DOI: 10.5061/dryad.kkwh70s34
title: Predation and parasitism on herbivorous insects change in opposite directions in a latitudinal gradient crossing a boreal forest zone

description: The Latitudinal Biotic Interaction Hypothesis (LBIH) predicts that the
 strength of various biotic interactions decreases from low to high
 latitudes. Inconsistency between studies testing this hypothesis may
 result from variations among different types of interactions and among
 study systems. Therefore, exploration of multiple interactions within one
 system would help to disentangle latitudinal patterns across individual
 interactions and to evaluate latitudinal changes in the overall impact of
 enemies on prey. We tested the prediction based on the LBIH that the
 pressure of natural enemies on herbivorous insects decreases with an
 increase in latitude across the boreal forest zone. We also asked whether
 the impacts of major groups of these enemies exhibit similar latitudinal
 patterns and whether these patterns are consistent across study
 years.  In 10 forest sites located from 60°N to 69°N in Northern
 Europe, each summer, from 2016–2019, we measured (i) mortality of three
 groups of leafmining insects caused by birds, ants, parasitoids, and
 unknown factors, (ii) bird attacks on caterpillar-shaped plasticine
 models, and (iii) birch foliar damage caused by defoliators and
 leafminers. Latitudinal patterns in both insect herbivory on birch and
 top-down pressure on herbivorous insects varied considerably and
 inconsistently among the four study years, so that only some of the
 year-specific correlations with latitude were statistically significant.
 Nevertheless, meta-analysis combining correlations across years, preys and
 enemies revealed general decreases in predation by birds (on both natural
 and model prey) and ants, but an increase in parasitism rates, from low to
 high latitudes. We found that the direction of latitudinal changes in the
 strength of biotic interactions was interaction-specific: predation and
 herbivory supported LBIH, whereas parasitism exhibited an opposite trend.
 Consequently, the overall impact of natural enemies on herbivorous insects
 did not change with latitude and was therefore an unlikely reason for the
 poleward decrease in herbivory observed in our gradient. Considerable
 among-year variation in the strength of the latitudinal patterns in all
 the studied interactions suggests that this variation is a widespread
 phenomenon.
description: The data are of three types: (1) mortality of leafminers: three
 data filed and three metadata files; (2) bird attack rates on artificial
 prey: one data file and one metadata file; and (3) herbivory on birches:
 one data file and one metadata file. <strong> 1.
 Mortality of leafminers</strong> We collected
 birch leaves mined by the larvae of <em>Eriocrania</em> spp.
 (between 10 and 29 June) and <em>Stigmella</em> spp. (between
 8 and 29 August), and rowan leaves mined by the larvae of several moth
 species (between 8 and 29 August); sampling was conducted at each of our
 study sites annually from 2016‒2019. The same experienced persons searched
 for the mined leaves at all sites and during all study years. All
 discovered mines had been collected; we attempted to obtain 20–50 mines of
 each of three groups of moths from each study site annually. The samples
 were classified as either empty mines left by larvae which had
 successfully completed their development (i.e. survived individuals) or
 mines containing dead larvae or mines that had been opened by predators.
 The mines containing dead larvae were classified as killed by parasitoids
 if a parasitoid larva or pupa was found inside the undamaged mine. If no
 signs of parasitism were discovered, then the death was attributed to
 unknown reasons. All larvae that failed to reach 10% of their final size
 were classified as having died from unknown reasons because they were too
 small to have been a target for predators or parasitoids. If the mine
 contained no dead larva and only a small part of a single epidermal wall
 of the mine was damaged (usually in the form of one or more small round
 holes), then the larva was classified as having been eaten by ants. If
 both walls of the mine were punched through and severely damaged, the
 mortality was attributed to birds. <strong>2.
 Bird attack rates on artificial prey </strong> We
 fabricated our prey models from non-toxic, unscented, soft modelling clay
 (Chemical plant ‘Luch’, Yaroslavl, Russia) of green and brown colours to
 imitate the natural colours of palatable caterpillars. Green wire 0.3 mm
 in diameter was used to attach two plasticine models (28–32 mm length and
 3.5–4 mm diameter) of different colours to thin branches of each of five
 haphazardly selected mature (&gt;3 m height) birches at a height of
 1.2–1.8 m and at least 50 cm apart. The models were exposed from 10–12
 June and checked on 26–30 June, 8–12 August and 23–29 August in 2016–2019.
 During each inspection, we counted plasticine caterpillars with beak marks
 left by birds and then remoulded the attacked models or replaced them if
 the damage was severe. When a model (including the wire) was not found,
 this record was considered as missing and a new model was placed on the
 tree. <strong>3. Herbivory on
 birches</strong> On 8–16 August 2016–2019, when
 the majority of insect herbivores had completed their feeding, we
 haphazardly selected five mature individuals of each of the two birch
 species at each site. Each individual was located at least 10 m apart from
 others of its species. From each tree, we collected a branch with
 approximately 80–120 leaves (median value: 100 leaves). We avoided the
 impact of unconscious biases on the values of foliar losses by selecting
 the branches while standing at a distance of 5‒10 m away, which prevented
 visual evaluation of leaf damage by insects. The branches were labelled
 with random numbers, so the person assessing herbivory was blinded with
 respect to the sample origin. The leaves on each branch
 (including the petioles of fully consumed leaves) were counted, and each
 leaf was carefully examined for the presence of insect damage. Each leaf
 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 defoliating
 insects: 0 (intact leaves), 0.01–1, 1–5, 5–25, 25–50, 50–75 and 75–100%.
 The plant-specific percentage of foliage lost to insects 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. The
 abundances of <em>Eriocrania</em> and
 <em>Stigmella</em> leafminers were quantified as their
 intensities, i.e. the number of mines divided by the number of leaves on
 the branch.