Size Matters: Diversity and Abundance of Small Mammal Community Varies with the Size of Great Cormorant Colony

Abstract

Cormorant colonies are often viewed negatively by fishermen and foresters due to their extremely high impact on aquatic and terrestrial ecosystems. In forests, the habitats of nesting territories are destroyed, with concomitant impacts on the animal communities. In 2011–2022, investigating three colonies of Great Cormorants (Phalacrocorax carbo), we aimed to test whether their effect on small mammals depends on colony size. In the largest colony in Lithuania, a low species richness, lower diversity and relative abundance, as well as poorer body conditions of the most abundant species was found in the nesting zone. However, once the cormorants left the nesting site, all the parameters recovered. Two small colonies had a positive impact, with higher species richness in the territory of the colony (seven and ten species), diversity (H = 1.56 and 1.49), and relative abundance (27.00 ± 2.32 and 25.29 ± 2.91 ind. per 100 trap days) compared with the control habitat (three and eight species; H = 1.65 and 0.99; 12.58 ± 1.54 and 8.29 ± 1.05 ind./100 trap-days). We conclude that up to a certain colony size, cormorant pressure is a driver of habitat succession and has similar effects on the small mammal community as other successions in disturbed habitats.