Expansion of great cormorant colony immediately increased isotopic enrichment in small mammals

Abstract

Abstract. Colonies of great cormorants (Phalacrocorax carbo) impact terrestrial ecosystems transporting nutrients from aquatic to terrestrial ecosystems. Deposited guano overload ecosystem with N and P, change soil pH, damage vegetation.However, ways how small mammals are impacted, are not known in details. We aimed to evaluate the effects of an expanding great cormorant colony, testing if the expansion immediately increases input of biogens to the forest ecosystem, and if influence of the colony is reflected in the basal resources (plants and invertebrates) and the hair of small mammals. Stable carbon and 5 nitrogen isotope signatures were analysed in granivorous yellow-necked mice (Apodemus flavicollis), omnivorous bank voles (Myodes glareolus) and basal resources of animal and plant origin from the territory of a colony of great cormorants, situated near Baltic Sea, in West Lithuania. We found that biogens transferred by great cormorants to terrestrial ecosystems affected the potential foods of the small mammals and led to highly elevated and variable δ 15 N values. Increase of the colony in 2015 caused isotopic enrichment of the small mammals in the zone of expansion compared to 2014 levels. For most of the resources 10 tested, the isotopic signatures in the established colony area were significantly higher, than in the ecotone (between colony and surrounding forest) and expansion zone with the first year on cormorant nests present. Surprisingly, in the colony δ