Increased autumn rainfall disrupts trophic interactions in fragmented boreal forests

We show strong effects of autumn climate (number of days with rainfall and with temperature<0°C) on food-store composition of pygmy owls. Increasing frequency of days with precipitation in autumn triggered a decrease in i) total prey biomass stored, ii) the number of bank voles (main prey) stored, iii) the body condition of pygmy owl females. Increasing proportions of old spruce forests strengthened the functional response of owls to variations in vole abundance and were more prone to switch from main prey to alternative prey (passerine birds) depending on local climate conditions. Long-term temporal stability in local vole abundance refutes the hypothesis of climate-driven change in vole abundance and suggests that rainier autumns could reduce the vulnerability of small mammals to predation by pygmy owls. As small rodents are key prey species for many predators in northern ecosystems, our findings raise concern about the impact of global change on boreal food webs through changes in main prey vulnerability.

Erkki Korpimäki