into nitrogen). Bacteria in the dirt and the plant debris, which collects at the dams, turns nitrates into nitrogen gas. The gas bubbles to the surface and mixes with the atmosphere once more. Salmon and trout Beaver dams and the associated ponds can provide nurseries for salmon and trout. An early indication of this was seen following the 1818 agreement between the British government of Canada and the government of America allowing Americans access to the Columbia watershed. The Hudson's Bay Company, in a fit of pique, instructed its trappers to extirpate the fur-bearing animals in the area. The beaver was the first to be made locally extinct. Salmon runs fell precipitously in the following years, even though none of the factors associated with the decline of salmon runs were extant at that time. There are several reasons why beaver dams increase salmon runs. They produce ponds that are deep enough for juvenile salmon to hide from predatory wading birds. They trap nutrients in their ecology and notably the nutrient pulse represented by the migration of the adult salmon upstream. These nutrients help feed the juveniles after the yolk sac has been digested. The dams provide calm water which means that the young salmon can use energy for growth rather than for navigating currents; larger smolts with a food reserve have a better rate of survival when they reach the sea. Finally, beaver dams keep the water clear which favours all salmonoids. Frogs Beaver dams have been shown to be beneficial to frog and toad populations, likely because they provide protected areas for larvae to mature in warmer, well-oxygenated water. A study in Alberta, Canada, showed that "Pitfall traps on beaver ponds captured 5.7 times more newly metamorphosed wood frogs, 29 times more western toads and 24 times more boreal chorus frogs than on nearby free-flowing streams." Birds Beaver dams help migrating songbirds. By stimulating the growth of species of plants that are critical to populations of songbirds in decline, beaver dams help create food and habitat. The presence of beaver dams has been shown to be associated with an increased diversity of songbirds. They can also have positive effects on local waterfowl, such as ducks, that are in need of standing water habitats. Disruption Beaver dams can be disruptive; the flooding can cause extensive property damage, and, when the flooding occurs next to a railroad roadbed, it can cause