Diet of the hedgehog population
In this study, beetles were found in 94% of samples. This is a common finding in both European (Obrtel and Holišová 1981; Grosshans 1983; Wroot 1984) and some New Zealand studies (Berry 1999; Jones et al. 2005). Hedgehogs’ heavy consumption of beetles may be of concern for the conservation values at our study site as two at-risk species were eaten. Both the large sand scarab beetle Pericoptus frontalis and the Alexandra chafer Prodontia modesta are considered to be vulnerable due to restricted range under the New Zealand Department of Conservation threat classification criteria (Molloy et al.
2002). Although both species occurred in relatively low frequencies in droppings, their restricted range means that sustained predation may have a significant impact on their viability.
Predation by hedgehogs on native lizards may also be of concern to conservation managers. Lizard remains were found in one in seven droppings in this study. The presence of 10 McCann’s skink feet in one sample indicates that at least three skinks were eaten in a single feeding bout. Jones et al. (2005) noted a lower frequency of occurrence of skink remains in hedgehog guts than found here, but found a marked intersexual difference, with female hedgehogs three times more likely to prey on lizards than males.
Hedgehogs’ ability to take full advantage of temporarily abundant foods is exemplified by the occurrence of large numbers of molluscan eggs in faecal samples obtained over a relatively short and well-defined period. Similar behaviour has been described in both New Zealand and Europe where hedgehogs took advantage of temporarily abundant grass grubs, cockchafer beetles (Amphimallon solstitialis Linnaeus, 1758) and aphid eggs (Campbell 1973; Obrtel and Holišová 1981; Wroot 1984).
Some studies have noted the importance of earthworms in hedgehog diets (Brockie 1959; Campbell 1973; Yalden 1976; Wroot 1984), but we found no evidence of earthworm chaetae to indicate consumption, nor were any earthworms seen when digging-in the 120 pitfall traps. The absence of earthworms was probably due to the very dry soil conditions at the time of the study. Earthworms require sufficient soil moisture to remain active near the soil surface where they would be accessible to hedgehogs.
Earwigs, the most preferred food in this study, were also considered a highly preferred food in Wroot (1984). The reason for this preference is unclear as earwigs have a relatively low energy content compared with other available foods (Wroot 1984; Reeve 1994). Possible explanations are that earwigs have nutritional value beyond their calorific content, that their smell or taste is particularly attractive to hedgehogs or that earwigs may represent a good “fallback” food source when drier conditions mean that soft-bodied prey are less abundant. Wroot (1984) noted a strong trend between earwig consumption and low rainfall and/or relative humidity, conditions which were manifest during our study.
Tenebrionid beetles were also a preferred prey, while carabid beetles were less so in spite of being a commonly eaten food in this and other studies. Carabids commonly employ chemical defences and are relatively fast moving, which may contibute to their being less preferred (Reeve 1994; Berry 1999). Although tenebrionids may also use chemical deterrents, they have a higher energy content per unit mass (Wroot 1984; Bell 1990), which may make them a more profitable prey.
Woodlice (Isopoda), Hymenoptera and cylindrical bark beetles were the least preferred foods. Woodlice have odour-producing lateral plate glands and contain relatively low levels of energy per unit mass, which may contribute to their low preference status in this and other studies of hedgehog diet (Dimelow 1963; Wroot 1984).
While ranking of foods, rather than absolute index values, is probably the most realistic indicator of preference (Lechowicz 1982; Norbury and Sanson 1992), in our study, there was an order of magnitude difference in E* values between earwigs and tenebrionid beetles and between the latter and all other food types. Most other foods had E* values of very close to 0. This would suggest that selection or avoidance of these foods is very weak, i.e. hedgehogs in this environment consume foods in relation to their abundances. This may be the result of limited opportunities to diversify in a habitat that is very moisture-limited and where most of the ground cover is dominated by a single species (T. vulgaris), thus limiting the complexity and diversity of the local terrestrial invertebrate community.
Both availability and use measures are subject to biasses that must be acknowledged. Pitfall trap results are biassed because actively moving surface fauna have a much higher probability of capture than sedentary forms, such as larvae (Cooper and Whitmore 1990). Numbers of individuals trapped allow inferences based on “activity-density” (Thiele 1977) which was considered a reasonable surrogate for availability in this study. Our pitfall traps were open continuously for 6 days so diurnally active species to which hedgehogs may not have had access could have been trapped. The potential for this was reduced by our use of dark wooden trap covers. Baars (1979) found that diurnally active species were caught less frequently in traps with dark covers than in those with light or transparent covers.
We also assumed that all local hedgehogs had equal access to all potential prey. Hedgehogs are non-territorial and show extensive intersexual and intrasexual home range overlap (Parkes 1975; Reeve 1982; Boitani and Reggiani 1984). Individuals in this study nested and were trapped throughout the study site.
In insectivore diet investigations using faecal analysis, variation in digestibility between hard- and soft-bodied prey types means that the latter are likely to be underrepresented, as are smaller prey items (Putman 1984; Dickman and Huang 1988). Prey remains are more broken down than in gut samples, and a large number of small fragments may be unidentifiable. In spite of these caveats, Dickman and Huang (1988) found good general agreement between ranked frequencies of taxa consumed and those recovered in droppings.
There was little individual variation in diet composition based on our analysis of faecal bulk contents. Only two animals’ diets overlapped by a Horn’s index value of less than 0.70: one male ate the smallest volume of earwigs, yet consumed the largest volume of bird and mammalian tissue, and another male was the only hedgehog to eat fruit. Some authors have dismissed the presence of fruit and other plant remains in hedgehog gut/faecal contents as resulting from accidental ingestion (Yalden 1976; Berry 1999), but this was clearly not the case here, with repeated, but temporally separate, samples containing large numbers of fruit. Other hedgehog species have been recorded deliberately eating fruit including Erinaceus amurensis (Schrenk, 1859) and Atelerix algirus, Lereboullet, 1842 (Liu 1937; Barquin et al. 1986 cited in Reeve 1994). We believe that the lack of individual variation in our studied animals’ diets reflects the relatively harsh and resource-poor habitat and that incongruities such as heavy fruit consumption by one individual represent hedgehogs’ ability to target energy-rich items that may become temporarily available. The non-selective feeding habit of hedgehogs means that they represent a significant threat to small isolated populations of prey, particularly where this species’ own abundance is maintained by other food types.