Resistance of a sub-arctic bird community to severe forest damage caused by geometrid moth outbreaks

Abstract

Outbreaks by geometrid moths periodically cause mass mortality of trees and state changes in understorey vegetation in sub-arctic mountain birch forest in northern Scandinavia. In order to assess the short-term impacts of such disturbance on forest bird communities, we took bird censuses in forest where almost all birch trees had been killed by moth outbreaks 2-4 years before the study and in undamaged forest. The study was repeated in two locations (Kirkenes and Tana) with contrasting forest structure and fragmentation. Using a hierarchical community model, we show that the total abundance of birds in Kirkenes was only about 25 % lower in damaged than undamaged forest and that species richness differed even less between the two forest types. Meanwhile, neither bird abundance nor species richness differed between damaged and undamaged forest in Tana. The observed patterns in abundance were mainly driven by a few very common species. Only a single species showed indication of being more abundant in damaged forest. Thus, our findings indicate that bird communities in sub-arctic mountain birch forest have a high degree of resistance to forest damage caused by moth outbreaks. We suggest that bird populations in outbreak-affected forest may be maintained by surviving trees and by standing dead tree trunks, which help maintain the vertical structure of the forest habitat. The fact that many of the studied bird species are habitat generalists may also explain their apparently weak responses to the damage caused by the outbreak. Our results do not point towards forest damage caused by moth outbreaks as a major driver of change in bird communities in the study system, although more long-term research is needed to substantiate this conclusion.

Elevationally biased avian predation as a contributor to the spatial distribution of geometrid moth outbreaks in sub‐arctic mountain birch forest

“…It is important to emphasise that the pied flycatchers and great tits inhabiting our nest boxes represent only a small subset of the bird community in the study system. At least 20 other passerine species occur in Scandinavian mountain birch forest (Vindstad et al ., ). Some of these, like the brambling ( Fringilla montifringilla ), the willow warbler ( Phylloscopus trochilus ) and the common redpoll ( Carduelis flammea ), are very common and prey heavily on moth larvae (Hogstad, ).…”

Cascading Effects of Moth Outbreaks on Subarctic Soil Food Webs

“…Sampling was replicated in two areas located approximately 70 km apart, both at the border of the outbreak range, but with slight differences in the defoliation year: Tana (70º03' N, 27º45' E.), defoliated during 2006-2007, and Kirkenes (69º46' N, 29º20' E) defoliated during 2007-2009. Differences in the forest characteristics between these two areas allow to control for the in uence of the initial forest characteristic on the effect of defoliation, that has been proved to be non-negligible in past studies 32,36,40 . In each area, a linear transect was previously established from highly impacted forest stands to undamaged stands 36,40 .…”

“…Differences in the forest characteristics between these two areas allow to control for the in uence of the initial forest characteristic on the effect of defoliation, that has been proved to be non-negligible in past studies 32,36,40 . In each area, a linear transect was previously established from highly impacted forest stands to undamaged stands 36,40 . In order to maximise the differences between defoliated and undamaged forests we selected the two stations (separated from at least 2km between them) at each extreme of the transects that were categorized as defoliated forest, i.e.…”

“…Drastic shifts in the composition of biological communities following defoliation have been already reported in these nutrient-limited soils where the dominance of the allelopathic dwarf shrub Empetrum nigrum in the understory leads to regressive succession that may inhibit soil microbial activity, organic matter decomposition, and thus nutrient availability [37][38][39] . These shifts correspond to a replacement of Empetrum nigrum by a grass Avenella exuosa 32 with subsequent effects on the diversity and abundance of organisms directly relying on plants, including vertebrate herbivores 31 , birds 40 , saproxylic beetles 36 , and fungal communities 41,42 . However, we still ignore whether moth outbreaks induced indirect effects across the soil food web, whether these effects are of comparable magnitude to those observed for vegetation, and nally, whether these effects have signi cant consequences on trophic interactions and ultimately on the whole soil food web structure.…”

Point of no return – absence of returning birds in the otherwise philopatric willow warbler Phylloscopus trochilus

“…Snow melting date (Martin and Wiebe 2004), sudden cold spells in spring and bad weather (Hendricks and Norment 1992, Newton 2007, Pérez et al 2016, rodent and lemming cycles (Sittler et al 2000, Bêty et al 2002 and outbreaks of autumnal moth (e.g. Epirrita autumnata) (Enemar et al 2004, Laksforsmo Vindstad et al 2015 are factors that can affect a territory's suitability unpredictably between years in the Subarctic. However, in ecological models a philopatric strategy can still perform well in unpredictable environments, given that mean quality within territories is equal among territories (Switzer 1993).…”