Overlap of food and microhabitat preferences among some native and nonnative slugs in mid-Atlantic forests of eastern North America

Abstract

Introduced competitors do not share an evolutionary history that would promote coexistence mechanisms, i.e. niche partitioning. Thus, nonnative species can harm a trophically similar native species by competing with them more intensely than other native species. However, nonnative species may only be able initially to invade habitats in which resource overlap with native species is small. The nonnative slug Arion subfuscus exists in close sympatry with the native philomycid slugs Philomycus carolinianus and Megapallifera mutabilis in central Maryland forests. Resource use by most terrestrial gastropods is poorly known, but seems to suggest high dietary and macrohabitat overlap, potentially placing native gastropod species at high risk of competitive pressure from invading species. However, A. subfuscus was introduced to North America 150 years ago, supporting the possibility that A. subfuscus initially entered an empty niche. We tested the hypothesis that P. carolinianus and M. mutabilis would exhibit greater overlap in food and microhabitat use with A. subfuscus than they would with each other. We established food preferences by examining the faecal material of wild-caught slugs, distinguishing food types and quantifying them by volume on a microgrid. We determined microhabitat preferences by surveying the substrates of slugs in the field. The overlap in substrate and food resources was greater between A. subfuscus and P. carolinianus than between the two native species. However, substrate choice was correlated with local substrate availability for P. carolinianus, suggesting flexibility in habitat use, and the slight overlap in food use between A. subfuscus and P. carolinianus may be low enough to minimize competition.