阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。
The polar bear is found in the Arctic Circle and some big land masses as far south as Newfoundland. While they are rare north of 88°,there is evidence _ 61 _ they range all the way across the Arctic, and as far south as James Bay in Canada. It is difficult to figure out a global population of polar bears as much of the range has been _ 62 _ (poor) studied; however, biologists calculate that there are about 20,000-25,000 polar bears worldwide.
Modem methods _ 63 _ tracking polar bear populations have been employed only since the mid-1980s, and are expensive _ 64 _ (perform) consistently over a large area. In recent years some Inuit people in Nunayut _ 65 _ (report) increases in bear sightings around human settlements, leading to a _ 66 _ (believe) that populations are increasing. Scientists have responded by _ 67 _ (note) that hungry bears may be congregating(聚集) around human settlements, leading to the illusion(错觉) that populations are _ 68 _ (high) than they actually are. Of _ 69 _ nineteen recognized polar bear subpopulations, three are declining, six _ 70 _ (be) stable, one is increasing, and nine lack enough data.