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

Abstract

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.

Shedding light on trophic interactions: A field experiment on the effect of human population between latitudes on herbivory and predation patterns

“…Only two studies have previously explored the interaction between HPD and latitude studying species diversity (Perez et al, 2022) or biotic interactions (Hernández-Agüero et al, 2023).…”

“…Only two studies have previously explored the interaction between HPD and latitude studying species diversity (Perez et al, 2022 ) or biotic interactions (Hernández‐Agüero et al, 2023 ). Opposite effects of latitude and urbanization have been recently detected on ant species diversity in a 60° absolute latitudinal range study across 63 cities (Perez et al, 2022 ).…”

“…In higher‐latitude cities, both species richness and community composition were more similar to the surrounding non‐urban ant communities. Regarding biotic interactions, the intensity of herbivory and predation in a global analysis decreased with an increase in HPD at lower latitudes, remained unaffected at intermediate latitudes, and increased at higher latitudes (Hernández‐Agüero et al, 2023 ). Cities are ideal places to explore the ecological consequences of possible future climatic change scenarios (Youngsteadt et al, 2015 ).…”

“…The objective of this study was to test whether the patterns detected in Hernández-Agüero et al (2023), what we called the 'opposite latitudinal-disturbance' hypothesis, can be detected in a field experiment. The specific aims of this research were (i) to investigate the effects of human population density on trophic interactions (bird predation and insect herbivory) and (ii) to compare how these effects change with latitude.…”

“…Similarly, Moreira et al ( 2019 ) found lower foliage mortality caused by leaf chewers in urban areas compared to rural areas, and again this effect was not influenced by latitude. In a systematic review conducted by Hernández‐Agüero et al ( 2023 ), the combined influence of latitude and urban factor was used as explanatory variables in order to understand changes in the intensity of herbivory and bird predation. The findings revealed a pattern where trophic interactions decreased with an increase in human population density at lower latitudes, no significant effect at intermediate latitudes, and an increase in trophic interactions with increasing urbanization at higher latitudes.…”