The Response of European Pine Marten Martes martes L. Feeding to the Changes of Small Mammal Abundance

1. What they did

The European pine marten eats a wide range of foods, so it is usually called an opportunist: a predator that eats whatever is most plentiful at the time. The authors tested this idea in a natural park in northwestern Spain. If pine martens are opportunists, they should eat more small mammals in the seasons when small mammals are most abundant.

• Scat collection (July 2004 to June 2005). Every month they walked 25 transects, each 300 m long, along roads in four zones of the park set far apart. Pine martens and other carnivores often leave scats on roads as scent marks. Each scat’s position was recorded with a Global Positioning System (GPS) unit so that no scat was counted twice.
• Identifying and ageing the scats. Pine marten scats were told apart from those of wildcat, red fox and wolf by size and shape. To rule out the similar stone marten, 90 fresh scats were tested with DNA analysis (PCR-RFLP), and camera traps, visual surveys and searches for dead animals were also used. All the DNA samples were from pine martens. Each scat was aged as fresh, medium or old to estimate when it was left.
• Diet analysis. In the laboratory, the remains in 209 scats were separated, weighed and identified from hair, teeth and bones. Correction factors were used to estimate how much of each food was actually eaten.
• Measuring prey abundance. Once each season, small mammals were live-trapped in deciduous forest, mature pine forest and brushwood, with 25 traps at each of three sites per habitat, for three nights.
• Comparison. For each season they compared how many scats held small mammal remains with how many would be expected if the martens ate small mammals in proportion to their abundance.

2. What they found

Overall diet

• Mammals made up 50% of the food eaten (by biomass), followed by fruit (28%), birds (21%), insects (0.8%) and reptiles (0.2%).
• Among mammals, rodents made up 65%, insectivores 22% and carrion (roe deer, wild boar and badger) 13%.
• Small mammals made up 83% of the mammal biomass, mainly Apodemus mice (46%), then Glis glis (11%), Talpa occidentalis (7%) and bank vole (7%).
• Rowanberries made up about 96% of the fruit eaten.

Small mammal abundance

• 201 small mammals were caught; 95% were wood mice.
• Abundance peaked in autumn (64.4 per hectare), followed by summer (22.2), winter (3.6) and spring (0).

Diet against abundance

• Pine martens ate small mammals out of step with their abundance.
• In spring and winter, when small mammals were scarce, martens ate more of them than expected (53% and 60% of the food eaten).
• In summer, they ate small mammals in line with their abundance (24%).
• In autumn, when small mammals were most plentiful, martens did not eat more of them (31%) and instead favoured rowanberries.
• The diet was most varied in spring and winter, and least varied in autumn.

3. What they concluded

• The European pine marten is not a strict opportunist. Its feeding strategy seems to lie between that of an opportunist and a specialist.
• Small mammals, especially Apodemus mice, are its main prey and are taken even when scarce, although squirrels, forest birds, insects, reptiles and carrion were also available.
• This may be partly because small mammals were weak during the especially cold, dry winter of 2004 to 2005, making them easier to catch.
• In autumn, rowanberries probably give a better energy return than hunting, because there is no chase.
• Because their main prey (wood mice) live mostly in forest, the pine martens appear to prefer forest habitat in this area.
• The authors suggest pine martens probably have a strong influence on small mammal populations in forests, especially Apodemus mice.

4. How tracking was or was not used

The study rests on scats found along road transects. Pine marten scats were identified in the field by size and shape, and the authors checked this with DNA from 90 fresh scats, all of which proved to be pine marten. Scats were aged by their signs: fresh scats (up to 4 days old) had a strong smell, a layer of mucus and no drying; medium-age scats (5 days to 1 month) had lost the smell and mucus but kept their shape; old scats (over 1 month) had no smell and had lost their shape. Prey were identified from hair, teeth and bones inside the scats, with hair compared against reference guides, hairs collected in the study area and a museum collection. No trailing was reported.

5. Tracker or observer certification

Not reported. The paper does not say who identified and aged the scats in the field. Gamekeepers Tomás Pérez and Ricardo Prieto are thanked for help with fieldwork. The DNA check on 90 fresh scats supports the field identifications in that sample.

6. Other technologies, skills or innovations

• DNA identification of scats (PCR-RFLP) to separate pine marten from stone marten.
• Camera traps to check for stone martens.
• GPS to record scat positions and avoid counting the same scat twice.
• Sherman live traps to measure small mammal abundance.
• Lockie’s correction factors to turn the dry weight of remains into estimated food eaten.