ABSTRACT
BACKGROUND: Migratory insects are important for the provision of ecosystem services both at the origin and destination sites but - apart from some iconic species - the migration routes of many insect species have not been assessed. Coastlines serve as a funnel where migrating animals including insects accumulate. Migratory behaviour and captures of dragonflies in bird traps suggest autumn migration of dragonflies along coastlines while the origin and regularity of this migration remain unclear. METHODS: Dragonfly species were caught at the bird observatory Kabli at the Baltic coast in Estonia in 2009, 2010 and 2015. For the 2015 data set, we used a stable hydrogen (H) approach to trace the potential natal origin of the migrant hawker (Aeshna mixta). RESULTS: 1079 (2009), 701 (2010) and 88 (2015) A. mixta individuals were caught during the study periods (35, 37 and 11 days in 2009, 2010 and 2015, respectively). The migration period lasted from end of August to end of September. Based on the results from our stable isotope analysis, we identified two populations of A. mixta: One (range of isotope signatures of non-exchangeable H [δ2Hn wing]: -78 to -112) had a local likely origin while the other (δ2Hn wing: -113 to -147) migrated from northerly directions even in headwind from the South. The former showed an even sex ratio whereas the actively migrating population was dominated by males. CONCLUSIONS: Our results suggest a regular southbound autumn migration of A. mixta along the Baltic coast. However, nearly half of the sampled individuals originated from the surroundings suggesting either no, partial or "leap-frog" migration. Contrary to our expectation, A. mixta did not select favourable wind conditions but continued the southbound autumn migration in the flight boundary layer even in case of headwinds. The dominance of males might indicate migration as a result of competition for resources. Further repeated, large-scale studies along the Baltic coast are necessary to pinpoint the migratory pattern and the reason for migration of A. mixta. Such studies should also comprise locations north of the known species range of A. mixta because of the rapid climate-change induced range expansion.
ABSTRACT
A fossil oyster bed (Ostrea edulis) was recently encountered offshore Helgoland (German Bight). Oysters are important filter feeders in marine environments and their habitat structure supports a large associated biodiversity. The European flat oyster Ostrea edulis has historically occurred in vast populations in the North Sea, but declined massively in the early 20th century. The ecological restoration of Ostrea habitats is a current focal point in the North Sea. To better understand the mechanisms that caused the local collapse of the oyster population, this study investigated the size structure, weight, and age of the shells, along with the spatial dimensions, seafloor properties, and environmental context of the oyster bed. The results show that the demise of the population occurred around 700 CE, ruling out excessive harvest as a driver of decline. Synchronicity of increased geomorphological activity of rivers and concurrent major land use changes in early medieval Europe suggest that increased sedimentation was a viable stressor that reduced the performance of the oysters. The shells provided no indication of a demographically poor state of the oyster bed prior to its demise, but manifested evidence of the wide-spread occurrence of the boring sponge Cliona sp. Our study challenges the assumption of a stable preindustrial state of the European flat oyster in the North Sea, and we conclude that the long-term variability of environmental conditions needs to be addressed to benchmark success criteria for the restoration of O. edulis.
