Summer food of sympatric red fox and pine marten in the German Alps

1. What they did

Red foxes and pine martens are both described as opportunistic, generalist feeders, though foxes are known to favour Microtus voles and pine martens Clethrionomys (bank) voles. Because the red fox is larger and uses a wider range of habitats, the authors expected the fox to have the wider feeding niche and to eat larger prey.

On Teisenberg mountain in the foothills of the Bavarian Alps, Germany, they searched the whole 79 km network of forest roads three times between June and August 1999 for red fox and pine marten scats. Since stone martens had not been recorded there since 1988, every marten scat was labelled pine marten. They mapped the scats and chose 55 fox and 64 marten scats, spread evenly across 100 m bands of altitude.

In the laboratory they washed, sieved and dried the scats and identified hair, teeth, feathers, insect parts, berries and seeds under a microscope with identification keys and reference collections. They estimated each item’s share of the scat by volume, then converted this into consumed biomass using digestibility correction factors. They compared the diets as frequency of occurrence and as consumed biomass, and measured niche breadth with two indices (Levins and Shannon-Wiener).

2. What they found

Red fox

• Insects and plants were the foods found most often, but insects made up only 3% of the consumed biomass. Plants made up 46% and mammals 51%.
• Roe deer, field vole and red squirrel were among the largest mammal contributors, but no single species made up more than 8% of the diet.
• Grasses and ground beetles were the main plants and insects. Birds hardly featured; the bird of prey remains may have come from carrion.

Pine marten

• Close to two thirds of the food items were insects, mostly ground beetles and ants.
• In consumed biomass, plants (mostly blackthorn, brambles and other rose family fruits, plus bilberry) made up 46% and mammals 41%.
• Water vole and red squirrel were the mammals found most often, while mountain hare, red squirrel, hazel dormouse and bank vole made up the most mammal biomass.
• Neither species’ scats held amphibians or reptiles.

Niche breadth and prey size

• Based on how often foods occurred, both indices gave the red fox a wider niche than the pine marten. Based on biomass, the two species scored very similarly.
• The Levins index made both species look like specialists (0.15 to 0.34), while the Shannon-Wiener index made both look fairly generalist (0.57 to 0.73).
• Foxes ate a wider range of mammal prey sizes (5 g to 21,000 g, median 35.5 g) than martens (5 g to 2,850 g, median 111.3 g), but the difference in prey size was not statistically significant.

3. What they concluded

• The red fox results matched other European studies: a varied diet with voles important, and plenty of grasses, berries and beetles. Birds were unusually rare.
• Pine martens ate more insects than foxes, but most of their biomass came from fruit and mammals, especially bank voles and dormice. This supports the view of the pine marten as a generalist.
• The two niche indices gave very different pictures, because they weight rare and common items differently. Frequency of occurrence overstates foods that pass through the gut little digested (insect shells, plant fibre), while biomass relies on digestibility values from controlled experiments. The authors consider biomass the better measure and placed more weight on the Shannon-Wiener results.
• Their preliminary results suggest that a strict line between generalist and specialist feeders is not justified for medium-sized carnivores in the Alps, especially as results depend so much on the index used.
• Several years of data, and knowledge of how much food is available and how much effort the predators spend feeding, would be needed to understand the relationship between the two species.

4. How tracking was or was not used

The study rests on finding and identifying scats. The team walked the forest roads three times to find red fox and pine marten scats, mapped them, and assigned each to species. The paper does not say how fox and marten scats were told apart in the field. Marten scats were all assigned to pine marten on the grounds that stone martens had not been recorded on the mountain, although stone marten remains later turned up in one marten scat. Inside the scats, prey were identified from hair, teeth, feathers and insect parts using published keys.

5. Tracker or observer certification

Not reported. Three people are thanked for help collecting and analysing the samples, but nothing is said about who identified the scats in the field or how reliable those identifications were. The whole diet comparison depends on fox and marten scats being told apart correctly.

6. Other technologies, skills or innovations

• Microscope identification of hair (scale patterns, medulla and cross sections), teeth, feathers, insect parts, berries and seeds with published keys and reference collections.
• Volume estimated on a 5 mm grid, converted to consumed biomass with digestibility correction factors.
• Scats autoclaved within 3 days of collection to stop them breaking down.