to genetically uniform, high-yielding crop breeds.

About 75 percent of the world's food is generated from only 12 plants and five animal species, the FAO has said.

Unlike large plantations which are monocultures, small farmers often plant several different species of staple crops, like potatoes, improving the resilience of their food and increasing its diversity.

While less efficient for some large farms, planting a variety of seeds can help food systems build resilience to pests or climate change, growers said.

"How many resources are going to monocultures and how many are going to diversifying food production systems?" Nicaraguan indigenous activist Myrna Cunningham asked during a U.N. conference on Monday.

"If we (start to) base food production on the richness of our diverse societies, we can improve the situation."

Small farmers are often the first to face hunger or displacement, but their role in preserving varied types of seeds is crucial, activists said.

The new research from 11 countries in Africa, Asia and Latin America underlines the importance of supporting small farmers, particularly women, who are leading the way in preserving diversity in their use of seeds, they said.

The 25 percent of seeds for food crops not held by small farmers are preserved in gene banks, researchers said.

"As a society, we are increasingly exposed to shocks and risks (in our food systems)," Adolfo Brizzi, director of policy for the International Fund for Agricultural Development (IFAD), told the U.N. conference.

"We need diversity as a base in case something goes wrong."