Why Warmer Winters Don’t Always Help Geese
Contrasting consequences of climate change for migratory geese: Predation, density dependence and carryover effects offset benefits of high-arctic warming (2019) Layton-Matthews et al., Global Change Biology, DOI: 10.1111/gcb.14773
Most of us know that climate change will bring warmer, shorter winters to most parts of the world. For many species in areas like the Arctic, it would be easy to interpret this as a good thing – plants grow earlier, so animals get more food, right? Naturally it’s never that simple. Many herbivorous species have evolved in sync with climate cycles so that their reproduction peaks when food becomes available. If season start dates change, these species may not be able to change their own cycles in time. Additionally, what happens if populations of their predators suddenly boom?
Today’s authors wanted to know what role a warming climate played in the population fluctuations of migratory barnacle geese (Branta leucopsis).
What They Did
The population of barnacle geese studied have their breeding grounds on Svalbard, an island north of mainland Norway that lies deep in the Arctic circle. The geese migrate here from the south (Scotland) every year, hatching around the start of July. Data on bird clutch size and success hatching data was used to determine reproductive success of the geese, whilst mark-recapture data was used to determine their survival chances at different life stages. Data was collected over the last three decades, with over three thousand nests and five thousand individuals monitored in this time.
Survival data was compared to climate data like date of spring onset, average temperature and variation in rainfall, which attempted to capture the likely amount of food the herbivorous geese had access to. Estimates of Arctic fox (Vulpus lagopus) predation at their breeding grounds in Svalbard were also used to see how predator populations affected the geese.
What They Found
Whilst egg production for the geese increased, it was offset by increasing mortality among young geese. This was likely driven by an increase in Arctic fox numbers. Any benefits that increased populations may have had also may have been countered by density-dependent effects during winter in Scotland – when populations increase resources dry up and reproduction rates suffer.
Did You Know: Mark-Capture Data
The survival of every individual in a population is almost always impossible to determine. Ecologists have adapted, and mark-recapture models have become one of the more popular ways to determine the chance of an individual having survived over a time period. It simply involves marking a series of captured individuals of a population, and seeing how many of them were recaptured after a given time. It may sound like a stab in the dark, but modern statistics has made it possible to obtain quite accurate survival data using this technique.
Whilst the researchers did have climate data for the wintering and spring stopover grounds for the geese, the data which came from Svalbard was much more detailed. whilst the results here are quite in-depth, it still remains that there’s a large chunk of very relevant data missing. Getting similar data from spring and winter grounds would naturally make the results more informative, but it would also take an enormous amount of time and resources.
More resources doesn’t just mean more geese. Areas like Svalbard have fairly simple food webs, with relatively few predator and prey species. But the interactions between these species can still be complex. More reindeer carcasses (another result of climate change, read more about it here) mean more foxes, which means more predation on geese. Less ice also means more polar bears getting stranded on Svalbard, many of which have started preying on goose eggs.
Results like these give much needed insight into how food webs will morph as temperatures increase. If we can develop models for simple ecosystems that are being affected more strongly than others now, it means we can create complex models when climate change starts hitting other regions harder.