It is well established that corridors can, in certain cases, help spread non-native species in terrestrial ecosystems. In a new study by Airoldi et al., the authors expand on this theme with a look at the spread of non-native species at a regional scale. This study compares artificial marine infrastructure (ex. harbors, dikes, piers, breakwaters, etc.) and natural reefs. Their extensive look at 500 kilometers of shoreline in the North Adriatic Sea provides clues as to how marine corridors contribute to the spread of native and non-native species alike.

Because marine infrastructure usually replaces sedimentary substrata with artificial hard surfaces such as stone or concrete, it encourages hard bottom, rocky species to grow where otherwise they might not be able to. The study focused on assemblages of ascidians (commonly referred to as sea squirts), and compared habitat type (natural vs. artificial), coastline type (sandy vs. rocky) and exposure (sheltered vs. exposed).

They found that on average, species richness was 2-3 times higher, and native species were 2-9 times as abundant in natural habitats compared to artificial habitats. Artificial sandy habitats had the lowest species richness, lowest occurrence of native species, and the highest abundance of non-natives, with no significant differences found between exposure types. Overall, most native species were virtually absent from any artificial habitats along sandy coastlines, and still appeared in lower abundances even when the artificial habitat was near a rocky coastline.

The strong differences between natural and artificial habitats suggest that marine urban sprawl increases connectivity for non-native species. Artificial habitats provides inadequate substrata for native species, particularly when built along sandy shores, while at the same time act as regional corridors for non-native species. The large-scale hardening of coastal regions may have long-term impacts not only on species assemblage, but overall population connectivity as well.