Melting sea ice and the earlier-tha

Melting sea ice and the earlier-than-normal movement of walruses (海象) have made scientists worried. Researchers 48) that this year might bring another mass migration of animals to shore.Walruses 49) on shellfish (贝类动物) and other animals they find by diving to the seafloor. Normally, the walruses, 50) females with baby walruses, rest on the floating sea ice during the summer. However, because of the global warming sea ice melts away and 51) over the relatively shallower waters where the walruses can feed. The 52) ice stays only over water that is so deep that the walruses cannot dive down to the bottom for food. When this happens, the animals 53) back onto the land near shallower water.In recent years, researchers have found unusual 54) of mass migration have happened at an 55) level, with walruses gathering in a(n) 56) number of more than 40,000. This can 57) the local ecosystem (生态系统) as the animals feed in a limited space, and many, generally the little walruses, get killed because it is too crowded. A recent study proved that the loss of sea ice was forcing more animals to swim longer distance, bringing danger to the life of their little ones.