Fencing the Aberdares

A satellite view of central Kenya and the Aberdare mountain range (the second deep-green zone from the right), located north of Nairobi. At the right of the Aberdares, also in deep green, is Mount Kenya, and on the left is the Mau complex. The blue area on the left is Lake Victoria Photograph: NASA

An Aberdare rhino. Twenty years ago, there were believed to be only about 100 black rhino left in Kenya – now there are estimated to be more than 600 Photograph: Raphael Muthama/Rhino Ark

Building in progress. The fence is probably the longest conservation barrier in the world. Its 250 miles (400km) stretches over hills, passes over rivers and runs along the edges of hundreds of communities Photograph: John Vidal

Buffaloes in a forest clearing. The seven-strand, eight-foot electric fence will be finished in a few months' time Photograph: UNEP

A young boy with a cow on the fence. On one side, the barrier protects farmed fields from wild animals – and on the other side it protects the park from livestock grazing Photograph: John Vidal

The fence will be 250 miles long, enclose nearly 772 sq miles and will cost more than $8m (£5.3m) Photograph: John Vidal

It is expected to become a blueprint for other parks in Africa Photograph: John Vidal

A combo picture of aerial surveys in 1987 (left), 2000 (middle) and 2006 (right) shows deforestation increasing in Aberdare range forests Photograph: UNEP

Cultivated fields in indigenous forests within the park are threatened not just by ivory poachers, but by squatters wanting to farm, illegal loggers, hunters, villagers wanting firewood and corporates trying to source illegal water for flower and vegetable farms Photograph: Aerial Survey/UNEP

One of the flower and vegetable farms, which have a great demand for water Photograph: John Vidal

Threats: charcoal production on the South-eastern slopes Photograph: Aerial Survey/UNEP

Logging of indigenous cedar trees on the south-western slopes Photograph: Aerial Survey/UNEP

Marijuana cultivation is another threat to the Aberdares. This picture shows a planted marijuana field on the eastern slopes Photograph: Aerial Survey/UNEP

A quarry in Kiganjo forest reserve Photograph: UNEP

The Karuru falls. The Aberdares provide 90% of Kenya's capital Nairobi's water supplies and the rivers coming off the slopes are the source of 40%-50% of all the power in the city. Without the fence, Nairobi would be facing water shortages and power cuts Photograph: Rhino Ark