Alien species of Mediterranean origin in the Baltic Sea Region: current state and risk assessment

Abstract

Due to globalization and climate warming, the introduction and establishment of alien species has increased in recent years. The Mediterranean Sea Region (MSR) has not been explored as a common donor of alien species to the Baltic Sea Region (BSR); however, in the context of global warming, the BSR could be more suitable for alien species from the MSR. We evaluated the alien species of Mediterranean origin present in the BSR, with emphasis on aggressive, aquatic and terrestrial species spread in at least two countries of the BSR. Introduction pathways and the year of first record in the BSR were assessed for Mediterranean species. Using an analytical hierarchy process, we also performed a risk assessment for aggressive Mediterranean species in the BSR. In total, 6145 alien species were recorded in the BSR, but only 3033 species were verified. For 292 of these species, there is evidence of impact in the BSR, 10 of these (3.4%) are native to the MSR (six animals (Alphitophagus bifasciatus, Chrysolina americana, Tenebrio molitor, Limax maximus, Oxychilus draparnaudi, and Limacus flavus) and four plants (Linaria repens, Veronica filiformis, Prunus cerasifera, and Viola odorata)). Based on the risk assessment, eight of them represent moderate risk (L. flavus, L. maximus, O. draparnaudi, T. molitor, L. repens, V. filiformis, V. odorata, and P. cerasifera). In total, 715 freshwater and terrestrial species are spread in at least two countries of the BSR: 131 of them (18.3%) are of Mediterranean origin, all of them are terrestrial species (123 plants and eight animals). In general, Mediterranean plants were recorded earlier than animals in the BSR, as most of the plants were recorded in the years 1651–1750 and 1801–1900. Seven of the eight Mediterranean animals were introduced as contaminants of food, plants, or nursery material. Most of the Mediterranean plants in the BSR escaped from agriculture or horticulture (46.1%) or were transported as contaminants on animals or as seed contaminants (33.6%). This study is a first evaluation of the flux of species from the MSR to the BSR and will help the stakeholders to make decisions to prevent and control alien species in the BSR.