A major challenge with climate change is predicting how it could cause myriad effects that ‘cascade’ through food chains and ecosystems. A new study about polar bear predation at Arctic bird colonies illustrates one way these knock-on effects can play out.

After observing bird breeding colonies in Greenland and Norway’s Svalbaard Archipelago for more than 40 years, scientists documented an increase in polar bear raids on bird nests in step with reductions in sea ice coverage.

In some years, bears ate eggs from more than 90% of all nests. At one Norwegian site (Nordenskiöldkysten), a researcher observed bears eat more than 200 eggs in two hours; in 2014 no eggs or chicks of barnacle goose, eider, or glaucous gull nests at this site survived the polar bear raids.

Immense numbers of migratory seabirds and shorebirds travel to the Arctic to capitalise on long, productive days of a fleeting summer. What does the future hold in a rapidly warming Arctic? So far only a small segment of the polar bear population has turned to the nesting birds, but with devastating results for the birds’ reproductive success. Yet some speculate that the bird colonies will outlast the polar bears, because the latter are so highly dependent on sea ice.

Winged Sentinels covers other cascading effects of climate change on birds:

  • In Central Arizona, an increase in alpine snowmelt means more deer and elk graze woody vegetation at higher elevations. As this leafy understory is reduced, more bird nests have become vulnerable to predators.
  • Warming in Canada’s Hudson Bay has caused sea-ice dependent Arctic cod to decline. Brunnich’s guillemots, the largest surviving species of auk, have shifted their diet from arctic cod to smaller fish, leading to slower growing nestlings that are less likely to survive.
  • Lemmings are a keystone prey species in the high Arctic, but their boom-bust population cycles are flattening or becoming less predictable in some regions, with warming Arctic winters a suspected cause. Where lemmings have declined, predators including snowy owls and artic foxes have also suffered, as well as turning their attention to ground-nesting birds.

Sources

Prop et al. (2015) Climate change and the increasing impact of polar bears on bird populations. Frontiers in Ecology and Evolution.

Hyman (2015) Polar bears turn to sea ice for sustenance. Science.

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