Structural complexity and prey availability shape spider communities under retention forestry

Retention forestry is promoted as a conservation-oriented management
strategy to sustain forest biodiversity by preserving key structural
elements, such as single old trees and deadwood. However, the
effectiveness of this approach in conserving the diversity of spiders as
generalist predators remains unclear. Particularly because the effect of
structural elements under retention forestry on spiders may be mediated by
its effect on prey availability. We sampled spiders (Araneae) and
potential prey (Diptera, Hemiptera, Collembola) in 55 one-hectare plots
across mixed temperate forests of the Black Forest, Germany. We used
pitfall traps targeting species active on the forest floor. We studied
spider abundance, taxonomic diversity, ecological diversity (combined
measure of functional and phylogenetic distance), community composition
along gradients of forest structure (canopy cover, proportion of conifers,
stand structural complexity, volume of lying deadwood, herb cover, and
understory plant richness). We also looked at how potential prey abundance
varied with forest structure and cascade to their predator. Spider
richness increased with stand structural complexity. Abundance declined
proportion of conifer and increased with understory plant richness.
Ecological diversity was not significantly related with forest structural
variables. Prey abundance increased with structurally complex stands and
tended to decline proportion of conifers. Higher prey abundance was
positively related to spider abundance and partly accounted for lower
spider abundance in high proportion of conifer stands. Community
composition shifted with canopy cover and conifers gradients, and
functional trait identity varied with canopy cover, volume of lying
deadwood, and stand structural complexity. Synthesis and applications. Our
findings suggest retention forestry practices that maintain structural
complexity through spatial and vertical heterogeneity, integrate deadwood,
and support diverse plant communities may support spider richness and
shape dominant ecological strategies, while influencing predator
populations through prey availability. Managers aiming to enhance
biodiversity in managed forests may benefit from prioritizing structural
complexity and understory diversity, while considering potential
trade-offs associated with stand compositions. These findings provide an
evidence-based foundation for integrating principles of structural
complexity, resources availability, and trait-based filtering into forest
management and conservation strategies.