Many of the world's tropical protected areas are struggling to sustain their biodiversity, according to a study by more than 200 scientists from around the world. But the study published in Nature includes research focusing on a reserve in Tanzania by University of York scientists that indicates that long-term engagement with conservation has positive results.

Dr Andy Marshall, of the Environment Department at York and Director of Conservation Science at Flamingo Land, compared the data he collected in the Udzungwa mountains with data collected more than 20 years previously by Jon Lovett, formerly of the University of York and now Professor of Global Challenges at the University of Leeds.

Conservation efforts in this biodiversity hotspot have paid dividends. It is one of the few sites that are relatively unchanged from a biodiversity point of view.

Professor William Laurance, from James Cook University in Cairns, Australia, and the Smithsonian Tropical Research Institute in Panama, said that "these reserves are like arks for biodiversity."

"But some of the 'arks' are in danger of sinking," he said, "even though they are our best hope to sustain tropical forests and their amazing biodiversity in perpetuity."

Professor Laurance and his team studied more than 30 different categories of species-from trees and butterflies to primates and large predators-in protected areas across the tropical Americas, Africa and Asia-Pacific.

They estimated how these groups had changed in numbers over the past two to three decades, while identifying environmental changes that might threaten the reserves.

Laurance said their conclusion was that while most reserves were helping to protect their forests, about half were struggling to sustain their original biodiversity.

In Udzungwa, more than 20 years ago, the forest was being heavily logged but now the area had been designated a national park.

Dr Marshall said: "Our findings indicate that concerted engagement with conservation in Udzungwa has had a positive effect in mitigating the impact of human activity on biodiversity."

Professor Lovett added: "We were able to make a good assessment of the condition of the reserve because Andy Marshall was able to follow up my work from the mid-1980s. The comparison showed that long term engagement with conservation has positive results."

Elsewhere, the picture is more worrying with many reserves suffering a decline in a wide array of species, including big predators and other large-bodied animals, many primates, old-growth trees, and stream-dwelling fish and amphibians, among others.

The researchers found that reserves that were suffering most were those that were poorly protected and suffered encroachment from illegal colonists, hunters and loggers.

Deforestation is advancing rapidly in tropical nations and most reserves are losing some or all of their surrounding forest.

The team found many nature reserves acted like mirrors-partially reflecting the threats and changes in their surrounding landscapes.

The bottom line, the researchers say, is that a better job needs to be done in protecting the protected areas - and that means fighting both their internal and external threats, and building support for protected areas among local communities. Such efforts will help ensure protected areas are more resilient to future threats such as climate change.

"We have no choice," said Professor Laurance, "tropical forests are the biologically richest real estate on the planet, and a lot of that biodiversity will vanish without good protected areas."