DOI: 10.5061/dryad.z8w9ghxcd
title: Latitudinal gradient in the intensity of biotic interactions in terrestrial ecosystems: Sources of variation and differences from the diversity gradient revealed by meta-analysis

description: The Latitudinal Biotic Interaction Hypothesis (LBIH) states that the
 intensity of biotic interactions increases from high to low latitudes.
 This hypothesis, which may partly explain latitudinal gradients in
 biodiversity, remains hotly debated, largely due to variable outcomes of
 published studies. We used meta-analysis to identify the scope of the LBIH
 in terrestrial ecosystems. For this purpose, we explored the sources of
 variation in the strength of latitudinal changes in herbivory, carnivory,
 and parasitism (119 publications) and compared these gradients with
 gradients in the diversity of the respective groups of animals (102
 publications). Overall, both herbivory and carnivory decreased towards the
 poles, while parasitism increased. The latitudinal gradient in herbivory
 and carnivory was threefold stronger above 50–60º than at lower latitudes
 and was significant due to interactions involving ectothermic consumers,
 studies using standardized prey (i.e. prey lacking local anti-predator
 adaptations) and studies aimed at testing LBIH. The poleward decrease in
 biodiversity did not differ between ectothermic and endothermic animals or
 among climate zones and was four-fold stronger than decrease in herbivory
 and carnivory. The discovered differences between the gradients in biotic
 interactions and biodiversity suggest that these two global
 macroecological patterns are likely shaped by different factors.
description: All data were extracted from published studies that fit the
 following criteria: i) the data were collected from natural ecosystems;
 ii) the data were collected from at least two study sites located at least
 2 degrees of latitude apart; and iii) the magnitude of the effect was
 available from the study or could be calculated from the data presented in
 the publication or provided by authors. We identified
 suitable studies based on earlier reviews of latitudinal changes in biotic
 interactions, and we searched for additional publications in the ISI Web
 of Science using the keywords ‘latitud*’, ‘geographic’, ‘biotic
 interactions’, ‘herbivor*’, ‘predat*’, ‘carnivor*’ and ‘parasit*’. The
 search was completed in December 2020. We included only studies that
 contained direct quantitative estimates of the intensity of herbivory (the
 percentage of plant biomass or leaf area lost to herbivores or the
 proportion of damaged leaves), carnivory (the mortality of prey or the
 predator attack rates) or parasitism (prevalence; i.e. percentage of
 infected hosts). We rejected studies where the interaction intensity was
 deduced from the abundance of herbivores/predators and where interactions
 could have been affected by human activities, such as herbivory by
 semi-domesticated reindeer or by large animals whose numbers have
 considerably decreased over the past decades due to human-induced changes
 in vegetation. The vast majority of studies exploring latitudinal patterns in
 biotic interactions employed sites located at about the same (usually low)
 elevations. If elevation varied greatly within a study, we removed sites
 with deviating elevations from effect size calculations to
 prevent their undue influence on latitudinal pattern. When a study
 reported data collected over several years, we selected the year with the
 highest number of sites or with the longest latitudinal gradient. However,
 if data from different years were pooled by the authors, then we used the
 pooled data. When a study reported data collected from multiple localities
 from two or three climate zones, we divided these data by climate zones
 and then by continents. Entire global gradients were excluded from the
 analyses when our focus was on gradients either located within one climate
 zone or crossing a border between the two adjacent zones.
 The larger part of the data on latitudinal changes in
 biodiversity was directly extracted from the published databases
 (Hillebrand, 2004; Kinlock <i>et al.</i>, 2018), resulting in
 166 effect sizes (ES, hereafter) for taxonomic groups involving
 herbivorous, carnivorous and omnivorous animals. However, studies
 conducted at high latitudes were underrepresented in these databases
 compared with our database on biotic interactions. This prompted us to
 search for additional studies from subarctic and arctic regions in the Web
 of Science and in elibrary.ru (published in English and Russian,
 respectively) using the keywords ‘latitud*’, ‘diversity’, ‘polar’,
 ‘arctic’, ‘tundra’ and ‘boreal’.
description: All explanations and references are provided in the README
 file.