Food-web structure and robustness to species loss in Arctic and Antarctic ice-shelf ecosystems

Background/Question/Methods

Over the next century sea-ice declines are predicted to range from 3-67% in the northern hemisphere and 8 to 64% in the southern hemisphere.  Due to the high dependence of polar species on the sea-ice, it is expected that reductions in annual sea-ice cover will lead to changes in the trophic structure of the sea-ice ecosystems.  We assembled high resolution food webs for Arctic and Antarctic sea-ice ecosystems to compare their topological structure and robustness to species loss. To cross validate our link data we compared our binary estimates of trophic position with those calculated using stable isotopes of nitrogen for 80 Arctic and 67 Antarctic species. The robustness of the food webs to species loss was determined for a series of sequential simulated species removals including removing the 1) most connected species, 2) least connected species, 3) highest to lowest trophic position species, 4) lowest to highest trophic position species, and 5) random species.

Results/Conclusions

The Arctic web included 162 taxa and the Antarctic web included 239 taxa. When aggregated into nodes with 100% similarity in predators and prey, trophic richness was 46% higher in the Antarctic than in the Arctic (TS_Arctic = 153, TS_Antarctic = 224). Connectance (C) was ~25% higher in the Arctic (C_Arctic = 3.9%, C_Antarctic = 3%) however the number of links per species was lower on average by almost one link in the Arctic (L/S_Arctic=5.96, L/S_Antarctic=6.71).  The mean trophic position was approximately the same in both web (TP_Arctic=3.8, TP_Antarctic=3.9), with the Arctic having a higher maximum trophic position (Max-TP_Arctic=5.62, Max-TP_Antarctic=5.45). Short-weighted trophic position predicted 43% of the variance in δ¹⁵N in the Arctic and 56.4% in the Antarctic (Arctic p<0.001, Antarctic p<0.001) showing that our binary link data was a good approximation to more quantitative energy-flow based methods.  Both webs showed low robustness to the loss of highly connected species and low trophic position species, with a maximum of 15% of the species needing to be removed to collapse the webs to 50% of their original size.  The webs showed little sensitivity to the removal of least connected, highest to lowest trophic position and random species, which resulted in a minimal number of secondary extinctions.  Our results highlight the sensitivity of Polar food-webs to primary species loss such as might occur with sea-ice decline and suggest that Polar food-webs are particularly sensitive to the loss of low trophic level species and highly connected species.