Bark beetle infestation spots as biodiversity hotspots: Canopy gaps resulting from insect outbreaks enhance the species richness, diversity and abundance of birds breeding in coniferous forests

Abstract

Natural disturbances are a major source of spatial stand heterogeneity, initiating forest community turnover. Besides large-scale events like fires, landslides or windthrows, small-scale canopy gaps resulting from the mortality of a single tree or a group of trees represent a distinct structure within a stand. Although the role of canopy gaps is well recognized in tropical ecosystems, the effect of small-scale openings on species richness, diversity and abundance has not been satisfactorily addressed in temperate forests. A specific example of a canopy gap is the bark beetle infestation spot, that is, a small group of trees that have died as a result of insect activity. We studied the effect of canopy gaps on the breeding bird assemblage in old-growth coniferous forests in the Tatra Mts. and Gorce Mts. (Carpathians, S Poland). Our hypothesis was that the gaps and the nearby closed-canopy forest would differ in species richness, diversity and relative abundance. We censused the breeding bird assemblages in 50 such gaps (median size 0.85 ha, quartile range 0.30–2.34 ha, range 0.03–10.0 ha) and 50 nearby closed-canopy reference plots using point counts. Almost 25% more bird species were recorded in the gaps than in the closed-canopy forest, and species richness correlated positively with the number of snags. The number of individual birds recorded in the gaps was 40% higher than in the closed-canopy forest and correlated positively with the number of fallen logs. Gap size was not correlated with the richness, diversity or relative abundance of species. Our results show that canopy gaps resulting from bark beetle outbreaks increase species richness, diversity and abundance by providing suitable habitats for cavity-nesting, ground-nesting and insectivorous birds. We suggest that by increasing habitat heterogeneity and enhancing food resources, even relatively small-scale canopy gaps improve the overall biodiversity of coniferous forests.