Approximately 26% of young males and females begin to reproduce during the breeding season of their birth, but overwintered adults are responsible for most of the reproduction. Some adult females may have up to two litters during one breeding season. Among overwintered adults, 90% of females and 100% of males are reproductively active. The age distribution studied among populations show a 1:1 replacement of adults by young. Most water voles only live through one winter, and die at the end of the second breeding season. Very few adults survive two winters. There have been several studies done on the habitat of the vole, given its very specific requirements, to find out if grazing or precipitation levels affect the vole populations so that we can protect them. It has been found that higher precipitation levels create increased availability of usable habitat for water voles. In years of that are wetter than usual it has been observed that young water voles become reproductively active sooner, and therefore can have more offspring. The mean number of pups in each litter increased as well. It has also been found that grazing affects the water vole populations as well. In areas of light or moderate grazing not as many young survived, and the population sizes tended to be much smaller than other colonies. Where there was severe grazing it affected the stream bank to an extent that it was no longer a suitable habitat. The presence of livestock presents numerous detrimental changes to the water vole's habitat: altered abiotic characteristics, compacted soil, increased runoff, fractured stream banks, erosion, as well as loss of vegetation as protective cover and a food source. The abundance of ferns, mosses and shrubs are imperative as ground cover to protect water voles from predation, and in areas of grazing these plants were scarce or non-existent, and therefore water voles weren't often found in these areas.