Complexity of leaf miner–parasitoid food webs declines with canopy height in Patagonian beech forests

Abstract

1. Consumer-resource species interactions form complex, dynamic networks, which may exhibit structural heterogeneity at various scales. This study set out to address whether host-parasitoid food web size and topology vary across forest canopy strata, and to what extent foliar resources and species abundances account for vertical patterns in network structure.2. The vertical stratification of leaf miner-parasitoid food webs was examined in two monotypic beech (Nothofagus pumilio) forests in northern Patagonia, Argentina. Quantitative food webs were constructed for separate canopy layers by sampling foliage from three tree-height classes at 0.5-1, 2-3 and 5-6 m above ground.3. Leaf miner abundance per unit leaf mass and foliar damage (%) did not differ across strata, although foliage quality and quantity increased from the understorey to the upper canopy. Parasitism rates and food web complexity decreased with canopy height, as reflected by reduced linkage richness, linkage density, mean interaction strength, and host vulnerability. 4. Null model analyses revealed that food web metrics, especially in the upper canopy, were often lower than expected when compared with randomly structured networks. Overall, these patterns held for two forests differing in vertical structure and in dominant miner morphotype and parasitoid species.5. These results suggest that vertical declines in network complexity may be driven by the parasitoids' limited functional response to host abundance and dispersal from pupation sites in the forest floor. A broader constraint on food web structure seemed to be imposed by host-parasitoid trait matching, a reflection of large-scale assembly processes.

Vertical canopy gradient shaping the stratification of leaf‐chewer–parasitoid interactions in a temperate forest

“…Therefore, the higher diversity recorded in the first level might be due to the involvement of understory species, supporting our original hypothesis. The higher diversity of the third level might be explained by the presence of species‐rich semiconcealed feeders, which were considerably less numerous in the first level, supporting the original hypothesis of the competitive advantage of the concealed feeders in the harsh upper canopy environment (Chaij et al., 2016; Paniagua et al., 2009; Ribeiro & Basset, 2007). However, in our study, due to the absence of the upper canopy per se, differences in the ecological conditions (e.g., solar radiation, temperature, wind speed; Basset, Horlyck, et al., 2003; Parker, 1995) between the second and third levels were blurred.…”

“…In comparison, the less host‐specific Ichneumonidae and Braconidae used their hosts nonrandomly, preferring the first canopy level. This phenomenon suggests some environmental (Basset, Hammond, et al., 2003; Ulyshen, 2011) and/or life history constraints within these groups, such as low dispersal capacity (Godfray, 1994) and/or limited number of eggs carried by females combined with the proximity of overwintering sites to the first canopy level (Chaij et al., 2016; Heimpel, Mangel, & Rosenheim, 1998). Moreover, most parasitoids depend on sugar resources, due to their high activity (Shaw, 2006), with sugar feeding increasing host encounter rates (Forsse, Smith, & Bourchier, 1992).…”

“…Moreover, no previous study focused on leaf‐chewing assemblages. Inconsistent results obtained by previous studies suggest that the vertical patterns of parasitism are guild‐, latitude‐, or site‐dependent (Chaij et al., 2016; Morris et al., 2015; Paniagua et al., 2009; Sobek, Tscharntke, Scherber, Schiele, & Steffan‐Dewenter, 2009). The parasitism rate decreased as the season progressed, in accordance with another study on leaf‐chewing Lepidoptera in a temperate forest (Le Corff, Marquis, & Whitfield, 2000).…”

“…However, only three studies have investigated the vertical stratification of host–parasitoid food webs (Chaij, Devoto, Oleiro, Chaneton, & Mazía, 2016; Morris et al., 2015; Paniagua, Medianero, & Lewis, 2009). Therefore, more studies on the quantitative food webs of herbivorous insect–parasitoid communities in forest canopies are needed to understand differences between forest types and the particularities of various herbivorous guilds.…”

“…In temperate forests, leaf miner–parasitoid food web complexity decreases with canopy height, as a reflection of decreasing parasitism rate (Chaij et al., 2016). We expect a similar pattern, reflecting vertical changes in quantitative network indices.…”