The impact of Great Cormorants on biogenic pollution of land ecosystems: Stable isotope signatures in small mammals

Studying the isotopic composition of the hair of two rodent species trapped in the territories of Great Cormorant colonies, we aimed to show that Great Cormorants transfer biogens from aquatic ecosystems to terrestrial ecosystems, and that these substances reach small mammals through the trophic cascade, thus influencing the nutrient balance in the terrestrial ecosystem. Analysis of δ¹³C and δ¹⁵N was performed on two dominant species of small mammals, Apodemus flavicollis and Myodes glareolus, inhabiting the territories of the colonies. For both species, the values of δ¹³C and δ¹⁵N were higher in the animals trapped in the territories of the colonies than those in control territories. In the hair of A. flavicollis and M. glareolus, the highest values of δ¹⁵N (16.31 ± 3.01‰ and 17.86 ± 2.76‰, respectively) were determined in those animals trapped in the biggest Great Cormorant colony. δ¹⁵N values were age dependent, highest in adult A. flavicollis and M. glareolus and lowest in juvenile animals. For δ¹³C values, age-dependent differences were not registered. δ¹⁵N values in both small mammal species from the biggest Great Cormorant colony show direct dependence on the intensity of influence. Biogenic pollution is at its strongest in the territories of the colonies with nests, significantly diminishing in the ecotones of the colonies and further in the control zones, where the influence of birds is negligible. Thus, Great Cormorant colonies alter ecosystem functioning by enrichment with biogens, with stable isotope values in small mammals significantly higher in the affected territories. - Highlights: • Cormorants transport nutrients from water to land ecosystems and pollute biogenically. • We studied stable isotope composition of small mammal hair in 3 cormorant colonies. • δ¹³C and δ¹⁵N were measured using elemental analyzer–isotope ratio mass spectrometer. • δ¹³C and δ¹⁵N values were higher in rodents inhabiting cormorant colonies. • Disruption of the ecosystem caused by Great Cormorant colonies affects small mammals.