SAN FRANCISCO -- Animals neither too big nor too small but just the right size face a lower risk of extinction than those on both ends of the scale, an international research team has found.

Their report in the Proceedings of the National Academy of Sciences of the United States of America, the official journal of the American National Academy of Sciences, concludes that worldwide, the largest and smallest species face a greater risk of extinction than mid-sized animals.

Based on their study of more than 27,000 vertebrate animal species in the "Red List", the International Union for Conservation of Nature's compendium of endangered species, researchers from the United States, Australia and Switzerland found that disproportionate losses at the large and small ends of the scale raise the likelihood of significant changes to the way natural ecosystems function in forests, grasslands, oceans and even rivers and streams.

Among the groups of animals evaluated were birds, reptiles, amphibians, bony fishes, cartilaginous fishes and mammals, including about 4,400 species currently threatened with extinction.

The largest animals are threatened principally with harvesting by humans.

"Many of the larger species are being killed and consumed by humans, and about 90 percent of all threatened species larger than 2.2 pounds or 1 kg. in size are being threatened by harvesting," William Ripple, lead author of the study and an ecology professor at Oregon State University, said in a news release.

"Harvesting of these larger animals takes a variety of forms, including regulated and unregulated fishing, hunting and trapping for meat consumption, the use of body parts as medicine and killing due to unintentional by-catch," the study said.

The smallest species with high extinction risk consist of tiny vertebrate animals generally less than about 1.2 ounces, or 35 grams, in body weight. Mostly threatened by loss or modification of habitat, these diminutive species include Clarke's Banana Frog, the sapphire-bellied hummingbird, gray gecko, hog-nosed bat and the waterfall climbing cave fish.

Small species that require freshwater habitats are especially imperiled.

Ripple said knowing how animal body size correlates with the likelihood of a species being threatened "provides us with a tool to assess extinction risk for the many species we know very little about."  (Xinhua)