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

Abstract

Classic research on elevational gradients in plant-herbivore interactions holds that insect herbivore pressure is stronger under warmer, less seasonal climates characteristic of low elevations, and that this in turn selects for increased defence in low-(relative to high-) elevation plants. However, recent work has questioned this paradigm, arguing that it overly simplifies the ecological complexity in which plant-insect herbivore interactions are embedded along elevational gradients. Numerous biotic and abiotic factors vary with elevation, and their simultaneous influences are the focus of current work on elevational gradients in insect herbivory and plant defences. The present review 1) synthesizes current knowledge on elevational gradients in plant-insect herbivore interactions; 2) critically analyses research gaps and highlights recent advances that contribute to filling these gaps; and 3) outlines new research opportunities to uncover underlying mechanisms and build towards a unified theory on elevational gradients. We conclude that the next generation of studies should embrace community complexity -including multi-trophic dynamics and the multivariate nature of plant defence -and to do so by combining observational data, manipulative experiments and emerging analytical tools.

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

“…Considering that mid-elevational peaks in herbivore species diversity and richness and herbivory have been reported in a number of studies from different continents (McCain, 2005;Moreira et al, 2018), this elevational pattern in plant defence might be widespread. Rather, we found that the elevational pattern of spinescence is unimodal, with plants from mid-elevations having the highest proportion of spinescence.…”

“…A number of studies have found higher levels of defence at lower latitudes and lower elevations (e.g. Kooyers, Blackman, & Holeski, 2017;Moreira, Petry, Mooney, Rasmann, & Abdala-Roberts, 2018). However, many other studies report null results (e.g.…”

“…Some authors have suggested that the use of inconsistent methods may have obscured latitudinal and elevational gradients in defence (Moles, Bonser, Bonser, Poore, Wallis, & Foley, 2011). Finally, most studies have not considered the confounding effects resulting from sampling across different life-forms and across defence traits that are effective against different types of herbivores (Abdala-Roberts et al, 2016;Moreira et al, 2018). For example, trichomes and phytoliths are less effective in deterring large mammalian herbivores than insect herbivores (Barton, 2016).…”

Elevational cline in herbivore abundance driven by a monotonic increase in trophic‐level sensitivity to aridity

“…Thus, understanding how climate affects species interactions is essential for predicting changes in community structure under future climate change (Gilman et al, ; Tylianakis et al, ). However, because experimental manipulations of multi‐trophic interactions along climatic gradients are rare, the exact mechanisms driving variation in species abundances and distributions are not well understood (Mooney et al, ; Moreira et al, ; Walther, ).…”

“…Species interactions have classically been proposed to be stronger at lower latitudes and elevations where the climate is warmer and less seasonal (Coley & Barone, ; Moreira et al, ; Schemske et al, ), but recent studies have questioned the generality of this paradigm (Moles et al, ; Moles & Ollerton, ; Zhang et al, ). Such discrepancies may arise in part because most tests for gradients in species interactions fail to adequately incorporate the ecological complexity in which the interactions are embedded (Moreira et al, ). Climate may alter interactions between two focal species through direct physiological effects on one or both of the species involved in the interaction (Gilman et al, ).…”