High mortality of beetle migrants along the Eastern Mediterranean Flyway

Abstract

Migration is costly in terms of increased energy expenditure and exposure to risks encountered en route. These factors can lead to a higher mortality among migrants compared with more sedentary life history stages. Insect migrants are incredibly numerous, but as they are less conspicuous than vertebrate migrants and as migration often occurs at high altitude and over a broad front, it can be difficult to study migration-related mortality. A major source of information on migration-related mortality comes from cadavers found on the strandline following unsuccessful sea crossings. Here, we analyse strandline mortality following a 100 km crossing of open ocean along the Eastern Mediterranean Flyway from the Middle East mainland to the island of Cyprus by tens of millions of insects. All strandline recordings were of two species of beetles, the seven-spot ladybird Coccinella septempunctata and the carabid beetle Calosoma olivieri, whereas only nine individuals of C. septempunctata were caught successfully arriving with the rest of the insect assemblage over the ocean. Major strandings were associated with easterly winds, suggesting origins from the Middle East mainland, with the individuals documented as extremely weak and unable to fly or dead. Our results suggest that beetles are weaker migrants than other members of the migratory insect assemblage, with the sea crossing too far for the great majority to fly, leading to high mortality. The impact of this high mortality on the marine ecosystems is discussed.