Global patterns of insect herbivory across forest canopies and understories: Insights from a tropical case study and a global comparison

Several studies have examined global patterns of insect herbivory, revealing variations with latitude, elevation, and temperature. However, less attention has been given to herbivory patterns at smaller spatial scales, particularly the comparison between forest canopies and understories. Understanding these finer-scale patterns is crucial for predicting ecological responses to both natural and anthropogenic changes. Differences in herbivory between the canopy and understory are expected due to distinct environmental conditions, with light availability being a key driving factor. To examine whether insect herbivory differs between the canopy and understory in different forest types, we estimated herbivory as leaf area loss [%] in the tree canopies and the woody understory of a montane dry – (MDF) and a montane rainforest (MRF) in southern Ecuador. For this, we collected around 20 leaves per plant of 918 plants in total in six 1 ha plots at two elevation levels (600 m, 1200 m) in the MDF and in nine 1 ha plots at three elevation levels (1000 m, 2000 m, 3000m) in the MRF. Additionally, we conducted a literature survey resulting in 64 studies used for the analyses to compare the herbivory pattern between the two strata in a multi-studies approach. Herbivory in the MDF was – as expected – higher in the understory compared to the canopy. In the MRF, however, the pattern was reversed. The multi-studies approach revealed a mean herbivory of 9.45 % ± 8.75 % (standard deviation) and no significant differences in herbivory between the canopy and the understory. Neither latitude as a proxy for temperature, nor precipitation, to account for different forest types, had significant effects on herbivory. Both the multi-studies and the MRF and MDF case study suggest that the variation in herbivory between forest types might be larger than between strata. However, for a better understanding of general herbivory patterns in the canopy and the understory of forest systems the need of much more case studies directly comparing both strata with each other is essential.