Beavers build dams to provide deep water access to their lodges and food resources. That provides security and safe access to food, especially during the winter when shallow ponds freeze. By doing so they considerably change the aquatic environment from a running water one to a still water one. This has many subsequent effects on the associated aquatic and terrestrial communities. Thus animals like the North American beaver, Castor Canadensis, are often refereed to as 'ecosystem engineers'.