Latitudinal variation in plant chemical defences drives latitudinal patterns of leaf herbivory

Abstract

A long‐standing paradigm in ecology holds that herbivore pressure and thus plant defences increase towards lower latitudes. However, recent work has challenged this prediction where studies have found no relationship or opposite trends where herbivory or plant defences increase at higher latitudes. Here we tested for latitudinal variation in herbivory, chemical defences (phenolic compounds), and nutritional traits (phosphorus and nitrogen) in leaves of a long‐lived tree species, the English oak Quercus robur. We further investigated the underlying climatic and soil factors associated with such variation. Across 38 populations of Q. robur distributed along an 18° latitudinal gradient, covering almost the entire latitudinal and climatic range of this species, we observed strong but divergent latitudinal gradients in leaf herbivory and leaf chemical defences and nutrients. As expected, there was a negative relationship between latitude and leaf herbivory where oak populations from lower latitudes exhibited higher levels of leaf herbivory. However, counter to predictions there was a positive relationship between leaf chemical defences and latitude where populations at higher latitudes were better defended. Similarly, leaf phosphorus and nitrogen increased with latitude. Path analysis indicated a significant (negative) effect of plant chemical defences (condensed tannins) on leaf herbivory, suggesting that the latitudinal gradient in leaf herbivory was driven by an inverse gradient in defensive investment. Leaf nutrients had no independent influence on herbivory. Further, we found significant indirect effects of precipitation and soil porosity on leaf herbivory, which were mediated by plant chemical defences. These findings suggest that abiotic factors shape latitudinal variation in plant defences and that these defences in turn underlie latitudinal variation in leaf herbivory. Overall, this study contributes to a better understanding of latitudinal variation in plant–herbivore interactions by determining the identity and modus operandi of abiotic factors concurrently shaping plant defences and herbivory.

Effects of insularity on insect leaf herbivory and chemical defences in a Mediterranean oak species

“…We sampled all populations at the end of the growing season at each site; this was late August in northernmost populations and early September in southernmost populations. This approach was aimed at reducing phenological differences in herbivory and plant defensive traits across sites (Moreira, Abdala‐Roberts, et al., ). In addition, we restricted our sampling to fully expanded mature leaves located on branches of similar height (see ahead) to minimize microhabitat effects on leaf chemistry and herbivory.…”

“…We then transferred the extracts to chromatographic vials. To perform the chromatographic analyses we used Ultra‐High‐Performance Liquid‐Chromatograph (UHPLC Nexera LC‐30AD; Shimadzu) equipped with a Nexera SIL‐30AC injector and one SPD‐M20A UV/VIS photodiode array detector (Moreira, Abdala‐Roberts, et al., ). Compound separation was carried out on a Kinetex ™ 2.6 μm C18 82‐102 Å, LC Column 100 × 4.6 mm, protected with a C18 guard cartridge.…”

“…Third, studies have usually measured herbivory or plant traits associated with herbivore damage, but not both (e.g., Adsersen & Adsersen, ; Burns, ; Givnish, Sytsma, Smith, & Hahn, ; Kavanagh, ), leading to an incomplete understanding of insularity effects on plant–herbivore interactions. This aspect is also important in relation to the previous point, as abiotic factors may concurrently influence herbivores and plant traits (Moreira, Abdala‐Roberts, et al., ). For example, climatic variables can directly influence herbivory by shaping herbivore abundance or activity, but at the same time may also exert indirect effects on herbivory by altering plant physical or chemical traits that affect herbivores (Huberty & Denno, ; Jactel et al., ).…”

“…We predicted that oak trees found on islands would have lower levels of insect leaf herbivory and phenolic compounds than their mainland counterparts, and that such patterns would be underlain by differences in climatic factors. Phenolic compounds have been found to confer resistance against insect herbivores in Quercus species (Feeny, ; Moreira, Abdala‐Roberts, et al., ; Moreira, Abdala‐Roberts, et al., ; Roslin & Salminen, ), and were thus considered a suitable proxy for chemical defences in holm oak. Overall, this study contributes to better understand the ecological and evolutionary effects of insularity on plant–herbivore interactions and its underlying drivers.…”

Effects of tree species diversity on insect herbivory and leaf defences in Cordia dodecandra

“…Tack et al, 2010; see discussion in Moreira et al, 2018) and results based on such measurement undoubtedly revealed contrasting effects of diversity on patterns of attack by beetles and caterpillars. Nonetheless, in the case of herbivory by beetle larvae, the amount of leaf damage per leaf was consistently high (> 50% of leaf area lost), which suggests that counts of damaged leaves were probably well correlated with the amount of leaf area lost or damaged by this insect.…”