Applying abundance/biomass comparisons on a small mammal assemblage from Barn owl (Tyto alba) pellets (Mount Soratte, central Italy): a cautionary note

Abstract

Abundance/biomass comparisons (or ABC curves) allow to obtain information on the level of (anthropogenic or natural) disturbance that may sensu lato affect specific communities. In order to verify the general validity of the ABC curves, we applied these comparisons to a small mammal dataset obtained by a sample of Barn owl (Tyto alba) pellets collected in a nature reserve of central Italy (Monte Soratte). From 43 pellets, we obtained 111 individuals belonging to 11 shrews and rodents spe- cies. Microtus savii, Apodemus sylvaticus, and Apodemus flavicollis were the species most frequently preyed upon by owls. Indices such as absolute (and normalized) species richness and Shannon-Wiener diversity consistently showed higher values than those obtained in similar studies in central Italy, highlighting the complex structure of this assemblage occurring in a landscape with low anthropiza- tion. From ABC curves, abundance was slightly lower than biomass, even though differences between paired cumula- tive frequencies were not significant. This pattern suggests that the small mammal community in our study area features a low level of disturbance. Nevertheless, whereas in undisturbed assemblages a higher biomass in ABC curves indicate a higher frequency of larger species at higher trophic level, in small mammal assemblages such as those revealed by owl pellets, large-sized species are her- bivorous, synanthropic and typical of disturbed habitats. Accordingly, we caution that in disturbed habitats an increase of small mammals with high body mass may results in ABC curves with biomass higher than abundance. Our results therefore suggest that there might be relevant exceptions to the general patterns so far developed for ABC curves.