Many arctic bird populations have declined substantially during the last decades. Preliminary analyses in connection with the development of an ecosystem-based assessment system for Norwegian arctic tundra have shown that this also applies to COAT’s intensive monitoring sites in Varanger Peninsula. Many bird species are particularly vulnerable during the breeding season, and especially those that nest on the ground can be subjected to high nest predation rates. A study published in Science last year showed a strong increase in nest predation rates on arctic shore birds since the 1990s. This increase have coincided with the recent increase in temperatures in the Arctic. The authors of the Science paper speculated that warming-induced changes in vegetation or in the small rodent population cycle could be involved.
In a paper published today in Nature Climate Change, COAT researchers provide evidence that vegetation change – through the phenomenon of Arctic greening – may result in such increased predation rates on ground nesting birds. The study also provides clues to where in the elevation gradients (tundra ecotones) birds may be most affected, and how the small rodent population cycle may now play a different role in the relation between nest predators and tundra birds, compared to a few decades ago.
Rock ptarmigan nest. Photo: Rolf A. Ims
Arctic greening is due to increasing plant biomass resulting from longer and warmer growing seasons. The level of greening/greenness is customarily quantified by remote sensing (satellite data). By means of 900 experimental nest that simulated egg clutches of ptarmigan and shore birds across a sample of tundra landscapes in Finnmark (70-71°N), northern Norway -with varying levels of greenness – the new COAT study shows that predation rates increased by 72% from the least green to the greenest landscapes in the sample. This result accords well with ecological theory that predicts that increased primary productivity should inflict higher predation rates at intermediate trophic levels in tundra food chains. The design of the study also included elevation gradients from the tree-line to the mid-alpine zone in each of the mountainous landscape areas. The predation rates were found to increase with increasing elevation suggesting that bird species nesting at high altitudes are particularly impacted by predation.
The study was done over a 5-year period encompassing all phases of the 4-year rodent cycle. As expected, the predation rate peaked the year after the peak phase of the rodent cycle. Unexpectedly, however, the predation continued to be quite high for 2-3 years after the rodent peak. Such an extended delayed response of the predators relative to the rodent cycle will act to increase the cumulative impact of predation.
It is naturally important to know the identity of the predator species that are involved, and especially if management actions are deemed necessary to halt bird population declines. An attempt was therefore made to identify the involved predator species by means of placing one egg made of plasticine in the experimental nests. Marks left on the plasticine eggs indicated that avian nest predators (to a large extent corvids) were responsible for most of the predation in the study. However, because this methodology is rather imprecise, there is a great need for developing better methods in studies of nest predation both on simulated and natural nests. COAT (through the project COAT Tools) is presently performing trials of such new methods. However, there is a lot of scope for more innovations in the future to gain a better understanding of the drivers of increased nest predation rates in tundra ecosystems.