DOI: 10.5061/dryad.xksn02vcf
title: Latitudinal pattern in community-wide herbivory does not match the pattern in herbivory averaged across common plant species

description: 1. The latitudinal herbivory hypothesis (LHH) predicts that plant losses
 to herbivores decrease from low to high latitudes. Although the LHH is a
 community-level hypothesis, it has been rarely tested with data on
 community-wide herbivory, i.e. the percentage of annual production of
 foliar biomass consumed by insects from all plant species at a given site.
 Therefore, we asked whether community-wide leaf herbivory follows the same
 latitudinal pattern as observed for an unweighted average of herbivory
 across common plant species. 2. We selected ten study sites in boreal
 forests from 60°N to 69°N along 1000 km long latitudinal gradient in NW
 Russia. We measured relative foliar losses to insect herbivores in seven
 woody plant species (jointly comprising over 95% of the community-wide
 aboveground biomass) and estimated their current-year foliar biomass. We
 averaged leaf herbivory for all seven species and calculated
 community-wide leaf herbivory by weighting the relative foliar losses of
 each plant species against the contribution of that species to the annual
 foliar biomass production. 3. Leaf herbivory was five-fold higher in
 deciduous species than in conifers. Latitudinal patterns in herbivory
 varied from a significant poleward decrease in all deciduous species to a
 significant poleward increase in Norway spruce. Herbivory values, averaged
 across seven plant species, decreased with latitude and followed the
 pattern observed in deciduous plants due to their higher foliar losses
 compared with conifers. By contrast, community-wide herbivory did not
 change with latitude. This discrepancy emerged because the proportion of
 deciduous plant foliage in the community increased with increasing
 latitude, and this increase counterbalanced the simultaneous poleward
 decrease in losses of these species to insects. 4. Synthesis The herbivory
 measured by averaging relative losses of individual plant species and the
 community-wide herbivory are likely to show different latitudinal patterns
 in various plant communities. The contributions of plant species to the
 total foliar biomass production should be taken into account in studies of
 spatial patterns of herbivory which test community-level hypotheses. This
 approach may provide new insight into macroecological research on biotic
 interactions and improve our understanding of the role of insect
 herbivores in ecosystem-level processes.
description: On August 17–22, 2014, when the majority of insect herbivores had
 completed their feeding, at each site we haphazardly selected five
 (deciduous plants) to ten (coniferous plants) mature individuals of each
 of the seven species. Each plant was located at least 10 m apart from
 others of its species. From each of the deciduous individuals, we
 collected a haphazardly selected branch with approximately 80–120 leaves.
 In conifers, we collected a haphazardly selected branch with 120–200
 current-year needles. To avoid the impact of unconscious biases on the
 values of herbivory, the tree branches and bilberry stems were selected
 while standing at a distance of 5‒10 m away, which does not allow visual
 evaluation of leaf damage by insects. Leaf herbivory was measured in the
 laboratory within two days from sampling. In broadleaved species, the
 leaves on each branch were counted, and each leaf was carefully examined
 for the presence of damage imposed by chewing insect herbivores (both
 miners and defoliators). In conifers, we examined 100 current-year
 needles, starting from the tip of the branch, for traces of insect
 feeding. Each leaf/needle (leaf hereafter) was assigned to one of the
 damage classes according to the percentage of the area of the leaf that
 was consumed or otherwise damaged by insects: 0 (intact leaves), 0.01–1,
 1–5, 5–25, 25–50, 50–75 and 75–100%. Petioles of fully consumed leaves
 were also recorded and attributed to 100% damage. The plant-specific
 percentage of foliage lost to insects (i.e. leaf herbivory level) was
 calculated as follows: the numbers of leaves in each damage class were
 multiplied by the respective median values of the damaged leaf area (i.e.
 0 for intact leaves, 0.5% for the damage class 0.01–1%, 3% for the damage
 class 1–5%, etc.); the obtained values were summed for all damage classes
 and divided by the total number of leaves (including undamaged ones) in a
 sample.
description: Columns in the data file (space-separated) contain the following
 information: Column 1: Latitude of study site (degrees
 N). Column 2: Plant name
 (genus_species). Column
 3: Plant-specific percentage of leaf area consumed by insects.