The complexity of ecological impacts induced by great cormorants

Following decades of global extermination, the general population of the great cormorant (Phalacrocorax carbo L.) is on the rise. The lack of regular predators, highly skilled fish rapacity, rapid metabolism, significant rate of excretion and ability to form large nesting colonies on relatively small areas lead to numerous environmental consequences of cormorant presence. Here we comprehensively review the occurrence and distribution of this species and, in particular, its multi-faceted impact on terrestrial and aquatic ecosystems and the main routes through which these impacts are being manifested. The bird-induced chemical loading and its biological and ecological consequences, and the effects on microbial pollution and pathogen dispersion are discussed in particular. The need for further investigation to fully elucidate particular effects is stressed throughout the paper. It is concluded that the environmental effects of great cormorants are rather complex, can lead to serious ecosystem modifications and that the presence of these birds should be taken into consideration in ecological assessment and monitoring.