Applying abundance/biomass comparison curves to small mammals: a weak tool for detect urbanization-related stress in the assemblages?

Urbanization is a form of pervasive human-induced disturbance. We tested the effectiveness of Abundance/Biomass Comparisons (ABC) as an approach in detecting stress due to landscape urbanization in large small mammal assemblages obtained from pellets of Barn Owl (Tyto alba; Strigiformes). We compared three assemblages sampled in not urbanized contexts (agro-mosaic landscapes) with three assemblages preyed in highly urbanized contexts. In all assemblages, the role of strictly synanthropic species (in our case: rodents) emerged since almost all of total biomass was assigned to these species: indeed, everywhere (both in agro-mosaic and urbanized sites) species of low trophic level (i.e. omnivorous/herbivorous rodents) significantly prevail in biomass when compared to insectivorous species (i.e. shrews, Soricomorpha) linked to less anthropized habitats. This biomass dominance in rodent species is highlighted by the data on evenness, showing lower values in biomass when compared to abundance. This pattern did not match with the classic assumption expressed by the ABC model (i.e., species with higher biomass are typical of undisturbed assemblage) and could be wrongly interpreted. Our study evidenced as ABC approach is a not reliable tool to detect the effect of urbanization as landscape disturbance acting on small mammal assemblages. Therefore we suggest that the ABC assumptions are not universal but limited only to assemblages where high body mass species coincide to species of a higher trophic level.