Local feeding specialization of the red fox (Vulpes vulpes) in response to eastern cottontail (Sylvilagus floridanus) introduction (NW Italy)

1. What they did

Red foxes are the classic generalist, eating whatever food is plentiful in each place and season. The authors asked what happens when a new, abundant prey arrives. The eastern cottontail, a rabbit native to the Americas, was introduced to Piedmont in north-western Italy in the mid-1960s and reached their study area, a 250 ha nature reserve on the River Po, in the mid-1990s. By the end of 2000 there were more than 15 cottontails per km² there. The native wild rabbit had died out in the area in the early 1990s, and hares were always scarce.

• Collecting scats. Each month from June 1998 to February 2000 they walked a 4.1 km transect through the main habitats and collected 115 fox scats.
• Analysing them. In the laboratory the scats were washed through sieves, and food remains were identified: mammal hairs under the microscope with reference keys, feathers, reptile and amphibian remains, insect parts, fruit seeds, and earthworm bristles.
• Comparing. They compared their results with fox and badger diets recorded in the same area in 1988 to 1989, before the cottontails came, and with badger diet in 1998 to 2000. They measured how wide each diet was (niche breadth) and how much the fox and badger diets overlapped.

2. What they found

Fox diet, 1998 to 2000

• Foxes ate mostly meat. Mammals were in 93.9% of scats and made up 87.7% of the diet by volume.
• Lagomorphs (rabbits and hares), which the authors attribute almost entirely to cottontails, were by far the main food: in 75.6% of scats and 68% of the diet by volume.
• Rodents made up 17.2%, mainly common dormice and water voles. Birds, mostly small songbirds, made up 6.6%. Everything else was under 5%; fruit was eaten only occasionally (1%).
• Lagomorphs were the main food in every season. Only insect eating changed with the season, peaking in summer.
• How often foxes ate lagomorphs did not change between the cottontail breeding season (February to September) and the rest of the year.
• The more lagomorphs foxes ate in a season, the narrower their diet. Diet was narrowest in winter and widest in spring. Birds were eaten more when rodents were eaten less.

Before and after cottontails

• Before cottontails arrived (1988 to 1989), lagomorphs were in only 27.3% of fox scats, against 75.6% afterwards, even though wild rabbits were plentiful then. Foxes also used to eat maize (in 24.2% of scats); after the cottontails arrived, none was found.
• Fox niche breadth halved, from 0.64 to 0.31.
• Badger niche breadth hardly changed (0.54 to 0.49). Badgers did eat more earthworms and frogs and less Prunus fruit in 1998 to 2000.
• Diet overlap between foxes and badgers fell from 0.59 to 0.13.

3. What they concluded

• In this area foxes act as local specialists on cottontails. Their diet is dominated by one food, and that food is used steadily through the year. This is a “realized” specialization of a typical generalist, which the authors explain with optimal foraging theory: predators should favour the most profitable prey.
• Cottontails are very profitable prey. They offer about 35 times the body mass of most voles and mice, breed quickly, and, unlike wild rabbits, do not dig burrows, so they (especially the young) may be more exposed to foxes.
• The few other foods foxes ate (dormice, water voles, songbirds) live in the same dense, bushy field margins and hedgerows that cottontails favour, so foxes may simply come across them while hunting cottontails.
• Because foxes now focus on cottontails while badgers eat mainly earthworms and maize, the two species compete less for food, especially in summer, when drought makes earthworms scarce and badgers look for other protein.
• Foxes may limit this introduced species, whose effect on native hares is still unknown. More research is needed on how fox specialization affects competition with other local predators, such as stone martens.

4. How tracking was or was not used

Fox scats were collected monthly along a fixed 4.1 km transect, and the food remains in them were identified in the laboratory. The paper does not say how the scats were recognized as fox scats in the field (for example by size, shape, smell or location), and no other check of identity, such as DNA, is reported.

5. Tracker or observer certification

Not reported. The paper does not say who collected the scats or how they were identified as fox scats, although all the diet results depend on that identification. Laura Gola and Barbara Zappia are thanked for help with the field research.

6. Other technologies, skills or innovations

• Scat analysis following Kruuk and Parish (1981) and Prigioni (1991): scats washed through sieves of 1.5, 0.3 and 0.1 mm mesh, with the finest sediment checked under a microscope for earthworm bristles.
• Mammal hairs identified at 20x and 40x magnification with a hair atlas; feathers, reptile and amphibian remains with published keys; insects and fruit seeds with reference collections.

7. Implications for conservation (from the paper)

• Introduced species can change native ecosystems in many ways, and serving as prey for native predators is probably the most overlooked.
• Cottontails were introduced for hunting but are rarely shot, because hunters prefer hares. Foxes may act as a limiting factor for this introduced species, whose impact on native lagomorph populations is still unknown.
• By specializing on cottontails, foxes may reduce competition for food with badgers.