National parks in Africa, originally set up to conserve endangered species, are failing to protect wildlife within their boundaries, a study claims.
Researchers say a decline in the number of large mammals, such as antelopes, was a result of increased pressures on the reserves' ecology.
They said the parks faced an uncertain future as a greater number of people increased the demand for resources.
The study has been published in the African Journal of Ecology.
"For years, wildlife managers and biologists in Africa have known that large mammals were disappearing outside reserves," ecologists Tim Caro and Paul Scholte wrote.
"But now a raft of studies are showing that we have moved beyond this to the next step - we are losing species from many of Africa's national parks.
"What the new data show is even relatively well-organised protected areas cannot be relied on as long-lasting conservation tools," they added.
Parks under siege
The pair, from the University of California, Davis, US, and Leiden University, Netherlands, examined a number of studies tracking the decline of antelopes.
The main cause behind the animals' decline was human activity, they concluded: "Many parks are subject to the ravaging impact of illegal hunters.
"Bushmeat hunting is often the most common factor pressing upon antelope populations. In the old days, this was for local consumption, now it includes tables in far-off cities that, incredibly, extend to London and Paris."
Another factor was marked increases in human populations and immigration, resulting in communities moving into reserves to farm.
Around smaller reserves, the increase in farming shut off migration corridors used by the animals, the researchers added.
They warned that there was "no easy solution" to halt the decline in antelope numbers.
"The old idea of setting aside large tracts of land in remote areas far from human populations is still a viable option in some parts of the continent.
"But it is a conservation approach increasingly outmoded by land-use change, demographics and policy reform," they wrote.
"We may have to get used to [a] relaxation in Africa's network of famous reserves, leaving a continent containing isolated pockets of large mammal diversity living at low population sizes - just like Europe."