Changes in red fox Vulpes vulpes diet due to colonisation by lynx Lynx lynx

1. What they did

Large predators such as wolves and lynx often kill prey bigger than they can eat in one meal, leaving carrion for scavengers. In the boreal forest of Sweden, red foxes were studied in the past when large predators were almost absent, and then they fed mostly on voles. During the 1990s, lynx recolonised the Grimsö Wildlife Research Area in south-central Sweden, going from virtually none to a resident population in one or two years. Roe deer are a year-round staple for lynx in this region, and red foxes are known to scavenge lynx kills and to follow lynx trails in search of food remains.

The authors asked whether the return of lynx changed what red foxes eat. They predicted that roe deer would make up more of the fox diet, and that the fox food niche would become narrower as foxes ate less of the food they presumably like less.

• Collecting scats (June 1999 to April 2002). Most fox scats were collected along forest roads, which were first cleared of old scats. Roads were searched monthly in summer (May to October), and once or twice in winter (November to April) when snow conditions allowed, plus once the snow had melted. Some scats were collected while snow tracking foxes and in live traps for foxes.
• Analysing the diet. They analysed 60 scats per season (all 50 found in summer 2000), chosen to spread evenly across the area and the season. Scats were dried, weighed and examined by eye and under the microscope, compared with reference guides and a reference collection. Food remains were sorted into eight categories, and the biomass eaten was estimated with standard correction factors.
• Comparing with the past. They compared the results with an identical study in the same area in 1974 to 1979, before lynx returned. They also used long-term counts of roe deer, hares, forest grouse (from pellet counts) and voles (from snap trapping).

2. What they found

Diet in 1999 to 2002

• Roe deer made up about half of the food consumed in winter (43 to 54% of biomass) and on average 38% in summer. They were the single most important food all year round.
• Other important foods, often 10 to 20% of what was consumed, were hares in winter, vegetables in summer, and voles and birds in both seasons.
• Roe deer remains appeared in scats throughout the summer. The summer amounts were too high to be explained only by foxes killing fawns, so foxes were also scavenging roe deer in summer.

Compared with 1974 to 1979

• Both summer and winter diets were different, mainly because there was more roe deer in the scats.
• In winter, foxes ate fewer invertebrates and vegetables. In both seasons, “other mammals” and “fish and reptiles” made up smaller shares. Hares, voles and birds did not differ between the two periods.
• Even though all three winters of the new study were mild, with shallow and short-lasting snow, the share of roe deer in fox food items was more than double that of the very harsh winter of 1976/77, when snow was over 70 cm deep for four months and many roe deer starved.
• The winter food niche was narrower than the summer niche. The summer niche did not differ between the two periods.

Why more roe deer?

• Roe deer were not more common: their numbers had actually declined, at least partly because of lynx.
• A parallel radio-tracking project found that lynx predation was the main cause of death for roe deer (except for young fawns, often taken by foxes). Of the other causes, few leave carcasses foxes can reach in winter. The authors conclude that the extra roe deer in the fox diet came mainly from lynx kills, at least in winter.
• Other prey known to matter to foxes (voles in particular, also hares and forest grouse) were generally less abundant than in the 1970s.

3. What they concluded

• Lynx feeding on roe deer give boreal red foxes a stable winter food supply and add to their summer diet.
• The heavy use of roe deer was probably a combined effect of a steady supply of deer carrion and few other prey.
• Roe deer replaced foods that are presumably less profitable for foxes (fish, reptiles, invertebrates and vegetables), and the narrower winter niche suggests better food availability, according to optimal foraging theory.
• The overall effect of lynx on red fox numbers may cut both ways. More food could improve fox breeding and condition, but lynx readily kill foxes, and fox population indices in the region suggest a decline after lynx returned.

4. How tracking was or was not used

The study rests on finding and identifying red fox scats, mainly along forest roads, and analysing the food remains in them. Some scats were collected during snow tracking of foxes. The paper does not say how fox scats were told apart from those of other species, or how the snow tracking was done. The authors also cite an older snow-tracking study (Haglund 1966) as the source for foxes following lynx trails in search of food remains.

5. Tracker or observer certification

Not reported. Nothing is said about who collected the scats or did the snow tracking, or how scats were identified as red fox, although all the diet results depend on correct identification.

6. Other technologies, skills or innovations

• Scat analysis by eye and under the microscope, using hair identification guides and a reference collection, with biomass estimated by eye and corrected with digestion factors.
• Long-term prey monitoring in one research area: pellet counts for roe deer, hares and forest grouse, snap trapping for voles, and den counts for foxes.
• Radio-tracking of roe deer in a parallel project, used to judge causes of death.

7. Implications for conservation (from the paper)

• Removing large predators may affect scavengers, because carrion becomes scarcer and less predictable. The authors give the rarity of arctic foxes and wolverines in the Scandinavian mountains as a possible example, and the tripling of carrion after wolves returned to Yellowstone, benefiting grizzly bears.
• The return of lynx creates an important, stable food source for red foxes, but lynx also kill foxes, so the effect on fox populations may be two-fold.