Feeding responses of the red fox (Vulpes vulpes) to different wild rabbit (Oryctolagus cuniculus) densities: a regional approach

1. What they did

The European wild rabbit is a keystone species in Iberian Mediterranean ecosystems and the most important prey for many predators there. Rabbit numbers fell sharply on the Iberian Peninsula over the past 50 years, mainly because of two viral diseases, myxomatosis in the 1950s and rabbit haemorrhagic disease (RHD) at the end of the 1980s. The red fox is the most widespread generalist predator on the peninsula. The authors asked how the share of rabbit in fox diets changes across a whole region with very different rabbit densities.

• Measuring rabbit numbers. In summer 2002, at 86 localities in central–southern Spain, two observers walked a 4 km transect and counted rabbit pellets in 40 small circular plots (0.47 m²) set 100 m apart, avoiding latrines. This gave a pellet abundance index for each locality.
• Collecting scats. All carnivore scats found along each transect were collected, dried and weighed. Of 150 carnivore scats, 114 from 47 localities were identified as red fox, by scent, shape, size, thickness, fresh and dry weight, and fox hairs swallowed while grooming.
• Analysing the diet. Scats were soaked, washed through a fine sieve and sorted. Mammal remains were identified from bones, teeth and hair using reference keys. Each food class was expressed as percent dry weight and percent occurrence.
• Testing the functional response. Using only the 18 localities with at least 3 fox scats (70 scats), they fitted the percentage of rabbit in the diet against rabbit abundance to Holling’s three types of functional response. Type I rises in a straight line, type II rises steeply then levels off, and type III is S-shaped: few rabbits eaten at low density, a rapid rise at intermediate density, then a plateau.
• Measuring diet diversity. They grouped scats into four rabbit-density classes (virtually absent, low, intermediate, high) and compared diet diversity with the Shannon diversity index.

2. What they found

Rabbit numbers and scats

• On average 1.32 fox scats were found per transect (range 0 to 8).
• The average rabbit index was 25.32 pellets/m², ranging from 0 to 259.41.

Functional response

• The S-shaped type III curve fitted best (R² = 0.79), ahead of type II (R² = 0.71) and type I (R² = 0.38).
• Foxes ate very few rabbits where rabbits were scarce, ate more as rabbit density rose to moderate levels, and the share of rabbit levelled off at high densities.

What foxes ate (percent dry weight)

• Rabbit made up 6.1% of the diet where rabbits were virtually absent, 29% at low density, 52.7% at intermediate density and 65.1% at high density (39.1% overall).
• Where rabbits were virtually absent, foxes ate more small mammals (26.2%), arthropods (20.7%), ungulates (12.4%) and birds (10.1%).
• Arthropods were found in about 9 out of 10 scats at every rabbit density, and birds in about half of all scats.

Diet diversity

• Fox diets became less diverse as rabbits became more abundant (Spearman’s R = −1). Diversity was lowest at high rabbit density and highest where rabbits were scarce.

3. What they concluded

• Red foxes behaved as opportunistic predators, eating rabbits according to their availability, with a strong tendency to eat rabbits where they were abundant.
• Foxes are best described as facultative predators: they specialise on rabbits when rabbits are plentiful and switch to other prey, and so a more varied diet, when rabbits are scarce. This switching does not support the idea that foxes are rabbit specialists.
• Because fox predation was density-dependent at low rabbit densities, it could hold rabbit populations around a lower equilibrium at moderate densities.
• Limitations: they did not know the number of prey eaten, so could not fully evaluate the functional response; some localities yielded few scats, which could underestimate rabbit in the diet where rabbits were scarce; and known biases of scat analysis were not specifically addressed. On the other hand, they sampled widely and at the same time across the full range of rabbit densities, which is uncommon in such studies.
• This is a first step towards understanding whether red foxes regulate rabbit populations in central–southern Spain. The fox’s numerical response, and experiments, are still needed.

4. How tracking was or was not used

No trailing was reported. Two kinds of sign were used:

• Rabbit pellets: counted in plots along transects as an index of rabbit abundance.
• Scats: carnivore scats found on the transects were identified to species by scent, shape, size, thickness and weight, and by the predator’s own hairs found inside. Only scats judged to be red fox were analysed. Prey were then identified in the laboratory from bones, teeth and hair in the scats.

5. Tracker or observer certification

Not reported. The paper does not say who identified the scats in the field or how accurate the identification was, although every diet result depends on the 114 scats being correctly assigned to red fox. Field assistants are thanked by name for help with the rabbit surveys.

6. Other technologies, skills or innovations

• Hair identification keys (Teerink 1991 and others) to confirm the predator and identify mammal prey.
• Curve-fitting software (TableCurve 2D v5.01) to compare the three types of functional response.
• Resampling (1,000 random sub-samples of 30 scats) to compare diet diversity across groups of unequal size.

7. Implications for conservation (from the paper)

• Rabbit specialists such as the Iberian lynx and Spanish imperial eagle have become seriously endangered, mainly through habitat fragmentation and the decline in rabbits, while generalist predators have increased.
• After RHD, some rabbit populations crashed or went locally extinct. Below certain rabbit numbers, predator pressure could be preventing recovery (the “predator pit” hypothesis).
• To know how far foxes regulate rabbits, both the functional and the numerical response must be studied, and experimental studies are needed on whether foxes hold back rabbit recovery after RHD.