Barcelona, Spain ? At least one-quarter of the world’s mammals in the wild are threatened with extinction, according to an international survey released Monday that blames the loss of wildlife habitat, hunting and poaching for the steep declines.

The survey, assembled over five years by 1,700 researchers in 130 countries, is the most comprehensive yet to assess the status and future of mammals on every continent and in every ocean.

The “baiji,” or Chinese river dolphin, faces extinction and already might have joined the species that have vanished from Earth.

Others are not far behind, such as the “vaquita,” a small porpoise that is drowning in fishing nets in the northern Gulf of California; the North Atlantic right whale, and various monkeys and other primates hunted by poachers in Africa.

Scientists have determined that about one-quarter of the world’s 5,487 species of mammals are threatened with extinction. The proportion of marine mammals in trouble appears to be higher, with an estimated one-third facing a serious threat of being wiped out. Many are killed when they are struck by ships or entangled in fishing gear.

About one-half of the world’s remaining apes, monkeys and other primates face threats from hunting or destruction of forests to make way for farming, said Russell A. Mittermeier, president of Conservation International.

Scientists find mammal extinction worrisome because a diversity of species stabilizes the planet. Each extinction disrupts this balance and ripples through the food chain, making it difficult for other species, including humans, to survive.