Monkey populations in threatened forests are far more sensitive to damage to their habitat than previously thought, according to new research.

An analysis of monkeys living in Tanzania's Udzungwa Mountains suggests that the impact of external factors, such as human activity, on species numbers is felt in forests as large as 40 square kilometres.

Researchers also found that the health of monkey populations is closely related to the type of habitat found between forest fragments, rather than the distance that separates them.

The findings have broader implications for conservationists as the number of monkeys and the variety of species is a visible indicator of the underlying health of their habitat.

The research was conducted by Dr Andrew Marshall, from the Environment Department at the University of York and Director of Conservation at Flamingo Land Theme Park and Zoo, in collaboration with colleagues from the University of York, the University of Copenhagen, the Tremto Museum of Natural History (Italy) and the Udzungwa Ecological Monitoring Centre (Tanzania).

Dr Marshall said: "This study suggests that while small forest fragments need protecting we should intervene at an earlier stage to protect larger forest areas that are under threat.

"It also supports the case for working with local communities on practical steps that will help forest species. These could include reducing dependence on bush meat and encouraging the planting of habitat that can form corridors between forest fragments."

The research investigated the distribution of seven species living in an area covering 10,000 km2 and has led to a wider conservation and education project in the area led by Dr Marshall, through Flamingo Land Theme Park and Zoo. The discovery of a new species of chameleon in this area was announced last year.

The latest research is published in the American Journal of Primatology.