Test of biotic and abiotic correlates of latitudinal variation in defences in the perennial herb Ruellia nudiflora

Abstract

Summary Geographic variation in abiotic factors and species interactions is widespread and is hypothesized to generate concomitant patterns of species trait variation. For example, higher rates of herbivory at lower latitudes are thought to select for increased plant defences, although latitudinal variation in defences may also be influenced directly by abiotic factors and indirectly by predators and parasitoids reducing herbivore pressure. We measured defences of the herb Ruellia nudiflora among 30 populations spanning a latitudinal gradient from northern Yucatan to southern Belize that vary substantially in leaf herbivory (fourfold), seed herbivory (25‐fold) and seed herbivore parasitism (14‐fold). These surveyed populations span one‐third of the species’ latitudinal distribution (5° of latitude), the entire precipitation gradient of its distribution, and one‐third of the temperature gradient of its distribution. Our prior work showed that leaf herbivory decreased with latitude and that seed herbivory increased with latitude. Here, we measured leaf trichome density and leaf and seed phenolics and tested whether latitudinal variation in climate, herbivory and parasitism explained latitudinal variation in these defensive traits. Patterns of variation in leaf trichomes fully supported predictions, with trichome density increasing with a parallel increase in herbivory towards lower latitudes. While seed phenolics were positively associated with herbivory, and seed herbivory tended to increase with latitude, the predicted (positive) association between latitude and defence was not detectable. There was no detectable association between parasitoids and seed defences. In addition, the association between leaf herbivory and phenolics was weak, and leaf phenolics were not associated with latitude. Importantly, variation in the abiotic environment was associated with plant defence, indicating that abiotic factors can play a major role in shaping plant defences, independently of herbivory. Synthesis. Latitudinal variation in abiotic factors may drive concomitant patterns of variation in plant defences, independently of herbivory. Collectively, these findings highlight the need for assessing geographic variation in plant defences from a multi‐factorial perspective, testing for the simultaneous influence of biotic and abiotic factors.

Plants are more likely to be spiny at mid‐elevations in the Qinghai‐Tibetan Plateau, south‐western China

“…However, although plant damage is influenced by many factors, including herbivore-and plant-related characteristics (Abdala-Roberts et al, 2016;Grubb, 1992), it is widely expected that higher herbivore richness would lead to higher herbivory due to the species complementarity effect or species identity effect (Salazar & Marquis, 2012). In accordance with this expectation, recent experimental or synthetic studies have shown that, concomitant with the increases in herbivore richness, the herbivore pressure also increases (e.g.…”

“…Geographical variation in species interactions is central to our understanding of large-scale patterns in species abundance, diversity and functional traits (Abdala-Roberts, Moreira, Rasmann, Parra-Tabla, & Mooney, 2016;Moeller et al, 2017). Traditional theories, such as the latitudinal herbivory defence hypothesis and the elevational herbivory defence hypothesis, are based on the prediction that biological interactions such as herbivory are stronger at lower latitudes and at lower elevations (Dobzhansky, 1950;Pellissier et al, 2012).…”

Variation in Plant Response to Herbivory Underscored by Functional Traits

“…An association between latitude and plant defense or herbivore load holds for many groups of plants [30, 31] although in some cases the correlation between defenses and latitude may be more reflective of underlying variation in abiotic factors rather than variation in biotic interactions[32–34]. The generality of a latitudinal gradient in herbivory and plant defense has been called into question[35], and a recent analysis found that across taxa most resistance traits were either unrelated to latitude or were greater at higher latitudes[36].…”

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

“…However, systemic induced responses may still take place and are not accounted for using this procedure. Accordingly, measured levels of chemical defences presumably represented a combination of constitutive defences and an unknown level of systemic induction due to herbivory occurred over the growing season (Abdala‐Roberts, Moreira, et al., ). After collection, we oven‐dried leaves for 48 hr at 40°C, ground them with liquid nitrogen, and stored the samples for subsequent quantification of phenolic compounds.…”

“…We tested whether the effect of insularity on insect herbivory was mediated by climate and/or changes in plant defences by running “mechanistic” models with insularity and phylogeographic group as fixed factors, as well as climatic factors or leaf chemical defences as covariates (PROC GLM in SAS 9.4). We tested for effects of defences on herbivory rather than the inverse because our analyses indicated that plant defences were negatively correlated with leaf damage (see section ), suggesting that defences drove herbivory (see Abdala‐Roberts, Moreira, et al., ). Specifically, we ran separate models including either climatic variables (temperature and precipitation) or plant defences, because running a single model with both types of predictors would not permit to tease apart the relative influences of climate versus leaf traits underlying the effect of insularity on herbivory.…”