Numerical responses of saproxylic beetles to rapid increases in dead wood availability following geometrid moth outbreaks in sub-arctic mountain birch forest.

Ole Petter Laksforsmo Vindstad,Lauri Kapari,Anne Sverdrup-Thygeson,Rolf Anker Ims,Jane Uhd Jepsen,Sabrina Schultze,Martin Biuw

doi:10.1371/journal.pone.0099624

Abstract

Saproxylic insects play an important part in decomposing dead wood in healthy forest ecosystems, but little is known about their role in the aftermath of large-scale forest mortality caused by pest insect outbreaks. We used window traps to study short-term changes in the abundance and community structure of saproxylic beetles following extensive mortality of mountain birch in sub-arctic northern Norway caused by an outbreak of geometrid moths. Three to five years after the outbreak, the proportion of obligate saproxylic individuals in the beetle community was roughly 10% higher in forest damaged by the outbreak than in undamaged forest. This was mainly due to two early-successional saproxylic beetle species. Facultative saproxylic beetles showed no consistent differences between damaged and undamaged forest. These findings would suggest a weak numerical response of the saproxylic beetle community to the dead wood left by the outbreak. We suggest that species-specific preferences for certain wood decay stages may limit the number of saproxylic species that respond numerically to an outbreak at a particular time, and that increases in responding species may be constrained by limitations to the amount of dead wood that can be exploited within a given timeframe (i.e. satiation effects). Low diversity of beetle species or slow development of larvae in our cold sub-arctic study region may also limit numerical responses. Our study suggests that saproxylic beetles, owing to weak numerical responses, may so far have played a minor role in decomposing the vast quantities of dead wood left by the moth outbreak.

Highlights

Insect outbreaks are major disturbance factors in many forest ecosystems, periodically causing defoliation and mortality of trees and other vegetation across vast areas [1]
We examine the responses of saproxylic beetles to a massive outbreak of the two geometrid moths (Lepidoptera: Geometridae) Epirrita autumnata (Bkh.) and Operophtera brumata (L.) that occurred in the mountain birch (Betula pubescens ssp. czerepanovii Orlova) forest of the Varanger region in northern Norway (Fig. 1A) during 2002– 2009
By comparing beetle counts between damaged and undamaged forest, we address the following specific questions: 1. What is the magnitude of the short-term numerical responses of saproxylic beetles to the superabundance of dead wood resources generated by the moth outbreak?

Summary

Introduction

Insect outbreaks are major disturbance factors in many forest ecosystems, periodically causing defoliation and mortality of trees and other vegetation across vast areas [1]. One important consequence of outbreaks is that they alter resource availability for many consumer species. Outbreaks can increase food availability for insectivorous birds [3,4,5], small mammals [6,7] and invertebrates [8,9]. The resulting increase in understory plant biomass may, in turn, provide resources for species that depend on understory vegetation for food or cover [13,14]. In all of these cases, outbreaks provide consumers with resource pulses, i.e. rare and ephemeral events of resource superabundance [15,16]. We still have very limited understanding of how such outbreak-induced resource pulses affect the dynamics of consumer communities

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Discussion

Conclusion