Abstract
The contribution of bird predation to the spatial variations in insect herbivory remains imperfectly understood, especially in Arctic ecosystems. We experimentally tested the hypothesis that the differences in insect herbivory between tundra and forest biomes, and between plant life-forms in these biomes, are associated with differences in the intensity of bird predation on defoliating insects. We observed substantial variation in herbivory (0% to 20% of foliage lost) among nine forest, mountain tundra, and lowland tundra sites in the Kola Peninsula (northwestern Russia) and among five woody plant species, but we found no consistent differences in herbivory between biomes and between plant life-forms. Bird attacks on artificial caterpillars were tenfold more frequent in forest than in tundra, while bird exclusion effects on herbivory did not differ between biomes, and the intensities of bird predation measured by these two methods were not correlated. Bird exclusion led to increases in insect herbivory, and this effect was significant in trees and tall shrubs but was not significant in dwarf shrubs in either forest or tundra sites. Bird predation, as measured in bird exclusion experiments, increased with an increase in the level of foliar damage inflicted by insects in forests but not in tundra habitats. We conclude that bird predation generally decreases plant losses to insects in both forest and tundra habitats, but birds are unlikely to shape the spatial patterns of plant losses to insects in Arctic ecosystems.
Highlights
Plant damage by herbivores greatly varies in both space and time
In forests, we found no differences in foliar damage inflicted by insects between trees and dwarf shrubs
We provide the first demonstration that the impact of insectivorous birds on defoliating insects may differ between plant life-forms within a habitat
Summary
Plant damage by herbivores greatly varies in both space and time. The most widely debated patterns in insect herbivory concern differences between latitudes (Adams and Zhang 2009; Kozlov et al 2015a; Lim et al 2015), altitudes (Galmán et al 2018), plant life-forms (Kozlov et al 2015c; Lim et al 2015), and plant ontogenetic stages (Barton and Koricheva 2010; Zverev et al 2017). Birds are the most important generalist vertebrate predators of herbivorous insects. The importance of birds in protecting forests from insect damage was proved scientifically about a century ago (reviewed by Bruns 1960; Inozemtsev 1978), and meta-analysis of controlled experiments has. Polar Biology (2020) 43:295–304 confirmed that birds generally reduce plant losses to insects (Mäntylä et al 2011). The data available so far do not allow for prediction of the sign of correlation between bird’s predation and insect herbivory: does bird impact decrease abundance of plant-feeding insects (Mäntylä et al 2011), but low insect availability decreases abundance of insectivorous birds (Martin 1987; Thiollay 1988)
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