An assessment of the local impact of native predators on an established population of British water voles (Arvicola terrestris)

1. What they did

Water voles are among Britain’s most threatened mammals. Habitat loss is the main cause of their decline, and introduced American mink have been blamed for many local losses. The author asked how much native predators also take, looking at two common, opportunistic hunters during winter, when water voles are thought to be most at risk.

• Study site. The National Wetlands Centre Wales, a wetland reserve on the Loughor estuary in South Wales with a large water vole population.
• Collecting scats and pellets (November 2003 to January 2004). All old fox scats and heron pellets were first cleared from the site. Then, once a month, fox scats were collected by walking about 16 km of hedgerows, ditches and paths, and heron pellets were collected from paths and bridges where herons regularly foraged.
• Analysing them. Each scat and pellet was dried, weighed and washed through sieves. Fox scats were confirmed by the fox’s own hair in them, and prey were identified with published keys and a reference collection of bones, fur and feathers. Heron pellets had few hard remains, so up to 30 hairs per pellet were identified to species.
• Reviewing earlier studies. The author gathered published local figures for water voles in the diets of British predators, and the causes of death of radio-tracked water voles in three earlier studies.

2. What they found

Fox scats (118 scats)

• Mammals were in 89.8% of scats, birds in 72.9%, invertebrates in 23.7% and plant material in 91.5%.
• Field voles were the main prey, in 55.9% of scats and 32.3% of the weight.
• Water vole remains were in 30.5% of scats and made up 13.1% of the weight.
• Other important prey were rabbits (20.3% of scats) and moorhens and coots.
• Plastic was in a third of the scats (33.9%).
• The rodent species occurred at significantly different rates.

Heron pellets (124 pellets)

• Almost half (47.6%) contained mammal fur, from five rodent species.
• Water vole fur was the most common, in 17.7% of pellets, followed by field vole (13.7%), wood mouse, bank vole and brown rat.

Earlier studies

• Published local figures for water voles in predator diets ranged from 0.5% to 14% for otters, 3.3% to 13% for mink, 20% to 30% for foxes, 17% to 91% for herons and 12% to 35% for barn owls.
• Radio-tracking studies showed that many water voles were killed by native predators, including heron, domestic dog, fox, stoat and other mustelids, as well as by mink.

3. What they concluded

• Foxes at this site ate water voles at a rate equal to or greater than most mink diet studies in Europe report. The author crudely estimates that one fox could eat 10 to 20 water voles a year.
• On the continent, water voles of the burrowing (fossorial) form spend much of the winter underground. At this site, water voles stayed active above ground all year where plant cover remained, which likely raises their risk of being caught in winter.
• About one in five heron pellets held water vole fur. Herons digest very efficiently, so pellets may overestimate mammals in their diet, but herons often return to the same sites to hunt and could have a strong effect on local water vole patches.
• The role of native predators in local water vole declines has been largely overlooked, with blame falling on mink. Small, fragmented water vole colonies may be especially vulnerable.
• The author has twice seen otters dig out water vole burrows, and notes that many otter diet studies do not identify mammal prey to species, so otter predation may be underestimated.
• Future work should study the diet and behaviour of all the predators, mink included, alongside water vole populations in different habitats.

4. How tracking was or was not used

The study rests on collecting and identifying signs: fox scats found along linear features such as hedgerows, ditches and paths, and heron pellets at paths and bridges known to be used by foraging herons. Fox scats were confirmed by finding fox hair inside them. Prey were then identified in the laboratory from bones, fur and feathers, using identification keys (one of them a field guide to the tracks and signs of birds) and a reference collection. No trailing was reported. The author also mentions two incidental observations of otters digging out water vole burrows.

5. Tracker or observer certification

Not reported. The author appears to have collected the scats and pellets, but how fox scats and heron pellets were recognised in the field is not described. Séverine Ricaud is thanked for help with some of the scat analysis.

6. Other technologies, skills or innovations

• Scats and pellets were frozen, oven-dried at 70 °C for 48 hours, weighed, and washed through 1.7 mm, 600 µm and 212 µm sieves.
• Hair identification to species with published hair keys, useful where pellets contain no bones.
• Prey reported both as frequency of occurrence and as percentage of dry weight, because frequency alone tends to overstate rare prey and understate common prey.

7. Implications for conservation (from the paper)

• Native predators, not only mink, can take significant numbers of water voles, which could slow the recovery of local populations where mink are absent and speed up colony losses where mink are present.
• With growing fox and, to a lesser extent, heron populations, conservation managers may have to make tough choices about predator control on a site-by-site basis if local water vole populations are to survive.