Climate change alters the structure of arctic marine food webs due to poleward shifts of boreal generalists

Abstract

Climate-driven poleward shifts, leading to changes in species composition and relative abundances, have been recently documented in the Arctic. Among the fastest moving species are boreal generalist fish which are expected to affect arctic marine food web structure and ecosystem functioning substantially. Here, we address structural changes at the food web level induced by poleward shifts via topological network analysis of highly resolved boreal and arctic food webs of the Barents Sea. We detected considerable differences in structural properties and link configuration between the boreal and the arctic food webs, the latter being more modular and less connected. We found that a main characteristic of the boreal fish moving poleward into the arctic region of the Barents Sea is high generalism, a property that increases connectance and reduces modularity in the arctic marine food web. Our results reveal that habitats form natural boundaries for food web modules, and that generalists play an important functional role in coupling pelagic and benthic modules. We posit that these habitat couplers have the potential to promote the transfer of energy and matter between habitats, but also the spread of pertubations, thereby changing arctic marine food web structure considerably with implications for ecosystem dynamics and functioning.

Keywords: biogeography; climate warming; fish community; food web topology; modularity; network structure.

Figure 1.

Food web diagrams of the Barents Sea for the (a) boreal, (b) arctic and (c) arctic II food webs. Each dot (node) represents a trophospecies. The lines connecting the nodes represent the feeding links between the trophospecies. The vertical position of the nodes indicates the trophic position of a species, and the horizontal position indicates the module affiliation of a species. The size of the nodes are proportional to the degree (no. of feeding links) of a species. The colour of the nodes indicates which functional group a trophospecies belongs to: grey, detritus; green, basal taxa; cyan, zooplankton; orange, benthos; blue, fish; magenta, sea birds; light pink, marine mammals. Schematic food web diagrams of the modular structure of the Barents Sea food webs: (d) boreal, (e) arctic and (f) arctic II food web. Each node (circle) represents a module in the corresponding food web. The size of the nodes indicates the number of trophospecies within each module. The colour of the nodes (pie charts) indicates the habitat affiliation of the trophospecies within the module: light blue, pelagic; red, benthic; dark blue, benthopelagic. The arrow width is proportional to the number of feeding links between modules in the direction of the arrowhead.

Figure 2.

Cumulative (a) in- and (b) out-degree distributions of the boreal (grey circles), arctic (black circles) and arctic II (open circles) food webs. The in-degree represent the number of prey items of a species, i.e. its generality. The out-degree represent the number of predators of a species, i.e. its vulnerability.

Figure 3.

Barents Sea maps of mean degree centrality (mean no. of feeding links) of 51 fish for the years (a) 2004 and (b) 2012. The dots indicate the position of sampling stations (approx. 400) and the size of the dots is proportional to the mean fish degree at the station. The coloured surface (colour code shown in the legend) indicates the mean degrees of fish, spatially interpolated on a regular grid. North of the 120 day isolines (black lines) sea ice was present for more than 120 days during the year. The red lines indicate the 2°C isoline, southwest of the isoline seawater temperatures are more than 2°C (atlantic water) and northeast of the isoline the temperatures are less than 2°C (mixed water and arctic water). See the electronic supplementary material, figure S2 for the position of the boreal and arctic study regions in the Barents Sea.

Figure 4.

Species topological roles with respect to modularity: (a) boreal, (b) arctic (c) and arctic II food webs. Dots are mean values based on 1000 estimates, and the error bars indicate the 95% CIs for z and PC. The topological network roles are: network connectors (upper right), module connectors (upper left), peripherals (lower left) and connectors (lower right). The colours, top left legend in (a), correspond to the functional affiliation of a trophospecies: grey, detritus; green, basal taxa; cyan, zooplankton; orange, benthos; blue, fish; magenta, sea birds; light pink, marine mammals. The larger blue dots in (a) and (c), upper right quadrant, are the two network connector hubs, cod and haddock. See the electronic supplementary material (tables S1 and S2) for lists of all taxa and their topological role.