The invasion of millions of field mice on farmland surrounding Hunan province's Dongting Lake is only the tip of the iceberg of a large-scale rodent assault on the mainland. Rising waters in Dongting Lake, one of the mainland's five major freshwater lakes, have forced up to two billion mice to abandon their nests near the lake and move inland in the biggest invasion of rodents on record, threatening paddy fields, dams, dykes and other infrastructure in the flood-prone areas. While the severity of the assault has caught farmers, the authorities and the wider community unprepared, many areas along the middle reaches of the Yangtze River and the country's west have been under siege from rodent plagues on a smaller scale because of excessive human activity. At the heart of the Qinghai-Tibet plateau, the source of China's major rivers, overgrazing since the 1990s has meant one third of the grassland has been lost to a plague of the Daurian pika - Ochtona daurica pallas - a plant-eating rodent. And that plague has shown signs of worsening since the Qinghai-Tibet railway was built, according to Zhang Meiwen , a deputy research fellow from the Chinese Academy of Sciences' Institute of Subtropical Agriculture. 'Because of the rail route, other mice species could find their way into the Qinghai-Tibet plateau, exacerbating the desertification process in the region,' the scientist said. He said several rodent species, including the brown and yellow-chested rats, previously found only in central mainland regions, had been spotted farther west since the railway started running in July last year. Mr Zhang said it was too early to say how much of a contribution the Qinghai-Tibet railway would make to mice plagues and desertification on the plateau, 'but transport networks have traditionally been blamed as opening up breeding grounds for mice plagues'. He warned that the railway line itself might be subject to a mice assault because the rodents would dig homes wherever they could. There is little data about rodent plagues on the mainland, but many scientists believe mice infestations have been worsening over the years because of climate change, excessive human activity and imbalances in the ecosystem. The number of natural mice predators such as snakes, hawks and owls has dropped drastically as a result of poaching and habitat loss and, in some cases, the ingestion of poisoned rodents. Plague-control authorities in some regions have tried to rehabilitate the ecosystem by luring some of the predators back into plague-stricken areas. Mr Zhang said the most effective way to contain the epidemic was to use environmentally friendly rat poison, but plagues would continue to haunt some parts of the country because of limited resources to combat outbreaks. Academy research fellow Wang Yong said farmers living near Dongting Lake would see more mice if water levels continued to rise. 'But the plague [at the lake] is basically under control,' Mr Wang said.