Kozlov, M. V. | bioRxiv Science

bioRxiv2023

The effects of human population density on trophic interactions are contingent upon latitude

Aim: Studies conducted at a global scale are necessary to make general conclusions on the effect of urbanization on trophic interactions and explore how these effects change along latitudinal gradients. Since biotic interactions are more intense at lower latitudes, we predict that the intensity of trophic interactions will decrease in response to urbanization (quantified by human population density), with higher impacts of urbanization at higher latitudes. Location: Global (881 study sites). Time period: 2000-2021. Major taxa studied: Birds, arthropods and woody plants. Methods: We compiled global data on insect herbivory and bird predation from individual studies using similar methodologies, and fitted generalized linear mixed models to test the effect of human population density, latitude and their interaction on these two response variables. Results: The intensity of herbivory and predation decreased with the increase of human population density at lower latitudes, remained unaffected at intermediate latitudes, and increased at higher latitudes. Main conclusions: The effect of human population density on insect herbivory and bird predation consistently varies across latitudes, with a reversal of the pattern at high versus low latitudes.

ecology↗

bioRxiv2020

Search for top-down and bottom-up drivers of latitudinal trends in insect herbivory in oak trees in Europe

AimThe strength of species interactions is traditionally expected to become stronger toward the Equator. However, recent studies have reported opposite or inconsistent latitudinal trends in the bottom-up (plant quality) and top-down (natural enemies) forces driving insect herbivory, possibly because these forces have rarely been studied concomitantly. This makes previous attempts to understand the effect of large scale climatic gradients on insect herbivory unsuccessful. LocationEurope Time period2018-2019 Major taxa studiedQuercus robur MethodsWe used scholar-based citizen science to simultaneously test for latitudinal variation in plant-herbivore-natural enemy interactions. We further investigated the underlying climatic factors associated with variation in herbivory, leaf chemistry and attack rates in Quercus robur across its complete latitudinal range in Europe. We quantified insect herbivory and the occurrence of specialist herbivores as well as leaf chemistry and bird attack rates on dummy caterpillars on 261 oak trees. ResultsClimatic factors rather than latitude per se were the best predictors of the large-scale (geographical) variation in the incidence of gall-inducers and leaf-miners as well as of leaf nutritional quality. However, insect herbivory, plant chemical defences (leaf phenolics) and bird attack rates were not influenced by latitude or climatic factors. The incidence of leaf-miners increased with increasing concentrations of hydrolysable tannins and decreased with those of condensed tannins, whereas the incidence of gall-inducers increased with increasing leaf soluble sugar concentration and decreased with increasing leaf C:N ratios. However, neither other traits nor bird attack rates varied with insect herbivory. Main conclusionsThese findings help to refine our understanding of the bottom-up and top-down mechanisms driving geographical variation in plant-herbivore interactions, and urge for further examination of the drivers of insect herbivory on trees.

ecology↗