Badger Meles meles spatial structure and diet in an area of low earthworm biomass and high predation risk

1. What they did

In Belarus, the Eurasian badger has been on the list of endangered species for more than 20 years, and protection has not worked well. Most of what is known about badgers comes from western Europe, where earthworms are plentiful and large predators are rare or gone. North-eastern Belarus is different: earthworm biomass is low, and wolves, lynx and brown bears are fairly common (about 8.5 large carnivores per 100 km²). The authors asked whether badgers here are limited mainly by food (a “bottom-up” model) or by predators (a “top-down” model), and how these two pressures shape where badgers live, how they move and what they eat.

The study took place in a 330 km² semi-natural forest area in north-eastern Belarus:

• Finding the setts. In 1997 they searched the whole area for badger setts, by direct inspection and by snow-tracking badgers in early spring, and rechecked all setts every year.
• Counting badgers (1998 and 2009). Setts were sorted into main and outlier setts. Group sizes came from live-trapping, watching from a hide, camera traps and morning checks for tracks at sett entrances, with the ground smoothed afterwards.
• Radio-tracking. Four breeding females (one per social group) were radio-tracked in 1998 and three lone adults in 2009, with a fix about every 15 minutes where possible. The team paid special attention to how badgers moved: did they stay near setts or travel straight between them?
• Diet. They analysed 1188 badger scats, collected mostly from latrines at setts between 1996 and 2007.
• Earthworm supply. They dug and weighed earthworms from 1 m² plots, 30 cm deep, in five main habitats every month from 1997 to 1999, and each mid-July from 1996 to 2002 in two habitats.

2. What they found

Population and home ranges

• In 1998 there were 30 badgers in 330 km² (9.1 per 100 km²): four social groups of 2 to 12 (average 7) plus two lone badgers. Group territories covered 16 to 34 km² (average 23.5 km²).
• This stayed fairly stable until 2008, when numbers dropped. In 2009 only nine badgers remained (2.3 per 100 km²): one group of six and three lone badgers.
• Lone badgers had small home ranges, about 4 km² for two females; the lone male’s range of 4.2 km² grew to 13.3 km² in June to August, when he walked directly to outlier setts.
• Neighbouring groups and lone badgers did not overlap.

How badgers moved

• Each badger used two to six permanent routes, forming a network within its home range. Routes were 0.8 to 7.5 km long (mean 3.7 km), and badgers travelled 1.5 to 9.1 km a day (mean 4.1 km).
• Badgers in groups mostly walked slowly and directly to outlier setts or to sites with setts nearby. Lone badgers more often stayed around their main sett.
• On average 96.6% of routes kept badgers close to a sett.

Diet through the seasons

The diet fell into five seasonal patterns, each significantly different from the next:

• Early spring, with snow (mid-March to mid-April): mostly carrion of wild ungulates (58.4% of biomass consumed). Snow-tracking showed badgers mainly searched for wild boar carcasses. Cranberries made up 16.1%.
• April, after snowmelt: mostly amphibians (44.0%), mainly common frogs at spawning sites, and earthworms (20.2%).
• May: earthworms (40.0%) and adult beetles (24.2%).
• June to mid-July: a broad diet, with mammals and birds together at 50.7% (birds 29.5%), probably mainly young animals, plus insects (24.1%).
• Mid-July to November: more plant food (fruits 16.7%, cereal seeds 10.2%), with insects (31.3%) and earthworms (19.0%).

Earthworms

• Earthworm biomass was highest in black alder swamps (mean 38.1 kg per hectare) and lowest in pine forest (2.5 kg per hectare), with a weighted mean of 17.5 kg per hectare for the study area. This is low compared with most western European badger sites (50 to 556 kg per hectare).
• Earthworms were patchy: about 10% of black alder swamp plots had over 300 kg per hectare, so roughly 1% of badger habitat was rich in earthworms.
• The share of earthworms in the diet rose and fell with earthworm biomass through the season (Spearman correlation 0.72).

Predation

From 2006 to 2009, badger remains were found in 3 of 293 wolf scats and 1 of 67 lynx scats. Badgers and wolves were often seen feeding on the same carcass on the same night.

3. What they concluded

• Badgers here act as generalist, opportunistic feeders: in each season they concentrate on a few foods that are most readily available. The authors found no evidence that they prefer any food, including earthworms.
• The authors argue that low food alone does not explain the low badger density: the area has sometimes supported larger groups, and badgers are usually in good condition (thick fat in autumn, emaciated badgers rare in spring).
• They propose that high predation risk, especially from wolves and lynx, is the more likely limit. To stay safe, badgers forage along straight routes between setts and other shelters (beaver burrows, fox earths). This keeps them away from many food-rich places, such as the scattered earthworm hotspots, and so helps keep density low.
• A simple extrapolation suggests the local wolves and lynx could kill about half of the badger population.
• Badger density in north-eastern Belarus is about 100 to 1500 times lower than in Great Britain. The authors suggest the very high British densities reflect man-made pasture landscapes with artificially high earthworm biomass, while the Belarus pattern (large territories, few badgers, many setts) may be the more natural one for the European forest zone.

4. How tracking was or was not used

Tracks and signs were part of the field methods, though the paper gives few details:

• Snow-tracking of badgers in early spring helped find all setts in the study area, and showed that badgers emerging in early spring mainly searched for wild boar carcasses.
• Sett inspection: setts were sorted into main and outlier setts by signs of denning or use for hibernation, and checked each year.
• Tracks at sett entrances: the ground was smoothed and checked each morning for fresh tracks, as one of the ways to count badgers.
• Scats were collected from latrines for diet analysis. Food remains were identified from earthworm bristles (chaetae), teeth and jaws, hair under the microscope, insect exoskeletons, feathers, bones and scales.
• Remains of badgers were identified in wolf and lynx scats.

5. Tracker or observer certification

Not reported. The paper does not say who did the snow-tracking or sett inspections. Radio fixes were taken by one or two researchers who were familiar with the area.

6. Other technologies, skills or innovations

• Radio-tracking with Telonics receivers and ATS radio collars, with telemetry error tested beforehand (transmitters on the ground, in a sett entrance or carried by a team member).
• Positions plotted on a 1:25,000 map with a Global Positioning System (GPS) unit.
• TrailMaster passive infrared camera traps at setts, live-trapping in cage box-traps and watching from a hide.
• Earthworm sampling by digging 1 m² plots to 30 cm, as a way to measure food supply.

7. Implications for conservation (from the paper)

• Badgers in Belarus have been listed as endangered for more than 20 years, and protection seems not to have worked; the authors link this to poor understanding of badger ecology in the area.
• Studies from western Europe, where earthworms are abundant and large predators are scarce, cannot simply be applied to badgers in Belarus.
• Predation, especially by wolves and lynx, may be an important limit on badger numbers where large carnivores are common.