Spatial heterogeneity in the relative impacts of foliar quality and predation pressure on red oak, Quercus rubra, arthropod communities

Abstract

Predation pressure and resource availability often interact in structuring herbivore communities, with their relative influence varying in space and time. The operation of multiple ecological pressures and guild-specific herbivore responses may combine to override simple predictions of how the roles of plant quality and predation pressure vary in space. For 2 years at the Coweeta LTER in the Southern Appalachian Mountains, we conducted a bird exclosure experiment on red oak (Quercus rubra) saplings to investigate the effects of bird predation on red oak arthropod communities. We established bird exclosures at six sites along an elevational gradient and estimated variation in foliar nitrogen and bird predation pressure along this gradient. Foliar nitrogen concentrations increased with elevation while our index of bird predation pressure was variable across sites. Greater arthropod densities were detected inside exclosures; however, this result was mainly driven by the response of phloem feeders which were much more prevalent inside exclosures than on control trees. There was little evidence for an effect of bird predation on the other arthropod guilds. Consequently, there was no evidence of a trophic cascade either in terms of leaf damage or tree growth. Finally, we found more variation in arthropod density among trees within sites than variation in arthropod density among sites, indicating the importance of micro-site variation in structuring arthropod communities.

Insect herbivory increases from forest to alpine tundra in Arctic mountains

“…Parasitism and predation on herbivorous insects and the density of predatory groups frequently decrease with elevation (Bowden & Buddle, 2010; Hodkinson, 2005; Pepi et al, 2017). However, other studies did not find any elevational trends in bird predation on arthropods (Schwenk et al, 2010; Zehnder et al, 2010). In particular, a bird exclusion experiment previously conducted in three of our six gradients showed similar effects of bird predation on insect herbivory in alpine tundra and in lowland forests (Zverev et al, 2020).…”

Elevational gradients in plant defences and insect herbivory: recent advances in the field and prospects for future research

“…1A). However, several other studies assessing changes in both enemy abundance and pressure have found no elevational patterns (McMillin andWagner 1998, Roininen et al 2006), and other authors that have reported expected decreases in parasitoid diversity or predator abundance with elevation have not found concomitant elevational gradients in parasitism or predation (Straw et al 2009, Tantowijoyo and Hoffmann 2010, Zehnder et al 2010. In these latter studies, elevational gradients in insect herbivore abundance and plant damage were presumably influenced by abiotic factors that directly (or indirectly via effects on host plants) influence herbivores rather than via indirect defence.…”

Meta‐analysis of elevational changes in the intensity of trophic interactions: Similarities and dissimilarities with latitudinal patterns

“…However, the strength or even the direction of elevational changes in these interactions can differ between trophic levels because this type of variation was previously detected in latitudinal gradients: herbivory and carnivory decreased from low to high latitudes, whereas parasitism increased (Zvereva & Kozlov, 2021). For elevational gradients, any particular predictions about variations among trophic levels are hampered by a lack of reviews of elevational changes in carnivory and by inconsistent results of a few primary studies that measured carnivory and herbivory within the same elevational gradients (Bito et al, 2011; Oksanen et al, 1981; Zehnder et al, 2010). Therefore, we could only anticipate that (2) the strength of elevational changes differs between herbivory, carnivory and parasitism.…”