Understanding drivers of population change is critical for effective species conservation. In the northeast Atlantic Ocean, recent changes amongst seabird communities are linked to human and climate change impacts on food webs. Many species have declined severely, with food shortages, and increased predation reducing productivity. Arctic skua Stercorarius parasiticus, a kleptoparasite of other seabirds, is one such species. The aim of the study was to determine relative effects of bottom-up and top-down pressures on Arctic skuas across multiple colonies in a rapidly declining national population. Long-term monitoring data were used to quantify changes in population size and productivity of Arctic skuas, their hosts (black-legged kittiwake Rissa tridactyla, common guillemot Uria aalge, Atlantic puffin Fratercula arctica, Arctic tern Sterna paradisaea) and an apex predator (great skua Stercorarius skua) over 24 years (1992-2015) in Scotland. We used digital mapping and statistical models to determine relative effects of bottom-up (host productivity) and top-down (great skua density) pressures on Arctic skuas across 33 colonies, and assess variation between three colony types classified by host abundance. Arctic skuas declined by 81% and their hosts by 42%-92%, whereas at most colonies great skuas increased. Annual productivity declined in Arctic skuas and their hosts, and reduced Arctic skua breeding success was a driver of the species' population decline. Arctic skua productivity was positively associated with annual breeding success of hosts and negatively with great skua density. Intercolony variation suggested Arctic skua trends and productivity were most sensitive to top-down pressures at smaller colonies of host species where great skuas had increased most, whereas bottom-up pressures dominated at large colonies of host species. Scotland's Arctic skua population is declining rapidly, with bottom-up and top-down pressures simultaneously reducing breeding success to unsustainably low levels. Marine food web alterations, strongly influenced by fisheries management and climate change, are driving the decline, and this study demonstrates severe vulnerability of seabirds to rapid change in human-modified ecosystems. Potential but untested conservation solutions for Arctic skuas include marine protected areas, supplementary feeding within colonies and management of great skuas.
Bumblebees are a key pollinator group globally which have declined over time. However, while some species remain common others are rare and have declined at a sharper rate. The Great Yellow Bumblebee (Bombus distinguendus Morawitz) is classified as 'Endangered' on the Irish Red List, and 'Vulnerable' at European level. We aimed to investigate the ecology of B. distinguendus within the wider bumblebee community in its only remaining Irish population to inform conservation management. Eight true bumblebee species were recorded in total in four habitat types surveyed, with highest species richness in species-rich grassland and highest abundance in urban habitats. Highest numbers of B. distinguendus were found in two designated nature reserves. However, outside these reserves the species was only found in three of twelve sites surveyed in both species-rich and coastal grassland habitats. B. distinguendus is a late emerging species in this region with a foraging preference for Common Knapweed (Centaurea nigra). Availability of key forage species was lowest when numbers of B. distinguendus were highest, potentially limiting survival of this species. Some smaller suitable habitats in urban areas and roadside verges were not used by the species, which suggests potential issues with habitat connectivity, patch size and/or availability of nesting and hibernation sites. Implications for insect conservation Based on our findings, we suggest that existing species-rich and coastal grassland sites should be conserved, and other areas restored, for B. distinguendus in Ireland. Conservation management should focus on delaying cutting grasslands until late September, winter grazing of stock and reducing pesticides and fertilisers with the aim of increasing forage availability in late season, and ensuring that suitable habitats are connected in the landscape to allow adequate habitat area as well as movement and dispersal of the species to new areas.
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