Resistance of a sub-arctic bird community to severe forest damage caused by geometrid moth outbreaks
Resistance of a sub-arctic bird community to severe forest damage caused by geometrid moth outbreaks
Ole Petter Laksforsmo Vindstad 0 1 2 3
Jane Uhd Jepsen 0 1 2 3
Rolf Anker Ims 0 1 2 3
0 Department of Arctic and Marine Biology, University of Troms , 9037 Troms , Norway
1 & Ole Petter Laksforsmo Vindstad
2 Communicated by Jarmo Holopainen
3 Norwegian Institute for Nature Research, Fram Centre , 9296 Troms , Norway
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.
Disturbance; Mountain birch; Operophtera; Epirrita; Transect; Hierarchical model
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Outbreaks by pest insects are major sources of disturbance
in many forest ecosystems (Barbosa et al. 2012). Outbreaks
can cause damage and mortality in trees and understorey
vegetation across enormous areas and thereby alter many
biotic and abiotic attributes of forest habitats, including the
amount of canopy cover (Dennison et al. 2010; Moulinier
et al. 2013), the spatial heterogeneity of the habitat (Muller
et al. 2008; Yang 2012) and the species composition of
vegetation communities (Man and Rice 2010; Karlsen et al.
2013). This, in turn, may alter habitat quality for a wide
range of forest-dwelling taxa (Lehnert et al. 2013).
However, 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
more extensive due to climate change (Jepsen et al. 2008,
2011; Weed et al. 2013).
Birds are one group of organisms that are known to be
sensitive to habitat changes caused by insect outbreaks.
Many species of birds forage, seek shelter from predators
and build their nests in trees or among understorey
vegetation. The severe impacts that outbreaks have on
forest vegetation can thereby cause cascading effects on
bird communities in outbreak-affected areas. 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).
In northern Scandinavia, outbreaks by the two geometrid
moths (Lepidoptera: Geometridae) autumnal moth
(Epirrita autumnata) and winter moth (Operophtera brumata)
periodically cause defoliation of mountain birch (Betula
pubescens ssp. czerepanovii) across areas of 1000s of
square kilometres (Jepsen et al. 2009a, 2013). Moth larvae
are an important food resource for many species of birds in
the mountain birch forest, and several bird species appear
to increase reproductive effort due to a superabundance of
food in moth outbreak years (Enemar et al. 1984, 2004;
Hogstad 2005; (...truncated)