Immediate increase in isotopic enrichment in small mammals following the expansion of a great cormorant colony

Abstract

Colonies of great cormorants (Phalacrocorax carbo) impact terrestrial ecosystems through the transport of nutrients from aquatic to terrestrial ecosystems. Deposited guano overload the ecosystem with N and P, change soil pH and damage vegetation. The ways in which small mammals are impacted, however, are little known. We aimed to evaluate the effects of an expanding great cormorant colony, testing if the expansion immediately increased the input of biogens into the forest ecosystem and, further, if the growing influence of the colony was reflected in basal resources (plants and invertebrates) and the hair of small mammals. δ15N and δ13C signatures were analyzed 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 the Baltic Sea in west Lithuania. We found that biogens transferred by great cormorants to the terrestrial ecosystem affected the potential foods of the small mammals and led to highly elevated and variable δ15N values. An increase of the size of the colony in 2015 resulted in isotopic enrichment of the small mammals in the zone of expansion in comparison to levels in 2014. The increase of δ15N in A. flavicollis was 7.5 % (p < 0.05) in the ecotone and 5.7 % in the expansion zone. The decrease in δ13C signatures in A. flavicollis was 4.5 % (p < 0.1) in the expansion zone and 3.1 % (p < 0.001) in the colony. In M. glareolus, the decrease in δ13C signatures was 8.5 % in the expansion zone, 3.3 % (p < 0.1) in the control zone and 2.6 % in the ecotone. Isotopic niches (central ellipses) of A. flavicollis in the colony and between the control and expansion zones were separated in 2014 and 2015, while they partially overlapped in the ecotone. The isotopic niches of M. glareolus in 2014 and 2015 were separated in the ecotone and had a small overlap in the colony. For most of the resources tested, the isotopic signatures in the established colony area were significantly higher than in the rest of cormorant-inhabited area. In the colony, the δ15N values in plants (16.9 ± 1.1 ‰) were higher than in invertebrates (13.6 ± 0.4 ‰). In the ecotone, the δ15N values were 12.0 ± 1.4 in plants and 14.7 ± 0.04 ‰ in invertebrates, while in the expansion zone they were 7.2 ± 3.0 and 9.9 ± 3.8 ‰, respectively. δ15N-rich resources led to increased δ15N values in the hair of A. flavicollis and M. glareolus. Thus, biogens from the great cormorant colony immediately affected small mammals through their food sources.