The giraffe population of Kenya's Masai Mara reserve has declined by up to 95% because of increased human settlement around the unfenced park, according to a new study.

Scientists at the Nairobi-based International Livestock Research Institute (ILRI) found that the numbers of giraffes, hartebeest, impala, warthogs, topis and waterbuck all fell "markedly and persistently" throughout the 1,500 sq km reserve between 1989 and 2003. Their sample counts were backed by government population estimates that showed actual losses as high as 95% for giraffes, 80% for wart­hogs and 76% for hartebeest for the period dating back to 1979.

The authors of the study, which was funded by the WWF and will be published in the British Journal of Zoology in May, say it presents the most detailed evidence yet of the rapid decline of ungulate, or hoofed, animals in the Mara, in south-western Kenya.

The main reason for the population decrease was the rapid expansion of human settlements on the land next to the reserve. The area was traditionally used by wild animals for seasonal grazing but is increasingly being turned over to livestock and crop production.

"The situation we documented paints a bleak picture and requires urgent and decisive action if we want to save this treasure from disaster," said Joseph Ogutu, a statistical ecologist at ILRI and lead author of the study. "If nothing is done, these declines will not just continue but accelerate as the human populations around the reserve increase."

The Mara reserve sits at the top of the Mara-Serengeti ecosystem, which hosts the spectacular annual migration of up to 2 million wildebeest and other animals. Bordering the park is an area known as the Mara ranchlands, which is mainly home to the Masai ethnic group.

Until the last few decades, the Masai semi-nomadic herders and their livestock coexisted easily with the wild animals that roamed their lands. But rapid population growth and a steadily more sedentary lifestyle for the Masai, driven in large part by decades of policy neglect, have seen the number of permanent settlements surge along the boundaries of the Mara. In Koyiaki, one of the ranches next to the reserve, the number of huts increased from 44 to 2,345 between 1950 and 2003. Individual titleholders now own what was once communal land.

Ogutu said this meant wild grazing animals that previously moved freely in and out of the reserve were increasingly competing for habitat with cattle and large-scale crop cultivation. The destruction of trees for firewood was also having a negative effect. "It's not just that wild animals are moving away from these areas; they are dying out," Ogutu said.

Among the other ungulates studied by the ILRI scientists, topi antelopes and impala also suffered sharp declines over the 15-year period. While zebra and waterbuck numbers increased, they were still down 87% and 57% respectively from their 1979 population estimates.

The study warned that killing of animals that damage crops and water supplies, break down fences and threaten humans and livestock was "common and increasing" in the ranchlands. Adding to the pressure on wildlife, more and more farmers were allowing their livestock to graze in the reserve, especially in times of drought. Hunting by mainly non-Masai ethnic groups within the Mara also remained a significant problem.

"The traditional livestock livelihoods of the Masai, who rarely consume wild animals, actually helped maintain the abundance of grazing animals in east Africa. And where a pastoral approach to livestock grazing is still practised, it continues to benefit wild populations," said Robin Reid, director of the Centre for Collaborative Conservation at Colorado State University, who co-wrote the paper.

She warned: "There appears to be a 'tipping point' of human populations above which former coexistence between Masai and wildlife begins to break down. In the villages on the border of the Mara, this point has been passed, but large areas of the Mara still have populations low enough that compatibility is still possible."

Since 2003, several wildlife conservancies have been set up in the Mara rangelands. The operators say these have helped increased animal numbers while providing employment and rental income to Masai landlords.