Tropical biodiversity \"arks\" meant to conserve species are vulnerable to encroachment and many are experiencing biodiversity loss, U.S. researchers say. Writing in the journal Nature, researchers report the protected forest areas aren\'t serving as the \"arks\" that some conservationists had hoped for, with four-fifths of the areas included in the survey showing declines in health and about half suffering serious losses to biodiversity. \"If you put a boundary around a piece of land and install some bored park guards and that\'s all you do, the park will eventually die,\" said researcher Daniel Janzen, a professor of conservation biology at Penn State. \"It\'s death from a thousand cuts.\" Environmental conditions and activities occurring outside of the reserves were strong predictors of how biodiversity in the boundaries would fare, the researchers said. Logging, declining forest cover and increasing fires outside the protected areas tended to pull down the health of the reserves themselves, they found. Many protected areas have found their borders slowly chipped away by development and human encroachment, the researchers said. The challenge, Janzen said, is to create the political and economic will to enact sustainable management and stave off threats from development and human activity in protected areas. And every reserve presents its own challenges, he said. \"You have to fine-tune and tailor-make your park to the particular circumstances of a place: the nature of the people, the resources and the organisms.\"