An Adapted Method for Sampling Earthworms at Night in Wildlife Studies

Abstract

Earthworms are a major food item of many vertebrates, and their high biomass in temperate ecosystems provides an abundant and high-energy food resource. Earthworm tissue has a protein content of 60–70% (dry weight) with a high amino-acid content that is well matched to the requirements of vertebrates (Lee 1985). In the Western Palearctic, earthworms are a regular part of the diet of 186 mammal-, bird-, reptile-, and amphibian-predator species (Granval and Aliaga 1988). Specifically, earthworms of the family Lumbricidae are the main prey of many nocturnal animals, including 8 species of bird (belonging to families Rallidae, Charadriidae, Scolopacidae, and Strigidae), 5 species of mammal: badgers (Meles meles), hedgehogs (Erinaceus europaeus), moles (Talpa europaea), and 2 shrews (Sorex araneus and Neomys fodiens), and 19 species of amphibian (Granval and Aliaga 1988). The predominance of vertebrate earthworm predators foraging at night is likely to be the result of a higher overall abundance of earthworms near the soil surface at night than during the day. Certain earthworms (especially anecic species, known as night crawlers) perform nocturnal vertical migrations, where they emerge from the soil and move on the surface during the night (Lee 1985). Earthworms are very sensitive to bright light and ultraviolet radiation (Edwards and Lofty 1972). Thus, earthworms come to the soil surface at night or during periods of very low light intensity during the day (late in the evening or early in the morning), to mate or in search of organic food litter (Lee 1985). Many ecological and behavioral studies (time–budget studies, energetic studies, optimal foraging theory) require data on the abundance and availability of prey species. However, it is time consuming and logistically difficult to collect such data on subterranean species such as earthworms. Earthworms are usually sampled according to the standardized method described by Bouche and Gardner (1984) and Bouche and Aliaga (1986), which gives an accurate estimate of earthworm biomass. This method combines 2 complementary extraction techniques: 1) a chemical extraction using diluted formalin, which makes earthworms surface, is sprayed either on bare soil or on short-cut grass and 2) a physical extraction of worms that did not respond to the formalin and/or died near the surface from soil cores. However, such a method can only be correctly used in the daylight because worms forced from the soil by the formalin are very difficult to find and collect at night with portable headlamps. Moreover, earthworm populations are known to be highly aggregated in patches (Poier and Richter 1992, Rossi et al. 1997) and samples spaced only a few meters apart can give large differences in abundance of earthworms. Hence, to account for the spatial heterogeneity of worms, it is generally advised to sample several plots (usually 3–6) at the same site (Lee 1985). Because this technique requires 45 min of formalin spraying per m plus 15 min of preparation time (grass cutting), it is extremely time consuming. Thus, sampling can take many hours depending upon the number of plots sampled. Such a long, involved sampling time is generally not compatible with the work of individual wildlife researchers, who need easy and fast methods to estimate the availability of prey to predators, and not necessarily the exact biomass of prey in the habitat. A habitat containing a high biomass of earthworms (measured by standard procedure) may be of little interest to the predator if prey are too deep to be captured. For example, shorebirds, like Eurasian oystercatchers (Haematopus ostralegus), are limited by the size of their bill and choose prey (bivalves) at a well-defined depth in the mud; therefore, knowing the total biomass of bivalves is useless if only the most superficial ones are potential prey (Zwarts et al. 1996). To our knowledge, the availability of earthworms at night has not yet been quantitatively assessed, and no study has focused on the reliability of the diurnal standard method for estimating nocturnal earthworm availability. We describe an original yet simple method to sample earthworms, designed for wildlife biologists studying nocturnal vertebrate predators. We tested this new sampling procedure as part of an ecological study on the Eurasian woodcock (Scolopax rusticola), a bird that mainly forages on earthworms. The few studies on habitat selection in woodcocks (including the closely related American woodcock, Scolopax minor) found a correlation between habitat selection and earthworm availability, in summer and winter (Hudgins et al. 1985, Hirons and Johnson 1987, Granval and Bouche 1993, Duriez et al. 2005c). On winter nights woodcocks use fields extensively, especially meadows (Cramp and Simmons 1983), which are among the richest habitats for earthworms in Europe (Edwards 1983, Binet and Trehen 1990, Binet 1993, Fraser 1994). Because woodcocks have a maximum 1 E-mail: o.duriez@wanadoo.fr