Positive tree diversity effects on arboreal spider abundance are tied to canopy cover in a forest experiment

Abstract

Human actions are decreasing the diversity and complexity of forests, and a mechanistic understanding of how these changes affect predators is needed to maintain ecosystem services, including pest regulation. Using a large-scale tree diversity experiment, we investigate how spiders respond to trees growing in plots of single or mixed species combinations (4 or 12) by repeatedly sampling 540 trees spanning 15 species. In 2019 (6 years post-establishment), spider responses to tree diversity varied by tree species. By 2021, diversity had a more consistently positive effect, with trees in 4-or 12-species plots supporting 23% or 50% more spiders, respectively, compared to conspecifics in monocultures. Spiders showed stronger tree species preferences in late summer, and the positive impact of plot diversity doubled. In early summer, the positive diversity effect was tied to higher canopy cover in diverse plots, leading to higher spider densities. This indirect path strengthened in late summer, with an additional direct effect of plot diversity on spiders. Prey availability was higher in diverse plots but was not tied to spider density. Overall, diverse plots supported more predators, partly by increasing available habitat. Adopting planting strategies focused on species mixtures may better maintain higher trophic levels and ecosystem functions.

Tree species identity, canopy structure and prey availability differentially affect canopy spider diversity and trophic composition

“…Previous studies suggested that especially non-flying taxa such as spiders benefit from structurally complex environments (Halaj et al 2000 ; Ramos et al 2022 ). Some studies even reported that canopy structure explains more variance in canopy spider abundance and species richness than prey availability (Halaj et al 1998 ; Butz et al 2023 ). Structural complexity and vertical vegetation layering promote arthropod abundances and diversity via higher space-filling, which increases total habitat and resource availability (Müller et al 2018 ; Knuff et al 2020 ; Rappa et al 2023 ).…”

“…Canopy structural heterogeneity is assumed to play a particularly important role for spiders (Araneae), as they rely on available structures for web attachment or shelter (Halaj et al 2000 ; Korenko et al 2011 ; Butz et al 2023 ). Conifers in particular feature beneficial fine-scale structures and high prey abundances, increasing spider abundances compared to broadleaved trees (Ozanne 1999 ; Korenko et al 2011 ).…”

Tree diversity enhances predation by birds but not by arthropods across climate gradients

“…diversity on the abundance and diversity of natural enemies (Ampoorter et al, 2020;Butz et al, 2023;Li et al, 2023;May-Uc et al, 2020;Nell et al, 2018;Penone et al, 2019;Stemmelen et al, 2022), but experimental tests involving assessments of predation pressure are still limited. The few available studies have reported mixed findings, with some supporting positive effects of tree diversity on predation (Leles et al, 2017;Nell et al, 2018), whereas others have reported no effects (Castagneyrol et al, 2017;Interian-Aguiñaga et al, 2022), or variable responses depending on the spatial scale or predator group studied (Muiruri et al, 2016;Yang et al, 2018).…”