Influence of Canopy Size, Conspecific Distance, and Local Topographic Variation on Insect Communities and Herbivory in Olive (Olea europaea L.)

Abstract

This study investigated how canopy size, conspecific neighbour distance, and local topographic variation influence insect communities and patterns of herbivory in olive (Olea europaea) plants in Almería, southeast Spain, within the framework of the Resource Concentration Hypothesis (RCH), which predicts that plants in dense, uniform stands experience greater herbivore pressure. Twenty focal olive plants were selected within a narrow elevation range (321–342 m), representing fine-scale variation in topographic position across the study site. Plants were georeferenced using GPS and measured for canopy size, neighbour distance, and leaf damage. Herbivorous and carnivorous insects were sampled using a combination of beat-sheet and sweep-net methods and identified with field guides and microscopy. Herbivory was quantified as the proportion of leaves with visible browsing damage. General Linear Models (GLM) were used to test the effects of environmental and structural variables on insect abundance, carnivore fraction, and diversity after appropriate transformations to meet model assumptions. Insect abundance decreased significantly with increasing elevation within the site (p = 0.005), suggesting sensitivity to fine-scale habitat variation. Carnivore abundance was positively related to canopy size (p = 0.042) but declined with elevation (p = 0.035), indicating that both vegetation structure and local topographic position influence predator distribution. Patterns of herbivory varied among plants, with higher levels generally observed in smaller canopies and in individuals located near conspecific neighbours. These findings highlight the importance of canopy structure and fine-scale habitat heterogeneity in shaping plant-insect interactions. The results have practical implications for olive management, as maintaining larger canopies may promote natural biological control and reduce reliance on chemical pesticides.