Climate Events Synchronize the Dynamics of a Resident Vertebrate Community in the High Arctic

In studying climate, scientists have been furthering their understanding of how climate events have been affecting a particular species.  But it is unclear how climate will affect communities of species as a whole.  Using the high Arctic as a case study, Hansen et al. (2013) describe changes in the weather patterns of Svalbard and how these events synchronize population fluctuations across the entire vertebrate community and cause a lagged effect on a secondary consumer, the arctic fox.  The synchronization of populations is theorized to occur when winter rains turn to ice, causing the vegetation to be encased in ice and therefore unavailable by herbivores.  This indirect bottom up forcing drives population dynamics across the vertebrates in Svalbard.  With global warming, such icing is expected to become more frequent in the Arctic and therefore strongly affect terrestrial ecosystems.  Hansen et al used statistical data based on population fluctuations and weather to demonstrate the effects of severe weather events on mixed populations. —Cameron Lukos

Hansen, B.B., Grøtan, V., Aanes, R., Sæther, B.-E., Stien, A., Fuglei, E., Ims, R.A., Yoccoz, N.G., Pedersen, Å.Ø., 2013. Climate events synchronize the dynamics of a resident vertebrate community in the high arctic. Science 339, 313—315.  

                  The community Hansen analyzed was the island of Svalbard.  The ecological community consisted of three herbivore species: Svalbard reindeer, Svalbard ptarmagin, vole and the secondary consumer the Arctic fox.  Hansen et al. found correlated population fluctuations of all four species.  The Arctic fox data were advanced one year due to a delayed population reaction to the change in herbivores.  Based on these data, Hansen et al. hypothesized that the climate events were affecting the plant species which limited the amount of forgeable food for herbivores.  This creates a bottom up effect which then causes dips in the arctic fox population.  In order to test this hypothesis, Hansen et al. ran linear regressions that modeled population growth rates as a function of population size and precipitation events.

                  The authors determined that after factoring in density dependence, number of rainy days in the winter months was the best predictor of annual population growth rates across species.  Winter rains caused a negative effect on all species.  Hansen et al. also found that summer temperatures had a positive effect on species growth rates.  This confirmed their hypothesis that climate events do enforce synchrony among herbivores and causes the lag response of artic foxes.  Increased summer temperatures increase green foliage which in turn supplies more food for herbivores and thus an increase in the number of secondary consumers.  Winter rains reduce food supply due to freezing vegetation and the soils.  This then causes increased mortality rates of old sick and very young individuals.  The changing mortality rates cause a decline in population followed by an increase due to less competition and in the case of ptarmagins and voles a drop in predation. 

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