Numbers of giraffe in the Masai Mara fell by more than 80%
Wild grazing animals in Kenya's Masai Mara National Reserve are steadily disappearing, a study has found.
Numbers of giraffe, warthog, impala, topi and hartebeest fell by 50% or more between 1979 and 2002.
The falls are linked to rapid growth of Maasai settlements around the reserve, say scientists from the International Livestock Research Institute (ILRI).
Their analysis is published in the British Journal of Zoology.
"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, the lead author of the study and a statistical ecologist at ILRI.
"Our study offers the best evidence to date that wildlife losses in the reserve are widespread and substantial.
"These trends are clearly linked to the increase in human settlements on lands adjacent to the reserve."
The loss of grazing animals is already having an impact on lions, cheetahs, and other predators, according to researchers.
"The carnivores which depend on these wildlife are the first casualties," said Dr Ogutu.
"The numbers of lions are going down. The cheetah numbers are declining. The wild dogs in the Mara system have become extinct."
The Masai Mara and the neighbouring Serengeti are world-famous for their exceptional wildlife population - including an annual migration of nearly two million wildebeest.
Lion numbers are declining in the Mara as their prey disappear
The Mara itself was recently voted one of the "seven modern wonders of the world".
But during recent decades, many species have come under threat from severe droughts, increased poaching, and more intensive grazing by Maasai pastoralists in the "ranchlands" at the fringes of the reserve.
Between 1989 and 2003 the ILRI scientists carried out monthly ground counts of seven ungulate species - giraffe, hartebeest, impala, warthog, topi, waterbuck, and zebra.
They found significant declines in giraffe, impala and topi, and even greater declines in warthog and hartebeest.
The trends they observed are backed up by a separate, aerial count of wildlife undertaken between 1979 and 2002, by the Kenyan government Department of Resource Surveys and Remote Sensing.
By 2002, numbers of giraffe in the reserve had fallen to 20% of their 1979 levels, the bulk of those losses occurring before 1989.
Topi and hartebeest in the reserve fell to less than half their 1979 levels, and almost disappeared in some of the neighbouring ranchlands where they once grazed.
Impala fell by 70% in the Mara itself, while warthog fell by more than 80%, although their numbers appeared steady since 1989.
The wildlife losses were most pronounced in the areas where human settlement has increased, even after factoring out the influence of drought.
Maasai have traditionally lived in harmony with wild animals
"Wildlife are constantly moving between the reserve and ranchlands, and they are increasingly competing for habitat with livestock," said Dr Ogutu.
"In particular, more and more people in the ranchlands are allowing their livestock to graze in the reserve - an illegal activity the impoverished Maasai resort to when faced with prolonged drought and other problems.
"The steady erosion of wildlife habitat caused by this intrusion is a key factor in the declines we observed.
"And since 2002 [when the survey ended] the number of settlements, human population and agriculture have continued to expand, so the declines can only be expected to accelerate."
Traditionally, most Maasai were semi-nomadic herders who co-existed easily with the wildlife in the region.
In the right circumstances, Maasai settlements can actually benefit populations of wild grazing animals, the researchers have found.
This is because human settlements can act as safe havens for wild grazing animals because human activity repels lions and other predators.
"The traditional livestock livelihoods of the Maasai, who do not consume wild animals, actually helped maintain the abundance of grazing animals in East Africa," said co-author Robin Reid, of Colorado State University in the US.
Maasai do not always benefit from the revenue the Mara wildlife tourism brings
"And where a pastoral approach to livestock grazing is still practiced, it continues to benefit wild populations."
But the growing communities of pastoralists and their exclusion from development of land policies have made their traditional way of life difficult to maintain.
Over the last few decades, many Maasai have left their traditional mud-and-wattle homesteads, known as bomas, and gravitated to more permanent settlements - a large number of which now crowd the "ranchlands" at the border of the reserve.
In just one of these ranchlands, the Koyiaki ranch, the number of bomas surged from 44 in 1950 to 368 in 2003, while huts increased from 44 to 2,735 in number.
As these permanent settlements increased, the abundance of wildlife decreased significantly, researchers note.
The ILRI scientists are helping to promote schemes where Maasai living next to game reserves receive rent payments from private game lodges in return for allowing wildlife to continue to roam on their property.
In one such conservancy, at Olare Orok, the numbers of lions "increased almost immediately", said Dr Ogutu.
"We know from thousands of years of history that pastoral livestock-keeping can co-exist with East Africa's renowned concentrations of big mammals. And we should look to these pastoralists for solutions to the current conflicts," said Carlos Seré, ILRI's Director General.
"With their help and the significant tourism revenue that the Mara wildlife generates, it should be possible to invest in evidence-based approaches that can protect this region's iconic pastoral peoples as well as its wildlife populations."