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Niche partitioning and indication of ontogenetic niche shifts in forest slugs according to stable isotopes

The turnover of slug assemblages suggests niche partitioning along gradients of microclimatic conditions and food availability (Kappes, 2006; Kappes and Schilthuizen, 2014). Within forests, most slugs such as those of the genus Arion feed on the leaf litter, but usually consume only a small portion of the annual input (e.g. Seifert and Shutov, 1981; Theenhaus and Schaefer, 1999). Some slug species can locally turn over up to 7.4% of the annual leaf litter input (Gupta and Oli, 1998). The attractiveness of microhabitats such as woody debris to slugs and snails is hypothesized to be partly caused by the presence of deadwood-associated fungi and microbial biofilms (Fog, 1979; Kappes, 2006). It appears that some slugs may forage on lichens and fungi (Frömming, 1954), and some generalists include varying portions of green plants, fruits, dead animals or faeces in their diets (Boback, 1952; Frömming, 1954; Richter, 1979). As a result, the trophic position of slugs is often difficult to identify. One method to visualize the breadth and overlap of nutritional niches makes use of the stable isotopic signature of the animals. For example, Schmidt et al. (2004) illustrated that stable isotopes can be used to characterize nutritional niches of field slugs and Meyer and Yeung (2011) analysed the isotopic composition of selected native and invasive gastropods on Hawaii. These authors included in their analyses samples of leaf litter and green plants as likely food items. Here, we provide stable isotopic data from different native forest slugs. As our study was performed on slugs from closed-canopy forests devoid of a herb layer, green plants were not included in our approach, but we sampled fungi as potential food sources. Environmental samples and slugs were collected in late autumn 2012 in beech-dominated forests northwest of Niedeggen, Eifel Hills, Germany. Environmental samples were leaf litter (n = 2), leaf litter and deadwood overgrown with visible fungal hyphae (each n = 1) and the fruiting body of a Basidiomycete (n = 1). The forests had a high canopy closure with no green plants on the forest floor. Slug taxa were Limax cinereoniger Wolf, 1803, Malacolimax tenellus (O. F. Müller, 1774) and Lehmannia marginata (O. F. Müller, 1774) (Limacidae) and Arion rufus (Linnaeus, 1758), A. intermedius Normand, 1852, A. silvaticus Lohmander, 1937 and A. fuscus agg. (Arionidae). All these taxa usually differ in size and colour: L. cinereoniger usually reaches more than 15 cm in length and is of black colour. The other large slug, A. rufus, is over 12 cm long and usually brilliant red when adult, whereas its juveniles are whitish and resemble adults of A. intermedius, which is usually about 3 cm long, more yellowish white and unbanded in the forests under study. Arion silvaticus is up to 4 cm extended and greyish with longitudinal bands, whereas A. fuscus agg. reaches up to 7 cm in length and is yellowish-brown with dark longitudinal bands. Malacolimax tenellus is up to 5 cm long and waxy-translucent yellow, and L. marginata is about 7 cm extended and grey-brown with obscure bands. We sampled three specimens from each taxon with the exception of A. rufus, of which we sampled three juveniles and three adults. Individuals were cooled and finally frozen. Slug tissue (foot muscle) and detritus were dried at 60 °C for more than 48 h before sample preparation and stable isotope analysis of δC and δN at the Stable Isotope Facility of the University of Davis, California. Isotopic signatures were visualized in a biplot. The calculation of isotopic niches is based on a Bayesian approach, especially suitable for small sample sizes. Briefly, a covariance matrix is calculated from the δC and δN values and its variation estimated by a Markov chain Monte Carlo (MCMC) sampling simulation. Ellipses around isotopic values of each species were calculated with SIBER (Jackson et al., 2011) in R v. 3.0 (R Development Core Team, 2007). Nonoverlapping ellipses indicate dietary differences (Jackson et al., 2011). As indicated by the ellipses, niche differentiation was larger between than within genera (Fig. 1). Lehmannia marginata showed the most extreme isotopic signatures; its highly negative δN values are typical of cyanobacteria. Cyanobacteria are symbionts of some lichens, but may also grow on the bark of trees. This result is in line with the well known feeding preferences of L. marginata (e.g. Frömming, 1954). However, L. marginata is not the only slug that is known to consume cyanobacteria. Selective grazing on lichen cephalodia with Nostoc has for example been described for boreal A. fuscus (Asplund and Gauslaa, 2010). The isotopic signatures of the other slug species suggest that they at least partially consume fungal hyphae. The isotopic values support observations by Frömming (1954) that M. tenellus, and to a lesser extent Limax cinereoniger, predominantly consume fungi. The three above-mentioned limacids are typical inhabitants of oldgrowth forests (Kappes, 2006). In general, isotopic composition of the species of Arion in the studied forests tended to be clustered and to overlap (Fig. 1). Interestingly, ontogenetic niche shifts are indicated in A. rufus. Juvenile A. rufus were somewhat similar to A. intermedius in terms of colour and size, but clearly differed in isotopic signature. It can be speculated that these differences in isotopic composition, and their changes during growth, reflect their occupied niches, i.e. avoidance of competition between parents and offspring on the one