Effect of climate change and invasive species on Arctic marine food-webs

Abstract

Climate warming induces poleward distributional shifts of boreal species in Arctic regions resulting in compositional changes with poorly understood consequences for Arctic food-webs. This thesis addresses the spatial variation in Barents Sea food-webs using previously compiled distributional data in polygons specified for the Nowegian and Barents Sea NoBa Atlantis model and a highly resolved Barents Sea metaweb dataset. Further, it studies implications of cod and snow crab invasion for Arctic marine food-webs organization. Spatial overlap across NoBa polygons was estimated for all pairs of interacting species in the metaweb and used as link weight for the metaweb to investigate zoogeographic patterns. Food-webs were compiled for the NoBa polygons based on compositional and metaweb data to address the spatial variation in food-web structure. For each food-web, a set of standard structural properties affecting food-web ecology was estimated. Environmental data and community weighted means of traits available for the species in the metaweb were compiled for each NoBa polygon to support analysis of spatial variation. The environmental, compositional, traits and food-web structure data were analysed by multivariate methods to map and assess spatial variation across NoBa polygons. Despite gradual change in environmental, compositional and traits characteristics along the ocean climate gradient separating Atlantic from Arctic regions, food-web organization displayed a discontinuity with a strong structural difference between the three Arctic polygons and the remaining food-webs. The structural difference concerned primarily food-web connectance, modularity and degree of omnivory, three properties that distinguish Arctic from Atlantic food-webs. Addition of cod to the three Arctic food-webs shortened path lengths connecting species, and strongly modified food-web structure, reducing modularity and increasing connectance and omnivory, thereby increasing their similarity to Atlantic food-webs. The addition of snow crab to Arctic polygons had a similar but far less pronounced effect due to its lower degree of generalism relative to cod and nearly exclusive affiliation with the benthic food-web compartment. The strong structural change in Arctic food-webs imposed by climate-driven cod invasion goes in the direction of reduced robustness against environmental perturbations. A reduced food-web robustness is concerning considering the likelihood of increased environmental perturbations in the Arctic due to higher climate variability and expanding human activities driven by climate change.