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

Ole Petter Laksforsmo Vindstad,Rolf Anker Ims,Jane Uhd Jepsen

doi:10.1007/s10342-015-0886-y

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.

Highlights

Outbreaks by pest insects are major sources of disturbance in many forest ecosystems (Barbosa et al 2012)
2005; Hogstad 2005), there has been no previous research on how bird communities are affected by the severe disturbance that outbreaks can cause to the vegetation in the mountain birch forest
Thereby, the pervasive damage that moth outbreaks can cause both to the tree layer and the understorey vegetation does not seem to have dramatic consequences for forest bird communities, at least not in the short term

Summary

Introduction

Outbreaks by pest insects are major sources of disturbance in many forest ecosystems (Barbosa et al 2012). The consequences of this have so far been studied for relatively few species, and there are still large uncertainties as to how outbreak-induced changes in habitat characteristics affect wildlife communities in most outbreak-affected systems. The importance of such impact studies is currently accentuated for boreal and sub-arctic forest ecosystems, where insect outbreaks recently appear to have become. Bird species which are associated with closed-canopy forest have typically been found to undergo population declines in areas experiencing outbreak-induced mortality of trees, while species that prefer clearings and disturbed habitats have usually been found to increase (Rabenold et al 1998; Gale et al 2001; Becker et al 2008). The same is true for species associated with the understorey vegetation, which often grows denser and taller after outbreaks, probably due to increased amounts of light reaching the forest floor or fertilization from insect frass and cadavers (Bell and Whitmore 1997; Canterbury and Blockstein 1997)

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Results

Conclusion