One of the problems, the report notes, is that none of the country's protected areas cover the full ranges of the migratory animals that inhabit the parks and reserves.
That means animals frequently move out of the parks and come up against the growing number of towns and farms cropping up around the edges of protected regions.
Such settlements are on the rise in Kenya, as land that was once communally owned is parceled out for individually owned farms and an increasing number of semi-nomadic Maasai herders decide to stay put.
Animal declines are "a problem across African wildlife areas mainly because of increasing human population," said Noah Sitati of the Eastern Africa Regional Programme Office for the conservation nonprofit WWF.
"Most of the former wildlife range has been turned into settlements and agriculture."
A drop in wild herd animals is particularly worrisome for Kenya, because wildlife tourism is central to the country's economy.
The loss of the famed wildebeest migration through the Masai Mara National Reserve, for instance, could lead tourists to go elsewhere, the report authors note. (Watch a wildebeest migration video.)
What's more, the recent increases in Kenya's elephant population, a trend hailed as one of the country's greatest conservation successes, may be hurting other animals, the study authors write.
In both Masai Mara and the Amboseli National Park, elephants have destroyed woodland where antelope forage and hide, exposing the antelope to more predation and forcing them to search for new territories.
"It's what we call ecological dislocation, which is a much bigger problem than anything else," study co-author Western said.
"It's the dislocation of all these processes that is leading to the loss of diversity and the loss of different sorts of species."
Kenyan officials say they are aware of species declines within protected lands, and they point to multiyear efforts to overhaul the country's decades-old wildlife policy.
One of the problems is that Kenya hasn't had the funding to support long-term wildlife monitoring and enforcement in its parks.
The Kenya Wildlife Service is starting an endowment fund that it hopes will attract foreign donations for wildlife conservation.
"The problem is not alarming, but sooner or later it's going to become alarming, given pressure of human population, loss of habitat, and dispersal of wildlife population," wildlife service spokesperson Paul Udoto said.
In the paper, Western and his co-authors point to studies that suggest greater community involvement in wildlife protection can help reverse the declines.
But some conservationists worry that community efforts might amount to little without strong government support.
"In a sense the question is, does Kenya value its wildlife sufficiently to do long-term monitoring?" said Allan Earnshaw, a safari operator and conservationist who is trying to implement a management plan for the Masai Mara reserve.
"I don't think [the government] values wildlife sufficiently to look after it, which is why it's in the perilous state it's in. All wildlife is suffering because of bad implementation of laws or bad policies."
SOURCES AND RELATED WEB SITES