Many Arctic species are shrinking in size. Say what? Yes, shrinking in size – polar bears included. Is this some clever climate adaptation (as the opposite holds true: larger body sizes are an adaptation to a cold climate), or is it a sign of increasing malnourishment?

Well, the latter – at least in the case of the red knot (Calidris canutus), a long-distance migratory bird that breeds on the Russian Arctic tundra and winters in tropical West-Africa, using temperate zone tidal wetlands as a refuelling stop on their long road.

Red knots are a sandpiper species – very long-distance migratory birds. They depend on the presence and health of several keystone ecosystems, not just their summer breeding grounds (the Taimyr tundra in Russia) and their tropical wintering grounds (Banc d’Arguin, Mauritania) – but also special feeding grounds halfway on their route north and south where huge flocks of red knots land twice a year to regain strength in between their seasonal migration extremes. The most important feeding ground is the Wadden Sea – the world’s largest intertidal area, where (Dutch part) this picture of flying red knots was shot. Image credit: Kasper Hendriks.

In fact, the decreasing body sizes of this Arctic-born sandpiper species places these birds at further disadvantage when finally they arrive in the tropics. The shellfish they feed on in summer on the coastal wetlands of Mauritania used to be in reach of the red knot’s bills – the smaller birds however have a problem, as their favoured summer food is out of reach…

This we learn from a special publication in Science from May 2016 titled ‘Body shrinkage due to Arctic warming reduces red knot fitness in tropical wintering range’ – a study that was performed by an international research team led by Jan van Gils of the Royal Netherlands Institute for Sea Research NIOZ.

Climate change-induced temporal mismatch migratory birds

To understand what’s going on we need to go back to what ecologists call a ‘temporal trophic mismatch’ caused by climate change – when two (groups of) species that are directly linked in the food chain have a differently timed response to warming, leading to a mismatch in the peak of their seasonal activity, essentially driving a wedge between these species.

Earlier in this series we discussed how European-African migratory birds can be seen as a canary in coalmine of ecological climate impacts – due to the relatively strong temporal mismatch these birds face, arriving too late for the spring food peak their successful nesting depends on.

Well, the even longer-distance migratory birds that breed in the Arctic and winter in tropical Africa, using Europe as a stepping-stone only, are probably at an even higher risk of a similar temporal mismatch. The reason is the relatively rapid warming of the Arctic, much faster than any other region on Earth – a phenomenon known as Arctic Amplification.

Red knots, like many other wetland migratory birds, are also dependent of the intertidal Wadden Sea in Northwest Europe – that they use as feeding grounds, and where large numbers can be spotted at the end of the summer and winter. Picture credit: Kasper Hendriks, Dutch Wadden Sea.

The red knots of course have to time their long annual travels according to seasonal climate development. When they depart from Africa at the end of winter, they have little way of knowing that spring on the Arctic is already well underway (while historically these birds used to build their nests in the snow still). Other species that stay on the tundra year round, some as eggs and seads, can of course respond much more rapidly to the rising temperatures. And that’s why, when finally the red knots have their hatchlings on their tundra nests, they are late to profit from the local insect peak. And because the young birds receive less food, they grow smaller than the generation before them.

For the red knot this creates another survival challenge during the summer:

“We show that an avian long-distance migrant (red knot, Calidris canutus canutus), which is experiencing globally unrivalled warming rates at its high-Arctic breeding grounds, produces smaller offspring with shorter bills during summers with early snowmelt.”

“This has consequences half a world away at their tropical wintering grounds, where shorter-billed individuals have reduced survival rates. This is associated with these molluscivores eating fewer deeply buried bivalve prey and more shallowly buried seagrass rhizomes.”

The study again illustrates how complex the ecological consequences of climate change can be.

The authors conclude: “We suggest that seasonal migrants can experience reduced fitness at one end of their range as a result of a changing climate at the other end.”

So can these species somehow evolve to cope with this survival challenge? The researchers describe what a future-proof red knot what look like: a smaller bird, with a relatively large bill. Chick that are somehow born with that trait my in turn produce offspring that can survive further warming. If however you compare the speed of the current rapid climate warming to the speed of species evolution…

The Wadden Sea, the world’s largest intertidal area, is a keystone ecosystem, acting as feeding grounds for millions of migratory birds between Greenland, Siberia and Africa south of the equator. Sadly this ecosystem is also threatened by climate change, through accelerating 21st century sea level rise. But that’s a different story. Video by We Love Earth.

Arctic Amplification, Arctic Shrinking, Arctic Starvation?

Back to the reduced body size for just a bit longer. Arctic shrinking is a universal development, currently witnessed among many species around the rapidly warming North Pole – including polar bears. Earlier research had suggested the shrinking size of polar bears is a pre-stage for their projected population decline – as the seasonal disappearance of sea ice hunting grounds forces this species to live off its fat reserves longer.

“Reductions in body size are increasingly being identified as a response to climate warming. Here we present evidence for a case of such body shrinkage, potentially due to malnutrition in early life,” the authors of the Science publication write.

That too may be a broader climate phenomenon we fear: Arctic starvation.

© Rolf Schuttenhelm | www.bitsofscience.org