Several thousand people who had settled illegally in Kenya's most important forest have left their homes at the beginning of an eviction plan designed to end rampant environmental degradation in the Rift valley.

Security officers this week entered the Mau forest, the country's largest water catchment basin, in the first stage of a government operation that will eventually see up to 30,000 families leave. More than a quarter of the 400,000-hectare forest has been lost because of human activity over the past 20 years, threatening Kenya's crucial tourism, tea and energy sectors and the livelihoods of millions of people reliant on the Mau ecosystem.

"We have no time to waste here," said Christian Lambrechts, a United Nations environment programme expert seconded to the government's Mau Secretariat. "The ecological services must be restored."

The dozen or so rivers that originate in the montane forest complex feed the Masai Mara Reserve and Lake Victoria, as well as the lush tea fields of Kericho. But in recent years the river flows have decreased or stopped during the dry season. At Lake Nakuru, Kenya's most visited national park, wildlife officials were forced to pump in water to supply the animals this summer when all the feeder rivers dried up.

A serious drought that has led to water and power shortages across the country was a contributing factor. But human destruction of the once-thick Mau Forest, which has caused its aquifer levels to fall significantly and seen soil erosion increase, played a major part. At its root, as so often happens in Kenya, is politics and corruption. Before the 1990s, the forest was a protected area. But then senior officials in President Daniel arap Moi's government grabbed large plots of the highly fertile land for themselves – Moi still owns a large tea farm in the Mau – profiting from the timber they cleared. They also removed protection from other parts of the forest where thousands of their supporters were allowed to settle and begin farming. Many of the plots were subdivided and then illegally sold on, sometimes to unwitting buyers.

Amid warnings that the entire ecosystem in the Rift valley and western Kenya was in danger due to the rapid deforestation, Kenya's government has made saving the Mau its number one environmental priority. A task force formed by the prime minister, Raila Odinga, last year recommended that all settlers in the forest be removed and that cleared areas be rehabilitated through mass tree planting. Only genuine titleholders – many of the titles in circulation are fictitious – are to be considered for compensation.

Some politicians from Moi's Kalenjin ethnic group, among them large beneficiaries of the land grab, have opposed the plan, describing it as an attack on their community. They have demanded alternative land for the nearly 1,700 families – about 8,000 people – identified as illegal squatters without title who are being targeted in the first phase of the operation. About 3,500 of them had left the Mau by this morning after being served with eviction notices. Some have complained they have nowhere else to go.

The next round of relocations, due in the next few months, will focus on those people with some sort of title to the land. The trickiest part will be dealing with the large landowners, including the politicians, who are unlikely to give up their farms without a fight.

It is likely that some forest dwellers, including a few thousand members of the Ogiek ethnic group who have lived in the Mau for generations, will be allowed to remain.