Thinner however resilient geese that thrive within the excessive Arctic

Barnacles in the Arctic have been on a diet. So many are now moving to northern breeding areas that there is less food in some places. The good news is that this doesn't seem to be limiting population growth just yet.

The world is changing dramatically, especially in the high Arctic. Climate change has caused spring to come earlier, but winters have become far more treacherous for arctic animals that hibernate there, with more rain and ice.

So what about birds that do not hibernate in the Arctic, but nest and raise their young there? Researchers have spent nearly three decades collecting data on a group of barnacles migrating to the Norwegian archipelago from Svalbard to see how the geese overall respond to environmental changes.

In one study, they found that previous sources related to climate change meant better conditions for the geese, but those benefits were offset by an increase in the number of hungry foxes – a precarious equilibrium that could change depending on future conditions.

Now researchers have looked at the geese's actual physical condition as their numbers have skyrocketed in their UK wintering grounds. There are now more than 40,000 birds compared to just 300 individuals in the 1940s when the birds were still hunted in the UK and elsewhere.

Barnacle Goose, Copyright David MacLeman, from the Surfbirds Galleries

However, more birds mean more competition for food in their summer nesting areas on Svalbard.

In fact, there is so much competition that the average body weight of birds has dropped 10 percent over the past three decades. In wildlife, reduced body weight often means reduced condition and poor ability to reproduce or survive. The question that the researchers have tried to answer in a new publication is: does this really matter?

The answer seems to be no. Not yet.

A long journey to nesting sites

Kate Layton-Matthews completed her PhD at the NTNU Center for Biodiversity Dynamics on Geese earlier this year. She worked with a team of researchers from NTNU, the University of Groningen in the Netherlands, the University of Sheffield in the UK and the Norwegian Polar Institute in Tromsø to model a long-term population study in the barnacle goose.

The barnacle geese that breed on Svalbard make a 3000km journey to their arctic breeding grounds from their wintering areas in Scotland every spring. This in itself takes a lot of energy, but the birds also need to be in good shape when they arrive in the Arctic to lay eggs and raise their young.

When the research team looked at their decades of data, they found that the birds' body weights had dropped 10 percent.

Therefore, Layton-Matthews used a complex statistical model that could process information about the geese's body condition, reproduction, and survival since 1990. Using this model, she was able to determine whether the net effect of lower body weights actually affected the birds' ability to survive and reproduce, and ultimately, if this led to changes in geese population numbers.

Thinner geese do not cause population decline

“We assume that body condition is so important for herbivores, and especially in the Arctic, that your body condition can limit reproduction and the likelihood of survival. Body weight or body condition could therefore be limited, whether or not they succeed in having offspring, and also whether young geese in particular survive their first migration, ”she said.

Barnacle Geese, Dumfries and Galloway, Copyright Glyn Sellors, from the Surfbirds Galleries

Surprisingly, although changes in body condition affect the reproduction and survival of young geese somewhat, this did not lead to any noticeable impact on population size.

"We were expecting something so important on an individual level to have a big impact on the population, but that's not the case," she said.

The researchers say this is because it is overridden by other key environmental factors that geese do not affect geese through body condition, such as the positive effects of previous sources on gosling production and the negative effects of arctic foxes on gosling survival.

However, since there is obviously a limit to how thin the geese can be and still perform well, population growth is likely to be constrained at some point if their summer habitat continues to deteriorate.

The good news, Layton-Matthews said, is that this still shows that geese populations are more resilient to changes in habitat quality and body condition than researchers previously thought.

Potential risk from hunting

The increase in geese numbers in their wintering areas has led to some places like Ireland where the birds can be hunted again. It was also discussed whether barnacle hunting should be allowed on Svalbard.

According to Layton-Matthews, history has shown that there is a potential risk if the overwintering birds are overhunted.

"When we start hunting a population that is very sensitive to hunting and very sensitive to climate change, as we know that population, we have to be very careful about what hunting levels are set," she said. "If they want to introduce hunting in wintering or breeding areas, we need to monitor and take into account the other processes, particularly climate change, so that they can set reasonable quotas that do not harm the general population."

Maarten J.J.E. Loonen began studying barnacles on Svalbard three decades ago to see how the breeding season would limit population size after population numbers increased due to changes in wintering areas. Winter populations performed well as hunting was banned in Scotland in 1950 and the grasslands on which they grazed were fertilized.

"In 1990 we thought that the Arctic would not change and that Svalbard as an island would lead to increased competition between geese and a decrease in breeding success," Loonen said. “In the poor years we knew that gosling growth slowed and these geese subsequently became smaller adults. However, after 30 years of data collection, this paper shows that a decrease in population size has not yet had any effect. "

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