Increased autumn rainfall disrupts predator–prey interactions in fragmented boreal forests

Abstract

There is a pressing need to understand how changing climate interacts with land-use change to affect predator-prey interactions in fragmented landscapes. This is particularly true in boreal ecosystems facing fast climate change and intensification in forestry practices. Here, we investigated the relative influence of autumn climate and habitat quality on the food-storing behaviour of a generalist predator, the pygmy owl, using a unique data set of 15 850 prey items recorded in western Finland over 12 years. Our results highlighted strong effects of autumn climate (number of days with rainfall and with temperature <0 °C) on food-store composition. 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, and (iii) the scaled mass index of pygmy owls. 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. High-quality habitat may allow pygmy owls to buffer negative effects of inclement weather and cyclic variation in vole abundance. Additionally, our results evidenced sex-specific trends in body condition, as the scaled mass index of smaller males increased while the scaled mass index of larger females decreased over the study period, probably due to sex-specific foraging strategies and energy requirements. 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.

Food supplementation and predation risk in harsh climate: interactive effects on abundance and body condition of tit species

“…It is possible that few pygmy owls were storing additional prey in natural cavities. In our study area however this seems unlikely since the number of suitable natural cavities seem to be low due to small coverage of old growth forests (see methods in Terraube et al , Morosinotto et al ).…”

“…In our study area, 230 forest patches with two nest‐boxes each are available for pygmy owls throughout the year and are regularly checked to determine food caching activity of owls (from late October to late March) and breeding activity (from mid‐April to mid‐June). During the study period the nest boxes were checked every month to ensure that all caching sites were detected (see also methods in Terraube et al ). Each pygmy owl can have 1–4 caching cavities (Korpimäki and Varjonen unpubl.)…”

“…Each pygmy owl can have 1–4 caching cavities (Korpimäki and Varjonen unpubl.) and the size and number of these stores vary greatly depending on the availability of voles (their main prey, Solheim ) and weather condition (Terraube et al ). It is possible that few pygmy owls were storing additional prey in natural cavities.…”

Mobbing behaviour in a passerine community increases with prevalence in predator diet

“…), snow cover and precipitation dramatically lowers the number of hoarded rodents (Terraube et al . ). This clearly suggests that the bird predation rate was higher during autumn and winter than during the spring.…”

“…comm.). Although it is known that this predator hoards prey such as voles or birds in cavities and nestboxes (Kellom€ aki 1977, Solheim 1984, Suhonen et al 2007, snow cover and precipitation dramatically lowers the number of hoarded rodents (Terraube et al 2016). This clearly suggests that the bird predation rate was higher during autumn and winter than during the spring.…”