Differential use of food and habitat by sympatric carnivores in the eastern Italian Alps

1. What they did

Red foxes, badgers, stone martens and pine martens are medium-sized generalist carnivores that live side by side in the Italian Alps. Species that share a place are expected to reduce competition by eating different foods, using different habitats or being active at different times. The authors set out to measure how these carnivores divide food and habitat in an Alpine valley.

They expected the biggest diet overlap between foxes and martens, because badgers would mostly eat earthworms and other invertebrates. They also expected foxes, about three times heavier than martens, to act as the dominant species, using more habitats and more kinds of food, and martens to keep to other habitats to avoid foxes.

• Collecting scats. Every month from June 1994 to June 1996, they walked four transects through the main habitats of the Fiemme Valley around Cavalese and collected 692 scats. Badger scats came from latrines. Fox and marten scats were told apart by shape and size. Pine and stone marten scats look the same, so they were grouped as Martes sp.
• Diet analysis. They washed each scat through sieves and identified the remains: hairs under the microscope, feathers, insect parts, fruit seeds and earthworm bristles. They estimated how often and in what volume each food was eaten, by season.
• Habitat use. They used the share of scats found in each habitat (the “marking ratio”) as an index of habitat use and compared it with how much of each habitat the transects crossed.
• Overlap. They calculated how broad each species’ diet was and how much the species overlapped in diet and in habitat.

2. What they found

Diet

• Fruits were the main food for all three. They made up about half of the fox diet (49%, almost all from the rose family), 73% of the badger diet and 58% of the marten diet.
• Foxes relied almost entirely on two fruits, rowan berries and rose-hips. They ate more mammals than the others, especially roe deer in spring and summer and hares in autumn and winter, and insects (mainly ground beetles) in spring and summer. About 40% of the meat foxes ate was roe deer (84% in summer), probably fawns in summer and carrion in winter and spring.
• Badgers ate surprisingly few invertebrates. Earthworms appeared almost only in spring, at under 5% of the spring diet. Instead, cultivated fruits (apples, plums, pears) and raspberries dominated in summer and autumn, and pine seeds in spring. No badger scats were found in winter, a time of heavy snow and little badger activity.
• Martens ate wild fruits of the rose family in spring and summer and other fruits, such as privet, in autumn and winter. Garbage (food of human origin) was important all year, at about 19% of their diet overall. Roe deer were almost absent from their diet.
• Rodents were rarely eaten by foxes and martens and not found in badger scats.

Diet breadth and overlap

• Badgers had the broadest diet and foxes the narrowest.
• Overall diet overlap was highest between badgers and martens and lowest between foxes and martens, the opposite of what the authors expected.
• Overlap was low overall because each species ate different kinds of berries and mammals and, to a lesser degree, ate the same foods in different seasons. In summer, when most fruits ripen, badger diet overlapped more with the other two.

Habitat

• Foxes used every habitat in proportion to what was available, except villages, where no fox scats were found.
• Badgers strongly preferred Alpine meadows and avoided villages and mixed woods.
• Martens did the opposite: they selected villages and mixed woods and avoided the other habitats.
• Habitat overlap was highest between foxes and badgers and lowest between badgers and martens. All overlaps were higher in summer.

3. What they concluded

• The low earthworm use by badgers was unexpected. The authors suggest the local rain, temperature and snow limit earthworms, so badgers switched to food “lying on the ground”, mainly fruit.
• Fruits seem to be an important alternative food for Alpine carnivores.
• As predicted, foxes used the widest range of habitats. Martens were the most selective, choosing habitats avoided by one or both of the other species. Together with the foxes’ more meaty diet, this suggests asymmetrical competition, with martens as the subordinate species, mainly towards foxes.
• Martens in town were probably almost all stone martens. Villages may offer them safe, warm resting places in attics and roofs, away from foxes, as well as garbage to eat. The authors suggest that urban areas and their surroundings can meet all of a stone marten’s needs.
• More research is needed to find out whether avoiding dangerous encounters with larger competitors explains why martens eat so little roe deer.

4. How tracking was or was not used

The whole study rests on scats found along transects:

• Badger scats were collected from latrines.
• Fox and marten scats were told apart by shape and diameter (marten scats under 10 mm, fox scats over 15 mm), following the field guide Animal Tracks (Bang and Dahlström 1974). The paper does not say how scats between 10 and 15 mm were treated.
• Pine and stone marten scats cannot be told apart by eye, so they were grouped as Martes sp. Because pine martens tend to leave scats on branches or at the base of trees below their dens, and may not use trails at low density, the authors think most collected marten scats were from stone martens.
• Scat counts per habitat (the marking ratio) were used as an index of habitat use. The authors note that the reliability of this method has been disputed, because it rests on assumptions about marking behaviour and on correct scat identification, but they consider scat counts an effective, low-cost index.

No genetic or other check of scat identification is reported, and no trailing was used.

5. Tracker or observer certification

Not reported. The paper does not say who collected and identified the scats or how their identification skill was checked. Renato Fumagalli is thanked for help in field research. This matters because the diet and habitat results depend on correct identification of fox, badger and marten scats.

6. Other technologies, skills or innovations

• Scat analysis with three sieves (1.5, 0.3 and 0.1 mm mesh), microscope identification of prey hairs with reference keys, and a search for earthworm bristles in the finest sediment.
• A Principal Components Analysis to show which foods set each species’ seasonal diet apart.

7. Implications for conservation (from the paper)

• Stone martens’ ability to live in towns could be key to their success in the Alps, where food becomes scarcer with altitude and competition among carnivores may increase.
• Towns may act as fox-free refuges for martens, offering shelter in old buildings and human food waste.