Quantitative assessment of plant-arthropod interactions in forest canopies: A plot-based approach

Abstract

Research on canopy arthropods has progressed from species inventories to the study of their interactions and networks, enhancing our understanding of how hyper-diverse communities are maintained. Previous studies often focused on sampling individual tree species, individual trees or their parts. We argue that such selective sampling is not ideal when analyzing interaction network structure, and may lead to erroneous conclusions. We developed practical and reproducible sampling guidelines for the plot-based analysis of arthropod interaction networks in forest canopies. Our sampling protocol focused on insect herbivores (leaf-chewing insect larvae, miners and gallers) and non-flying invertebrate predators (spiders and ants). We quantitatively sampled the focal arthropods from felled trees, or from trees accessed by canopy cranes or cherry pickers in 53 0.1 ha forest plots in five biogeographic regions, comprising 6,280 trees in total. All three methods required a similar sampling effort and provided good foliage accessibility. Furthermore, we compared interaction networks derived from plot-based data to interaction networks derived from simulated non-plot-based data focusing either on common tree species or a representative selection of tree families. All types of non-plot-based data showed highly biased network structure towards higher connectance, higher web asymmetry, and higher nestedness temperature when compared with plot-based data. Furthermore, some types of non-plot-based data showed biased diversity of the associated herbivore species and specificity of their interactions. Plot-based sampling thus appears to be the most rigorous approach for reconstructing realistic, quantitative plant-arthropod interaction networks that are comparable across sites and regions. Studies of plant interactions have greatly benefited from a plot-based approach and we argue that studies of arthropod interactions would benefit in the same way. We conclude that plot-based studies on canopy arthropods would yield important insights into the processes of interaction network assembly and dynamics, which could be maximised via a coordinated network of plot-based study sites.

Spatial covariance of herbivorous and predatory guilds of forest canopy arthropods along a latitudinal gradient

“…The vegetation of each plot was surveyed, and all stems with a diameter at breast height (DBH) ≥ 5 cm were tagged, mapped and identified to species. The plots were then gradually felled and sampled for arthropods (see detailed protocols in Volf et al ., 2019). We took advantage of ongoing logging operations (CZE, USA) or subsistence shifting agriculture (PNG) at our sites to avoid contributing to net deforestation.…”

“…In addition, the standardisation of arthropod abundances per unit of forest biomass is advisable for consistency in analyses but has previously been omitted. Such a crucial knowledge gap is perhaps not surprising, considering that arthropods are predominantly studied at the level of individual plant stems and/or limited sets of species, rather than in plot‐based surveys (but see Volf et al ., 2019). Thus, the extent to which arboreal arthropod abundance patterns are driven by plant biomass along fundamental ecological gradients remains relatively unknown.…”

Common ground: restoring land health for sustainable agriculture

“…A richer understanding of agricultural landscapes and the relative biodiversity retained in different production systems is needed to ensure the optimum use of agricultural land (Norris, 2008). According to a meta-analysis conducted in Europe, the density of hedges also appeared to be critical as a refuge for arthropods with direct impacts on pest control, pollination and yields (Volf et al, 2019). However, there is uncertainty over the implications for agricultural production at scale, and therefore the potential costs and trade-offs to farmers and society (Fahrig et al, 2011).…”

Notre terrain d’entente: rétablir la santé des terres pour une agriculture durable

“…(Sirami et al, 2019). Selon une méta-analyse réalisée en Europe, la densité des haies semblait également être critique en tant que refuge pour les arthropodes, avec des impacts directs sur la lutte contre les ravageurs, la pollinisation et les rendements (Volf et al, 2019). Cependant, une incertitude persiste quant à ses implications pour la production agricole à grande échelle et, par conséquent, aux coûts et compromis potentiels pour les agriculteurs et la société (Fahrig et al, 2011).…”