Diet is a fundamental aspect of an organism’s ecology and is one of the primary means through which animals engage with their environment. While an individual organism’s diet may seem trivial, the small-scale ecological behaviors associated with feeding can result in larger-scale consequences for nutrient cycling in the environment (Flecker 1996; Hall et al. 2003; Kielland et al. 2006; Butler and Kielland 2008; Strickland et al. 2013). This is especially true for invertebrate species since many exert considerable influence over their ecosystems (Prather et al. 2013). However, despite invertebrate animals constituting the most diverse clades, little is known about the specific foraging ecology of most species.
Terrestrial gastropods break down organic matter and contribute to soil nutrient cycling, which is arguably one of their most notable ecological functions (Mason 1970a; Oli and Gupta 2000; Meyer et al. 2013). These organisms decompose organic matter either directly via diet (Mason 1970a), or indirectly through synergistic relationships with other invertebrates and microbes (Theenhaus and Scheu 1996; de Oliveira et al. 2010). Terrestrial organic matter decomposition is critical because it facilitates the recycling of essential minerals like potassium and magnesium that would otherwise be removed from the ecosystem via burial (McTiernan et al. 1997; Krishna and Mohan 2017). Terrestrial gastropods also play a key role by mobilizing calcium found in soil and rock, making it readily bioavailable to other organisms of higher trophic levels (Graveland 1996; Hotopp 2002). Despite their ecological importance, the dietary preferences, and ecological interactions of most species of terrestrial gastropods are virtually unknown, with most species being considered herbivores, a few species are regarded as omnivores, and a minority are considered carnivores (Graham 1955; Chatfield 1976; Speiser 2001; Barker and Efford 2004).
The pressing challenge in investigating gastropods’ diet is that most species are relatively small in size (< 5 mm) and are nocturnal, which make direct observation in the wild difficult or inefficient. Other methods that are employed to study gastropod diets include fecal and stomach content analyses (e.g., Cook & Radford, 1988; Paustian & Barbosa, 2012), and laboratory feeding experiments (e.g. Baur, 1987; Agongnikpo, Karamoko & Otchoumou, 2010). However, inferences of gastropod feeding habits derived from these methods either depict dietary snapshots or diets under artificial conditions. An alternative approach to study animal diets is stable isotope ecology, a methodology that has been successfully applied to many other animals (e.g. Cerling et al. 2004, Galetti et al. 2015, Madigan et al. 2015) but is rarely used for terrestrial gastropods. The carbon (δ13C) and nitrogen (δ15N) stable isotope composition of an organism’s tissue reflect the isotope composition of the consumed and assimilated foods. However, animal tissues are often enriched in the heavier isotopes with respect to ingested food, resulting in an isotopic offset between a consumer’s tissue and its diet, which is known as “isotope fractionation” (DeNiro and Epstein 1978, 1981). Because isotope fractionation between consumer and diet is relatively constant and predictable, the isotope values of an organism’s diet can be inferred from the isotope composition of the animal’s body tissue (DeNiro & Epstein, 1978, 1981).
Published field observations and controlled feeding experiments examining the dietary habits of terrestrial gastropods suggest that the majority of species regularly consume vascular plants and contribute to leaf litter decomposition (Grime and Blythe 1969, Mason 1970a, 1970b, Jennings and Barkham 1979, Schmidt et al. 2004, Meyer et al. 2013), whereas a few species exhibit carnivorous or omnivorous feeding behavior, with the ability to consume other gastropod species and invertebrates (Davis and Butler 1964; Efford 2000; Stringer et al. 2003; Barker and Efford 2004; Dourson 2008; Meyer et al. 2008; Meyer and Cowie 2010; Boyer et al. 2011; Holland et al. 2012; Curry and Yeung 2013). However, the specific dietary habits, food preferences and resource partitioning tendencies of most terrestrial gastropod species in the wild are virtually unknown, especially for those living in temperate forests of the North American Midwest.
While most medium to large (> 5 mm) terrestrial gastropod species have been traditionally assumed to be generalized herbivores (Speiser 2001), recent investigations have observed that some taxa living in forested environments follow more complex diets than previously thought. For example, Dourson (2008) conducted direct field observations of the feeding habits of Triodopsis platysayoides in West Virginia and found that this species consumes a wide variety of plant and non-plant food resources, including vascular plants, fungi, lichens, mosses, fecal matter, and even body tissues of other invertebrate animals. Yanes et al.(2018) used carbon and nitrogen stable isotopes to investigate the diet of Neohelix major and documented that, in addition to vascular plant matter, approximately 48% of the species’ diet consisted of fungi, and also included significant proportions of lichen and mosses. Additionally, cannibalism, especially of eggs and young ontogenetic stages, has been noted among some species including Zonitoides nitidus, Arion cf subfuscus, Oxychilus draparnaudi, Arianta arbustorum, Sphincterochila boisseri, and Helix pomatia, among others (Yom-Tov 1971; Pollard 1975; Baur and Baur 1986; Baur 1990; Barker and Efford 2004). Interestingly, even apparently herbivorous species, like Achatina fulica, may prey upon other invertebrates under certain ecological conditions (Meyer et al. 2008).
With upwards of 30 sympatric species of terrestrial gastropods living closely together in a given microenvironment (Nekola 2005; Schamp et al. 2010), it remains unclear how such large numbers of species can coexist without extreme interspecific competition. In the wild, some animals are able to minimize interspecific competition by dividing resources, an ecological strategy known as “niche partitioning” (Schoener 1974; Hector and Hooper 2002). Previous research has suggested that sympatric gastropod species tend to develop similar morphological traits in response to environmental filtering (Schamp et al. 2010; Astor et al. 2014). However, with such morphological similarity among species, organisms must find ways to limit direct competition(Scriven et al. 2016). Evidence of the mechanisms behind interspecific competition and its influence in structuring gastropod communities remains elusive, particularly with respect to niche partitioning (see Watz and Nyqvist, 2022).
Sympatric species of terrestrial gastropods may attempt to reduce interspecific competition by partitioning the microhabitat (Paustian and Barbosa 2012; Astor et al. 2017), temporal portioning(Attia 2004) and food resources (Cook and Radford 1988; Paustian and Barbosa 2012). The few published studies that have quantified the diets of sympatric gastropod species have used different approaches and documented conflicting results. For example, Cook and Radford (1988) conducted fecal analyses and concluded that Limacus ecarinatus, Limacus flavus, Limax maximus, and Lehmannia marginata living in Northern Ireland appear to partition resources. Paustian and Barbosa (2012) found that Arion cf subfuscus, Philomycus carolinianus, and Megapallifera mutabilis in North America appear to partition resources as well. In sharp contrast, other gastropods from the Czech Republic did not exhibit significant niche partitioning tendencies based on stable isotope analyses (Němec et al. 2021). All in all, the dietary habits of most terrestrial gastropod species are uncertain. This study aims to address this knowledge gap by presenting the most extensive dietary analysis yet of a community of native and introduced terrestrial gastropods from a temperature deciduous forest in western Ohio, American Midwest. We test the hypothesis that different sympatric species of terrestrial gastropods with seemingly comparable ecology and behavior partition resources to avoid direct competition. To do this, we analyze the carbon and nitrogen isotopes of body tissues and potential food resources. Isotopic results are used here to assess probabilistic estimates of species’ diet, identify ecological niches, and are discussed in the context of gastropod communities from other regions in the world.